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Pneumatic Theory

Pneumatic systems, like hydraulic power systems, are classified as fluid power systems. The term "hydraulics" refers to a liquid used as a fluid, while the term "pneumatic" is taken from a Greek word meaning "unseen gas." From this, a fluid system that uses a compressed gas is called a pneumatic system. The gas most commonly used is compressed air, although some applications may use other gases such as nitrogen or carbon dioxide.

1.3.1 Energy
1.3.2 Power Vane Motors
2.10.2 Drills
2.10.4 Hammers
2.10.5 Hoists

Advantages of Fluid Power

  • The extensive use of fluid power to transmit power is due to the fact that properly constructed systems possess a number of favorable characteristics.

They eliminate the need for complicated systems of gears, cams, and levers. Motion can be transmitted without the slack inherent in the use of solid machine parts. Fluids used are not subject to breakage as are mechanical parts, and the mechanisms are not subjected to great wear.

The different parts of a fluid power system can be conveniently located at widely separated points, and the forces generated are rapidly transmitted over considerable distances with small loss.

  • These forces can be conveyed up, down, and around corners with small loss in efficiency and no complicated mechanisms. Very large forces can be controlled by much smaller ones and can be transmitted through relatively small lines and orifices.

If the system is well-adapted to the work it is required to perform and is not misused, it can provide smooth, flexible, uniform action without vibration and is unaffected by variation of load. In case of an overload, an automatic release of pressure can be guaranteed so that the system is protected against breakdown or strain.

Fluid power systems can provide widely variable motions in both rotary and linear transmission of power. Also, the need for control by hand can be minimized. In addition, these systems are economical to operate.

Transmission of Force through Fluids

When the end of a solid bar is struck, the main force of the blow is carried straight through the bar to the other end, as shown in Figure 1A. This happens because the bar is rigid. The direction of the blow almost entirely determines the direction of the transmitted force. The more rigid the bar, the less force is lost inside the bar or transmitted outward at right angles to the direction of the blow.

When a force is applied to the end of a column of confined liquid (Figure 1B), it is transmitted straight through to the other end. This force is also transmitted equally and undiminished in every direction throughout the column (forward, backward, and to the sides) so that the containing vessel is literally filled with pressure.

If a gas is used instead of a liquid, the force is transmitted in the same manner. The one difference is that gas, being highly compressible, provides a much less rigid force than the liquid, which is practically incompressible. This is the main difference in the action of liquids and gases in fluid power systems.

Figure 1: Transmission of Force

Figure 2 shows an example of this distribution of force. The flat hose takes on a circular cross-section when it is filled with water under pressure. The outward push of the water is equal in every direction. An automobile tire and a balloon are examples of this distribution of force through the use of gases.

Figure 2: Distribution of Force

Pascals Law

The foundations of modern hydraulics and pneumatics were established in 1653 when Pascals law was developed. Pascals law states:

    "...If a vessel full of water and closed on all sides has two openings, one a hundred times as large as the other, and if each is supplied with a piston that fits exactly, then a man pushing the small piston will exert a force that will equal that of one hundred men pushing the large piston and will overcome that of ninety-nine men."

For Pascals law, illustrated in Figure 3, to be made effective for practical applications, it was necessary to have a piston fit exactly. It was not until the late 18th century that methods were found to make these snugly fitted parts required in fluid power systems. This was accomplished by the invention of machines that were used to cut and shape the closely fitted parts and by the development of gaskets and packing. Since that time, such components as valves, pumps, actuating cylinders, and motors have been developed and refined to make hydraulics and pneumatics two of the leading methods of transmitting power.

Figure 3: Illustration of Pascal's Law

One of the consequences of Pascals law is that the shape of the container in no way alters pressure relations. Thus, as seen in Figure 4, if the pressure due to the weight of the liquid at one point on the horizontal line (H) is 8 psi, the pressure is 8 psi everywhere at level (H) in the system.

Figure 4: Pressure Relationship with Shape

Pressure due to the weight of a fluid depends, at any level, upon the vertical height of the fluid above. The vertical distance between two horizontal levels in a fluid is known as the head of the fluid. In Figure 4, the liquid head of all points on the level (H) with respect to the surface is indicated.

Pressure due to fluid head also depends upon the density of the fluid. Water, for example, weighs 62.4 pounds per cubic foot, or 0.036 pounds per cubic inch, while certain oil might weigh 55 pounds per cubic foot, or 0.032 pounds per cubic inch. To produce a pressure of 8 psi, it would take 222 inches of head using water but 252 inches using oil (see Figure 5).

Figure 5: Pressure and Density Relationship

This fluid head, which is sometimes referred to as gravity head or altitude head, also applies to gases. Atmospheric pressure at any given altitude is the result of the weight of the air above that altitude. In this case, however, several miles of vertical height are required to produce approximately 14.7 psi at sea level. Therefore, when considering a cubic foot of gas, the gravity head is negligible. For example, a cubic foot of compressed air (100 psi) at 70°F produces a gravity head that is less than 1% of that produced by a cubic foot of water.

Force and Pressure

    The terms force and pressure are used frequently in the preceding paragraphs.

To understand how Pascals law is applied to fluid power, a distinction must be made between force and pressure. Force may be defined as a push or pull. It is a push or pull exerted against the total area of a particular surface and is expressed in pounds. Pressure is the amount of force on a unit area of the surface acted upon. In hydraulics and pneumatics, pressure is expressed in pounds per square inch (PSI). Thus, pressure is the amount of force acting upon 1 square inch of area.

Pressure may be exerted in one direction, several directions, or in all directions. A solid exerts pressure downward, while a liquid will exert pressure on all the surfaces with which it comes in contact. Gas will exert pressure in all directions because it completely fills its container. Figure 6 shows how these pressures are exerted.

Figure 6: Exertion of Pressure

Pressure is normally read in pounds per square inch gauge (PSIG). However, in some instances, it may be necessary to view pressure in its absolute form, or pounds per square inch absolute (PSIA). Pressure in PSIG may be converted to PSIA by adding 14.7 (atmospheric pressure), while pressure in PSIA may be converted to PSIG by subtracting atmospheric pressure.

Computing Force, Pressure, and Area

Although there appears to be three formulas, there is only one formula that can be written in three variations. In this formula, P refers to pressure, F indicates force, and A represents area.

Force equals pressure times area.

Thus, the formula is written:

F = P x A

Pressure equals force divided by the area.

By rearranging the formula, this statement may be condensed into:


By rearranging again, area equals force divided by pressure; the formula is written:


Sometimes, the area may not be expressed in square inches, and dimensions are given instead.

If it is a rectangular surface, the area may be found by multiplying the length (in inches) by the width (in inches). The majority of areas to be considered in fluid power calculations are circular in shape. Either the radius or the diameter is given. The radius (in inches) must be known to find the area; the radius is one-half the diameter. Then, the formula for finding the area of a circle is used. This is written:


A = area

π = 3.1416

r2 = radius squared

Force and Pressure in Fluid Power Systems

In accordance with Pascals law, any force applied to a confined fluid is transmitted in all directions throughout the fluid regardless of the shape of the container. Consider the effect of this in the system shown in Figure 7. This is a modification of Figure 1(B) in which the column of fluid is curved back upward to its original level with a second piston at this point. If there is a resistance on the output piston (2) and the input piston is pushed downward, a pressure is created throughout the fluid that acts equally and at right angles to all surfaces in all parts of the container.

Referring to Figure 7, if the force (1) is 100 pounds and the area of the input piston (1) is 10 square inches, then the pressure in the fluid is 10 psi (100/10). It must be emphasized that this fluid pressure cannot be created without resistance to flow, which, in this case, is provided by the 100-pound force acting against the top of the output piston (2). This pressure acts on the output piston (2) so that, for each square inch of its area, it is pushed upward with a force of 10 pounds. In this case, a fluid column of uniform cross-section is considered so that the area of the output piston (2) is the same as the input piston (1), or 10 square inches. Therefore, the upward force on the output piston (2) is 100 pounds, the same as was applied to the input piston (1). All that has been accomplished in this system is to transmit the 100-pound force around a bend. However, this principle underlies practically all mechanical applications of fluid power.

At this point, it should be noted that since Pascals law is independent of the shape of the container, it is not necessary that the tube connecting the two pistons should be the full area of the pistons. A connection of any size, shape, or length can be used as an unobstructed passage. Therefore, the system shown in Figure 8 where a relatively small bent pipe connects two cylinders will act exactly the same as that shown in Figure 7.

Figure 8: Transmitting Force Through Small Pipe

Multiplication of Forces

In Figure 7 and Figure 8, the systems contain pistons of equal area, and the output force is equal to the input force. Consider the situation in Figure 9, where the input piston is much smaller than the output piston. Assume that the area of the input piston (1) is 2 square inches. With a resistant force on piston (2), a downward force of 20 pounds acting on piston (1) creates 10 psi (20 lbs/2in) in the fluid. Although this force is much smaller than the applied force in Figure 7 and Figure 8, the pressure is the same. This is because the force is concentrated on a relatively small area.

Figure 9: Multiplication of Forces

Differential Area

In Figure 10, a somewhat different situation is illustrated. A single piston (1) in a cylinder (2) has a piston rod (3) attached to one side of the piston. The piston rod extends out one end of the cylinder.

Fluid under pressure is admitted to both ends of the cylinder equally through the pipes (4, 5, and 6). The opposed faces of the piston (1) behave like two pistons acting against each other. The area of one face is the full cross-sectional area of the cylinder, 6 square inches, while the area of the other face is the area of the cylinder minus the area of the piston rod, which is 2 square inches. This leaves an effective area of 4 square inches on the right face of the piston. The pressure on both faces is the same: 20 psi.

Applying the rule previously stated, the force pushing on the left face of the piston is its area times the pressure, or 120 pounds. Likewise, the force pushing on the right face of the piston to the left is its area times the pressure, or 80 pounds. Therefore, there is a net unbalanced force of 40 pounds acting on the left face pushing the piston to the right. The net effect is the same as if the piston and the cylinder were just the same size as the piston rod, since all other forces are in balance.

Compressibility and Expansion of Gasses

    Two of the major differences between liquids and gases are compressibility and expansion.

While liquids are practically incompressible, gases are highly compressible. Gases tend to completely fill any container, while liquids fill a container only to the extent of their normal volume.

Although both liquids and gases expand when heated, gases expand much more than liquids (approximately nine times as much as water). Unlike liquids, all gases expand approximately the same. Because of these characteristics, there are several laws concerning the compressibility and expansion of gases. These laws are discussed in the following paragraphs.

The high compressibility characteristics of gases provide an excellent means for storing energy. A gas may be compressed to a high pressure in a small volume and contain potential, or stored energy. When the gas is released, the resultant decrease in pressure allows the gas to expand and, in a well-designed pneumatic system, perform work.

Boyles Law

Compressibility is an outstanding characteristic of gases. The English scientist Robert Boyle was among the first to study this characteristic, which he called the springiness of air. By direct measurement, he discovered that when the temperature of an enclosed sample of gas was kept constant and the pressure doubled, the volume was reduced to half the former value. As the applied pressure was decreased, the resulting volume increased. From these observations, he concluded that for a constant temperature, the product of the volume and the pressure of an enclosed gas remains constant. This became Boyles law, which is normally stated:

    "The volume of an enclosed dry gas varies inversely with its pressure, provided the temperature remains constant."

This law can be demonstrated by confining a quantity of gas in a cylinder, which has a tightly fitted piston. A force is then applied to the piston so as to compress the gas in the cylinder to some specific volume. When the force applied to the piston is doubled, the gas is compressed to one-half its former volume, as indicated in Figure 11.

Figure 11: Gas Compressed to Half its Former Volume by a Doubled Force

In equation form, this relationship may be expressed as:

V1 P1=V2 P2

Where V1 and P1 are the original volume and pressure, and V2 and P2 are the new volume and pressure.

Example of Boyles Law:

Four cubic feet of nitrogen are under a pressure of 100 psig. The nitrogen is allowed to expand to a volume of 6 cubic feet.

What is the new gauge pressure? Remember to convert gauge pressure to absolute pressure by adding 14.7.

Formula or equation:

V1 P1=V2 P2




P2=76.47 psia

Converting from absolute pressure to gauge pressure:

76.47-14.7=61.77 psi (Answer)

Changes in the pressure of a gas also affect the density. As the pressure increases, the volume decreases; however, there is no change in the weight of the gas. Therefore, the weight per unit volume (density) increases; it follows that the density of a gas varies directly with the pressure if the temperature is constant.

Charles Law

The French scientist Jacques Charles provided much of the foundation for the modern kinetic theory of gases. He found that all gases expand and contract in direct proportion to the change in the absolute temperature, provided the pressure is held constant.

Expressed in equation form, Charles law may be expressed as:

V1 T2=V2 T1


V1⁄V2 =T1⁄T2 where V1 and V2 refer to the original and final volumes and T1 and T2 indicate the corresponding absolute temperatures.

Since any change in temperature of a gas at constant pressure causes a corresponding change in volume, it is reasonable to expect that if a given sample of a gas were heated while confined within a given volume, the pressure should increase. By actual experiment, it was found that for each 1°Celsius increase in temperature, the increase in pressure was approximately 1/273 of the pressure at 0°C. Because of this fact, it is normal practice to state this relationship in terms of absolute temperature. In equation form, this part of the law becomes:

P1 T2=P2 T1 or:

P1⁄P2 =T1⁄T2

In other words, the absolute pressure of a gas varies directly with the absolute temperature, provided the volume is held constant.

Examples of Charles Law:

A cylinder of gas under a pressure of 1,800 psig at 70°F is left out in the sun and heats up to a temperature of 130°F.

What is the new pressure the cylinder?

The pressure and temperature must be converted to absolute pressure and temperature.

Formula or equation:

P1⁄P2 =T1⁄T2

Absolute temperature cube expressed using the Rankine system. Temperature given in degrees Fahrenheit is converted to absolute, or Rankine, temperature by adding 460°.

Using the Rankine system:

70℉=530° absolute

130℉=590° absolute


(1800+14.7) ⁄ P2 = 530/590



P2=2020 psia

Converting absolute pressure to gauge pressure:

2020-14.7=2005.3 psig (Answer)

By combining Boyles law and Charles law, a single expression can be derived that includes all the information contained in both. It is referred to as the General Gas Law, which states:

    "The product of the initial pressure, initial volume, and new temperature (absolute scale) of an enclosed gas is equal to the product of the new pressure, new volume, and initial temperature."

It is a mathematical statement whereby many gas problems can be solved involving the principles of Boyles law and/or Charles law.

The equation may be expressed as:

P1 V1 T2=P2 V2 T1


(P1 V1)/T1 =(P2 V2)/T2

The general gas law applies with exactness only to ideal gases in which the molecules are assumed to be perfectly elastic. However, it describes the behavior of actual gases with sufficient accuracy for most practical purposes. Two examples of the general equation follow:

    Two cubic feet of a gas at 75 psig and 80°F are compressed to a volume of 1 cubic foot and then heated to a temperature of 300°F. What is the new gauge pressure?

Formula or equation:

(P1 V1)/T1 =(P2 V2)/T2

Using the Rankine system:

80℉=540° absolute

300℉=760° absolute


((75+14.7)(2))/540=(P2 (1))/760




P2=252.5 psia

Converting absolute pressure to gauge pressure:

252.5-14.7=237.8 psig(Answer)

    Four cubic feet of a gas at 75 psig and 80°F are compressed to 237.8 psig and heated to a temperature of 300°F. What is the volume of the gas resulting from these changes?

Formula or equation:

(P1 V1)/T1 =(P2 V2)/T2

Using the Rankine system:

80℉=540° absolute

300℉=760° absolute






V2=2ft3 (Answer)


When friction is neglected, the work output is always equal to the work input. Work is a form of energy and all forms of work can be converted to and from energy. Work always involves actual movement, but energy can be at rest and still exist as energy as long as it is capable of doing work.

Energy can exist in many different forms, but all have one thing in common; they are all interchangeable with each other and with work. Some of the many forms that energy can take and their interchangeability are illustrated by a hydroelectric plant, as shown in Figure 12. Here, a body of water is held back by a dam. In this case, the water represents potential energy because it is not doing work at the moment but is capable of doing work if it is released. If an opening is provided, water will rush out in a high velocity jet, representing energy of motion or kinetic energy. If this jet is directed against the blades of a water wheel, it will push them around, producing a continuous rotary motion. This is work in its true sense because a force is moving through a distance.

Figure 12: Potential and Kinetic Energy

The water wheel can be connected to an electric generator, which converts the work into electricity. This electricity can be converted back into work by the use of an electric motor; or it can be converted into light by the use of an electric bulb or into heat in an electric iron. By means of a motor and a pump, the energy can be transformed back into its original form of potential energy existing as a body of water at an elevation. Thus, all of these forms of energy are interchangeable with each other. In actual mechanisms, there is always some loss in the form of heat, which is produced by friction at every exchange. However, the total energy, useful and wasted, will always add up to the original input energy.

Many hydroelectric facilities operate as discussed in the preceding paragraph. The water wheel is turned by flow from a reservoir, and the electric generator provides power for distribution to the surrounding area. When power usage in the area is low, electrical power is used to operate motors and pumps that move water back up to a reservoir, where it remains as potential energy. This potential energy may then be used later during peak power usage periods by opening gates and allowing the stored water to turn the water wheel.

    For simplicity, friction was disregarded in the preceding discussion.

However, it is well-known that there is always some friction in actual machines. It is also known that heat is produced whenever work is accomplished against friction. Therefore, heat is a form of energy because it can be produced from work. Likewise, heat in the form of fire under a boiler can be converted into work through the medium of a steam engine.

Friction represents a loss of efficiency, but this does not mean an annihilation of energy itself. It means only that some of the energy put into the system has been converted into another form, which is not useful for the particular problem at hand. The energy is not usable or available, but it still exists as dissipated heat.

    Any work or energy added to the system must, in turn, come from somewhere else, because it is not possible to create or destroy energy. All that can be accomplished is to change it from one form into other forms so as to make it more or less applicable to the purposes at hand.

In the case of a hydraulic jack, since there is always some friction both within the liquid and between adjacent parts, the useful work output will not exactly equal the work input, but the difference will always exist somewhere in some other form of energy. In this case, it will appear as heat that must escape from the system somewhere at some time. In other words, while the usable work output does not equal the input, the total energy output in all forms will always exactly equal the total energy input (Law of the Conservation of Energy).


    Work and energy are measured in the same units, the foot-pound in the English system and the gram-centimeter in the metric system.

Power is "the rate of doing work." More power is required to do a given amount of work where less time is used. The unit for measuring power in the English system is the horsepower, which is 33,000 foot-pounds per minute, or 550 foot-pounds per second. The metric system uses the centimeter-gram per second as the unit of power.

Fluid Flow

    In the operation of fluid power systems, there must be a flow of fluid. The amount of flow will vary from system to system.

To understand fluid power systems, it is necessary to become acquainted with some of the elementary characteristics of fluids in motion. Among these are volume and velocity of flow, steady and unsteady flow, and streamline and turbulent flow. Even more important are the force and energy changes that occur in flow and the relations of different kinds of energy to each other in fluid power systems.

Volume and Velocity of Flow

The quantity of fluid that passes a given point in a fluid power system in a unit of time is referred to as the volume of flow. Volume of flow can be stated in a number of ways; for example, it can be stated as 100 cubic feet per minute, 100 gallons per minute, 100 gallons per hour, etc. Gallons per minute is the usual method of expressing volume of flow in hydraulic systems, while cubic feet per minute is common in pneumatic systems. The relative pressure of the fluid is usually considered when expressing the volume of flow. This is especially important when considering the volume of flow of gases, since they are compressible. For example, at the same temperature, a cubic foot of gas at 100 psi contains twice as many molecules as a cubic foot of gas at 50 psi.

Velocity of flow means "the rate or speed at which the fluid moves forward at a particular point in the system." It too can be variously measured, but the usual method is in feet per second. Volume and velocity of flow are often considered together.

With volume of flow constant, the velocity of flow increases as the cross-section or size of the pipe decreases, and the velocity of flow decreases as the cross-section increases. In a stream, velocity of flow is slow at wide parts of the stream and rapid at narrow parts, even though the volume of water passing each part of the stream is the same. In Figure 13, if the cross-sectional area of the pipe is 16 square inches at point (A) and 4 square inches at point (B), the velocity of flow at (B) is four times the velocity at (A).

Steady and Unsteady Flow

A fluid may flow as a single continuous stream, or the volume of flow may increase, decrease, or fluctuate from moment to moment. Such changes in volume constitute unsteady flow. For example, when a faucet is first opened, the initial flow is unsteady during the short time that the rate of flow of the water is increasing from the initial zero rate to the full rate of flow. The flow then becomes steady and is maintained if the pressure remains constant. If the pressure changes, the rate of flow once more becomes unsteady until a new balance is reached.

Streamline and Turbulent Flow

At quite low velocities or in tubes of small diameter, flow is streamline, meaning that a given particle of fluid moves straight forward without crossing the paths followed by other particles and without bumping into them. Streamline flow is often referred to as a laminar flow, which is defined as a flow situation in which fluid moves in parallel lamina or layers. As an example of streamline flow, consider Figure 14, which illustrates an open stream flowing at a slow uniform rate with logs floating on its surface. The logs represent particles of fluid. As long as the stream flows at a slow uniform rate, each log floats downstream in its own path without crossing or bumping into the others.

However, if the stream narrows and the volume of flow remains the same, the velocity of flow increases. If the velocity increases sufficiently, the water becomes turbulent, as shown in Figure 15. Swirls, eddies, and cross-motions are set up in the water. As this happens, the logs are thrown against each other and against the banks of the stream, and the paths followed by different logs will cross and re-cross.

Particles of fluid flowing in pipes act in the same manner. The flow is streamline if the fluid flows slowly enough and even at greater velocities when the diameter of the pipe is small. If the velocity of flow is increased or size of pipe is increased sufficiently, the flow becomes turbulent.

One effect of turbulent flow is shown in Figure 16, where the length of the horizontal arrows indicates the relative velocities of flow at different places in the pipe, from the center to the edge when the flow is streamline and when the flow is turbulent. In both instances, the rate of flow varies from the center of the pipe to the edge, but the streamline flow varies more in velocity than the turbulent flow. For streamline flow, the average velocity is about one-half the maximum velocity, while for turbulent flow it is about four-fifths. Velocity of flow varies from the center of the pipe outward in all directions. In both streamline and turbulent flow, the fluid next to the wall of the pipe has no velocity.

While a high enough velocity of flow will produce turbulence in any pipe, other factors contribute to turbulence. Among these are the roughness of the inside of the pipe, obstructions, the degree of curvature of bends, and the number of bends in the pipe. In setting up or maintaining fluid power systems, care should be taken to eliminate or minimize as many causes of turbulence as possible, since the energy consumed by turbulence is wasted.

While designers of fluid power equipment do what they can to minimize turbulence, to a very considerable extent it cannot be avoided. For example, at 68°F, flow becomes turbulent at velocities over 6 inches per second in a 4-inch pipe or about 3 inches per second in a 6-inch pipe. These velocities are far below those commonly encountered in fluid power systems where velocities of 5 feet per second and above are common. In streamline flow, losses due to friction increase directly with velocity, while with turbulent flow, these losses increase much more rapidly.

Factors Involved in Flow

An understanding of the behavior of fluids in motion, or solids for that matter, requires an understanding of the term "inertia". Inertia is the term used by scientists to describe that property possessed by all forms of matter that makes the matter resist being moved if it is at rest, and likewise, resists any change in its speed if it is moving.

The basic statement covering the action of inertia is:

"A body at rest tends to remain at rest, and a body in motion tends to continue in motion with the same velocity and in the same direction."

Simply put, one must push an object to start it moving and offer an opposition to stop it again.

A familiar illustration is the effort a pitcher must exert to make a fast pitch and the opposition the catcher must put forth to stop the ball. Similarly, the engine to make an automobile begin to roll must perform considerable work; although, after it has attained a certain velocity, it will roll along the road at uniform speed if just enough effort is expended to overcome friction. However, brakes are necessary to stop the motion. Inertia also explains the kick or recoil of guns and the tremendous striking force of a bullet.

Inertia and Force

  • To overcome inertia, some force that is not otherwise canceled must act upon an object. This unbalanced force must be applied whenever fluids are set in motion, increased in velocity, or stopped.

Ignoring friction, if the force (A) in Figure 17 produces a velocity of 10 miles per hour (mph) when it is applied to a body for 5 seconds, it will produce a velocity of 20 mph when it is applied for 10 seconds. The same result of 20 mph would be obtained if a force (B) equal to twice force (A) were applied to the body for 5 seconds. Again ignoring friction, the body would be returned to rest from a velocity of 20 mph if force (C), equal to (A) but acting in the opposite direction, were applied to it for 10 seconds, or if a force (D) equal to twice force (C) were applied to it for 5 seconds.

There is a direct relationship between the magnitude of the force exerted and the inertia against which it acts. This force is dependent on two factors: the mass of the object (which is proportional to its weight) and the rate at which the velocity of the object is changed. The force in pounds required to overcome inertia is equal to the weight of the object, multiplied by the change in velocity measured in feet per second, and divided by 32.2 times the time in seconds required to accomplish the change. Thus, the rate of change in velocity of an object is proportional to the force applied. The number 32.2 appears because it is the conversion factor between weight and mass.

As discussed previously, fluids are always acted upon by the force of gravity or, in other words, by their own weight. Also, previously explained is the fact that fluids are acted upon by atmospheric pressure, or the weight of air over the system, if they are exposed to it - if, that is, the system is not enclosed. The action of specific applied force was also explained, and, in addition, it was pointed out that whenever there is movement, there is always some friction. Inertia, just described, completes the list of forces that control the action of fluids in motion.

There are five physical factors that can act upon a fluid to affect its behavior. All of the physical actions of fluids in all systems are determined by the relationships of following five factors:

  1. Gravity, which acts at all times upon all bodies, regardless of other forces.
  2. Atmospheric pressure, which acts on any part of a system exposed to the open air.
  3. Specific applied forces, which may or may not be present, but which, in any event, are entirely independent of the presence or absence of motion.
  4. Inertia, which comes into play whenever there is a change from rest to motion or from motion to rest, or whenever there is a change in direction or rate of motion.
  5. Friction, which is always present whenever there is motion.

Kinetic Energy

It was previously pointed out that a force must be applied to an object to impart velocity to it or to increase the velocity it already has. Out of necessity, the force must act while the object is moving over some distance. It was also previously stated that force acting over a distance is work, and that work and all forms into which it can be changed are classified as energy. Energy is then required to give an object velocity. The greater the energy used, the greater the velocity will be.

Likewise, disregarding friction, for an object to be brought to rest or its motion slowed down, a force opposed to its motion must be applied to it. This force also acts over some distance. In this way, energy is given up by the object and delivered in some form to whatever opposes its motion. The moving object is therefore a means of receiving energy at one place (when its motion is increased) and delivering it to another point (when it is stopped or retarded). While in motion, the object contains this energy as energy of motion, called kinetic energy.

Since energy can never be destroyed, it follows that if friction is disregarded, the energy delivered to stop the object will exactly equal the energy that was required to increase its speed. At all times, the amount of kinetic energy possessed by an object depends upon its weight and the velocity at which it is moving. Thus, in Figure 18, force (F) is applied to a body (A), which is at rest. Disregarding friction, after it has moved 1 foot, it will possess kinetic energy equivalent to 1. During each succeeding foot of movement, it gains an equal increment of kinetic energy, so long as the force is applied. If it meets a resistance after moving 5 feet, kinetic energy equivalent to 5 is available to do work. Accelerated motion has been a means of receiving energy, while force (F) was applied to (A) and delivering it to do work at the point (A) reached at that time.

Figure 18: Kinetic Energy

The mathematical relationship for kinetic energy is stated in the rule:

    "Kinetic energy in foot-pounds is equal to the force in pounds that created it multiplied by the distance through which it was applied." Kinetic energy is also equal to the weight of the moving object in pounds, multiplied by the square of its velocity in feet per second, and divided by 64.4.

Relationship of Force, Pressure, and Head

In dealing with fluids, forces are usually considered in relation to the areas over which they are applied. As previously discussed, a force acting over a unit area is a pressure, and pressure can alternately be stated in psi or in terms of head, which is the vertical height of the column of fluid with weight that would produce that pressure.

In most of the applications of fluid power, applied forces greatly outweigh all other forces and, in most systems, the fluid is entirely confined. Under these circumstances, it is customary to think of the forces involved in terms of pressures. Since the term head is encountered frequently in the study of fluid power, it is necessary to understand what it means and how it is related to pressure and force.

All five of the factors that control the actions of fluids can be expressed either as force or in terms of equivalent pressures or head. In each situation, however, the different factors are commonly referred to in the same terms, since on this common basis they can be added and subtracted to study their relationship to each other.

At this point, some terms in general use should be reviewed. Gravity head, when it is of sufficient importance to be considered, is sometimes referred to as head. The effect of atmospheric pressure is referred to simply as atmospheric pressure. Inertial effect, because it is always directly related to velocity, is usually called velocity head, and friction, because it represents a loss of pressure or head, is usually referred to as friction head.

Static and Dynamic Factors

Gravity, applied force, and atmospheric pressure apply equally to fluids at rest or in motion, while inertia and friction apply only to fluids in motion.

The first three are the static factors, and the latter two are the dynamic factors. The mathematical sum of gravity, applied force, and atmospheric pressure is the static pressure obtained at any one point in a fluid at any given time. Static pressure exists in addition to any dynamic factors that may also be present at the same point and time.

Remember, Pascals law states that a pressure set up in a fluid acts equally in all directions and at right angles to the containing surfaces. This covers the situation only for fluids at rest or practically at rest. It is true only for the factors making up static head. When velocity becomes a factor, it must have a direction so that Pascals law alone does not explain to the dynamic factors of fluid power.

The dynamic factors of inertia and friction are related to the static factors. Velocity head and friction head are obtained at the expense of static head. However, a portion of the velocity head can always be reconverted to static head. Force, which can be produced by pressure or head when dealing with fluids, is necessary to start a body moving if it is at rest and is present in some form when the motion of the body is arrested. Therefore, whenever a fluid is given velocity, some part of its original static head is used to impart this velocity, which then exists as velocity head.

Bernoullis Principle

Consider the system shown in Figure 19. Chamber (A) is under pressure and is connected by a tube to chamber (B), which is also under pressure. The pressure in chamber (A) is static pressure of 100 psi. The pressure at some point (X) along the connecting tube consists of a velocity pressure of 10 psi exerted in a direction parallel to the line of flow, plus the unused static pressure of 90 psi, which still obeys Pascals law and operates equally in all directions.

As the fluid enters chamber (B), it is slowed down, and in so doing, its velocity head is changed back to pressure head. The force required to absorb its inertia equals the force required to start the fluid moving originally, so that the static pressure in chamber (B) is again equal to that in chamber (A), although it was lower at the intermediate point.

The situation shown in Figure 19 disregards friction and would, therefore, not be encountered in actual practice. Force, or head, is also required to overcome friction, but unlike the inertia effect, this force cannot be recovered again, although the energy represented still exists somewhere as heat. Therefore, in an actual system, the pressure in chamber (B) would be less than in chamber (A) by the amount of pressure used in overcoming friction along the way.

At all points in a system, therefore, the static pressure is always the original static pressure less any velocity head at the point in question and less the friction head consumed in reaching that point. Since both velocity head and friction represent energy that came from the original static head, and since energy cannot be destroyed, the sum of the static head, velocity head, and friction head at any point in the system must add up to the original static head. This is known as Bernoullis principle, which states:

  • "For the horizontal flow of fluid through a tube, the sum of the pressure and the kinetic energy per unit volume of the fluid is constant."

This principle governs the relations of the static and dynamic factors concerning fluids, while Pascals law states the manner in which the static factors behave when taken by them.

Minimizing Friction

As mentioned previously, fluid power equipment is designed to reduce friction to the lowest possible level.

Volume and velocity of flow are made the subject of careful study. The proper fluid for the system is chosen. Clean, smooth pipe of the best dimensions for the particular conditions is used, and it is installed along as direct a route as possible. Sharp bends are avoided. Valves, gauges, and other components are designed so as to interrupt flow as little as possible. Careful thought is given to the size and shape of the openings. The systems are designed so they can be kept clean inside, and variations from normal operation can easily be detected and remedied.

Advantage of Pneumatics

Due to the similarity between pneumatic and hydraulic operating principles, pneumatic systems possess many of the same advantages as hydraulic systems, including the following common advantages:

  • Complicated gear, cam, and lever systems are eliminated.
  • Motion is transmitted without the inherent slack of solid machine parts.
  • Fluids are not subject to mechanical breakage.
  • Mechanisms are not subjected to great wear.
  • Systems can transmit force in any direction over long distances with little loss.
  • Very small forces can control large forces.
  • They can provide smooth, flexible, uniform action without vibration.
  • They are easily protected from overloads.
  • They can provide a wide variety of rotary and straight-line motion.
  • They are economical to operate.

In addition, using fluid power does not present a spark hazard when used in an explosive atmosphere. Fluid power systems and tools can also be used in wet environments without presenting an electric shock hazard. Another advantage of pneumatics is that air is readily available and can be stored in a tank and transported to a remote area for use. Finally, in pneumatic systems, return lines can be omitted because the discharged air does not need to be returned to a sump in order for the system to continue to operate.

Pneumatic Applications

A pneumatic power system can be designed to perform virtually any task that requires a force or speed to be generated, which exceeds the capabilities of a normal human being. Some common pneumatic applications are:

  • Devices that produce straight-line motion
  • Devices that produce rotary motion
  • Various hand tools
  • Process control devices

There are many common examples that everyone has probably come in contact with at one time or another. Of these, pneumatic tools are probably seen the most often. These include items like a dentists drill, the jackhammer used by a road construction crew, and the impact wrench a mechanic uses to change a tire. Items that may not be recognized as being pneumatic include the brakes on a semi-truck or train and the operating mechanisms on large power circuit breakers.

Low-pressure pneumatic control devices are also used extensively by the process control industry. For virtually every electric control relay made, there is a comparable pneumatic control relay that can perform the same function. Because of this, pneumatic devices can be found controlling everything from valve sequencing, air-conditioning systems, and even providing panel alarms and indications.

Pneumatic Symbols

The study of how a pneumatic system operates is easier to understand by using a diagram of the system. A schematic diagram uses symbols to represent components instead of three-dimensional representations.

The following information about a pneumatic installation can be obtained from the schematic diagrams:

  • Individual components used
  • Component connections
  • Component functions
  • System operation

For a person to be able to extract this information from a schematic, an understanding of the symbols must be obtained first.

The task of figuring out what the individual symbols stand for is much easier than it was before standardization. The International Standards Organization (ISO), Joint Industry Council (JIC), and the American National Standards Institute (ANSI) provide standard sets of symbols for representing fluid power components. These three sets of symbols are similar for nearly all applications. Some of the symbols are common to both hydraulic and pneumatic systems, while others only apply to one or the other. The standard symbols provided by ANSI are shown in Figure 20 through Figure 24.

There are several rules that apply to symbols used in fluid power (both hydraulic and pneumatic) diagrams. The symbols can show connections, flow paths, and functions, as well as conditions that occur during transition from one flow path arrangement to another. Symbols cannot show the locations of ports, direction of shifting spools, and the position of the control elements on actual components. They also do not indicate construction or values such as pressure and flow rate.

Pneumatic systems offer an alternate means of controlling large industrial systems that does not require a large number of electrical components.

Forces can be quickly transmitted over considerable distances efficiently with little loss. Large components can be controlled with very small forces and with smooth, uniform action. Naturally occurring air is used as a median that is cheap and readily available. Overall, pneumatic systems are economical, easy to operate, and virtually foolproof.

Pneumatic Components

There are many individual components necessary for the operation and control of a pneumatic system. Each individual component is available in more than one design. The actual design chosen for use will depend on the application, operating pressure, and maintenance requirements. Types of pneumatic components include:

  • Purification equipment
  • Piping and connectors
  • Cylinders
  • Valves
  • Compressors and motors
  • Tools

Purification Equipment

Purification equipment is the general group of components with the function to treat or condition the air used in a pneumatic system.

Air treatment is required, because most pneumatic systems draw air from their immediate surroundings as a supply for the system. If the air is not properly treated or conditioned, the useful service life of the system components will be shortened dramatically. Purification of the air used in a pneumatic system consists of the following individual steps:

  • Preliminary Filtering
  • Moisture Removal and Drying
  • Additional Filtering
  • Lubrication

Preliminary Filtering

The first step in air purification forces the air to pass through a filter before it enters the compressor suction. This will remove most of the dirt and solid contaminants before they enter the system. There are two types of filters commonly used: dry-type and wet-type.

Dry-Type Filters

Dry-type filters are available in numerous shapes, sizes, and configurations. Figure 25 shows a typical dry-type filter. Most dry-type filters contain a felt or cotton material that is packed into a wire screen or other open retainer. Other types of dry filters use replaceable or cartridge elements.

All of these types of filters can be cleaned. The recommended method of cleaning the packed-type filters is to wash them in a solvent recommended by the manufacturer. Gasoline should never be used as a cleaning solvent. After cleaning, the filters are dried by blowing them out with compressed air.

Blowing them out with compressed air can clean most of the cartridge-type filter elements. The air should be applied in the direction opposite to the normal flow through the element. Another method used on some cartridge filters is to wash in mild detergent and then dry. Rather than spend the man-hours to clean the cartridge-type filters, many organizations find it more cost effective to replace them.

Wet-Type Filters

Wet-type filters use a slightly different type of construction and operating principle. Figure 26 shows a typical wet-type filter. This particular filter uses a maze air filter that is mounted in a shallow oil reservoir.

Figure 26: Wet-Type Filter

The filter operates by forcing the air that enters the top of the filter housing downward into the oil. The air then moves upward through the filter medium before passing into the intake pipe. Any oil that is entrained with the air is trapped on the filter along with the dirt, dust, and other contaminants.

Moisture Removal and Drying

The presence of moisture or water vapor in a pneumatic system can pose a number of problems. The most obvious of these is that rust and corrosion can occur whenever moisture is present. Rust and corrosion result from a chemical reaction that occurs between the iron in the piping and components and the moisture in the air, which flows through them. The corrosion can occur even if the vapor does not condense into water. In addition to the rust, water will mix with impurities in the oil and form corrosive acids. These acids then corrode the inside surfaces of the pneumatic system.

Another problem associated with water in compressed air systems is that water will dilute and wash away the lubricants from the operating surfaces of the pneumatic system components. This, in turn, leads to increased wear and reduced service life. Water can also clog small passages in instruments and controls and make them inoperative. Because of these factors, it is easy to see the importance of keeping a pneumatic system as dry as possible.

Water Removal

There are several different ways to remove moisture and water vapor from compressed air systems. Probably the simplest and most common method is to condense the vapor. Once it is condensed and in a liquid form, it can be collected and drained. Other methods of removing moisture involve circulating the air through some kind of dryer. The dryer will remove any water vapor from the air without condensing it. Dryers are not normally used unless the application requires extremely dry air. Condensing the vapor using an aftercooler will usually remove a sufficient amount of moisture.

There are two different types of aftercoolers that can be used: air-cooled and water-cooled.

Air-Cooled Aftercoolers

An aftercooler, which uses air to condense water vapor, can consist of simple finned tubes or a fan-cooled radiator. These types are shown in Figure 27.

The amount of air that can be cooled by each individual aftercooler depends on three factors:

  • Size of the aftercooler
  • Temperature difference across the cooler
  • Speed at which the air is moving

Because of the fact that most air-cooled units use the air in the room in which they are located as the cooling medium, they cannot lower the temperature enough to condense all of the water vapor. This means that some water vapor will still be in the compressed air when it enters the air receiver and the lines.

Once the water vapor is condensed, it still must be removed from the system. This is the function of the moisture separators that are installed on the aftercooler and air receiver. Depending on the size of the system, a single moisture separator may be used for both the aftercooler and receiver.

Water-Cooled Aftercoolers

The air-cooled units generally have the lowest equipment cost. In contrast to this, the water-cooled units normally prove to be more economical in respect to operations. This is because water will remove heat from a cooling surface at a much faster rate than air. The size of the compressor and the load on the system usually dictate which type of cooler will be used. Small- and medium-sized compressors that do not operate continuously can normally achieve satisfactory operation using an air-cooled after-cooler. Large compressors and those that operate continuously will usually require a water-cooled aftercooler.

The typical water-cooled aftercooler consists of a shell-and-tube-type heat exchanger. The compressed air generally flows through the tubes, and the water flows through the shell and across the tubes to remove heat. This allows for less expensive construction, because the entire shell does not have to withstand compressed air pressure. This type of cooler is shown in Figure 28.

The amount of cooling that can be obtained by using a water-cooled heat exchanger depends on four variables:

  1. Amount of air to be cooled
  2. Amount of water used
  3. Temperature difference across the heat exchanger
  4. Size of the heat exchanger

Because aftercoolers must remove as much moisture as possible, as well as reduce the volume of compressed air, they are generally quite large. A moisture separator in a similar fashion must also collect the condensed water vapor to the air-cooled units.

Refrigerated Air Dryers

Another means of moisture removal by condensation is the refrigerated air dryer. Figure 29 shows a simple refrigerated air dryer. Warm, moist air from the compressor first enters a heat exchanger. This causes its temperature to be slightly lowered. The air leaving the heat exchanger then flows through the refrigeration unit, where it comes into contact with the refrigeration coils. This will cool the air to approximately 30oF, which causes the water vapor to condense. The condensate is removed by the moisture separator, and the air flows back through the heat exchanger and out into the system. This type of dryer removes almost all of the water vapor from the air.

Moisture Separators

As mentioned previously, the water that is condensed in the aftercoolers must be removed. This is the function of the moisture separator. Most systems or aftercoolers use mechanical separators to remove the condensate. Most of the separators force the air to swirl or make sharp changes in the direction of flow to separate the water from the air. Other types discharge the air stream against a flat surface or use a large chamber to lower the air velocity, which allows the water particles to drop to the bottom. This last type is called gravity separation.

Figure 30 shows the separating action used by the various mechanical separators. All of these types cause some type of swirling action, which causes the heavier water particles to be deposited on the outside wall. The water particles then flow down into the moisture trap, located at the bottom of the separator. These types of mechanical separators will remove up to 95 percent of the liquid from a stream of compressed air.

After the water has been removed from the compressed air, it must then be removed from the separator. The bottom of the separator contains a trap and float valve assembly, as shown in Figure 31. As the bowl fills with water, the float will rise and open the drain valve. When the water drains out, the float will drop back down and close the valve. The drain valve is positioned above the bottom of the bowl so that those solid particles cannot lodge between the valve and its seat, which might prevent it from closing. Any sludge or solid particles that collect in the bottom can be removed by opening the drain plug.

Figure 31: Moisture Separator and Trap Assembly

Air Dryers

Certain pneumatic components, like gauges and sensing devices, require a completely dry supply of air. This quality of air cannot be achieved by using aftercoolers, so these applications will require the installation of an air dryer. There are two types of air dryers currently available: deliquescent and adsorption.

Figure 32 shows a deliquescent, or chemical, air dryer. This type of dryer gets its name because of the chemical reaction that takes place as the desiccant inside the dryer absorbs moisture. As the desiccant absorbs moisture, the desiccant will slowly dissolve and become a liquid. As the desiccant dissolves, it is said to deliquesce.

During dryer operations, moist, dirty air enters the dryer at the bottom and is dispersed in the tank by an inverted dished plate. At this point, some of the liquid and solid particles will settle to the bottom. The air is then pre-dried in the intermediate section of the dryer. This occurs as the air passes through the deliquescent mist, which settles out from above. The mist will collect some of the finer vapors and form drops that are large enough to settle to the bottom of the dryer. The air will then flow through the desiccant bed, which chemically attracts the moisture and removes it from the air. The moisture will wet the desiccant, causing it to dissolve and form droplets of moisture that settle to the bottom of the tank. The air that leaves the dryer is clean, dry, and non-toxic. This type of dryer can handle a large volume of air. Its biggest disadvantage is that it must be refilled as the desiccant dissolves.

An adsorption-type air dryer is slightly different. This type of dryer is commonly used in high volume applications and is a little more complicated than any of the drying equipment discussed up to this point. Figure 33 is a simplified drawing of an adsorption-type dryer.

In a dual dryer arrangement such as that shown, only one dryer is used at a time. The air enters through a four-way valve and is directed into the left-hand chamber. Once in the chamber, the air passes down through a desiccant bed and then up through a return tube in the center of the desiccant bed. The air leaves through a three-way valve and flows back into the compressed air line as very dry air.

Besides the flow path, the actual mechanics of how this dryer operates are different, as well. As the incoming air passes through the desiccant bed, the moisture is attracted to the surfaces and pores of each individual granule of desiccant material. It is important to note that the moisture is not absorbed; it is just held on the surface. This process is called adsorption and is the result of a physical attraction between the moisture and unsaturated desiccant.

The temperature, speed of movement, and purity of the incoming air affect the amount of moisture that is removed by the dryer. Slow-moving cold air increases the adsorption capacity of the desiccant. Air purity is also very important. If oil and other impurities are not removed before the air passes through the desiccant bed, they will clog up the minute passages in the individual granules and permanently reduce the capacity to remove moisture.

While the air is being dried in the left chamber, the desiccant is being regenerated in the right chamber. Regeneration is accomplished by the property of the desiccant bed to release moisture when heated. The heat source can be an electric bed heater, heated purge air from the discharge of the online chamber, or heated purge air from an external source.

Regardless of the method used, a quantity of dry air will be forced through the chamber being regenerated in the reverse direction. This air absorbs the moisture released by the bed and purges it to the atmosphere. After a suitable length of time, the heat is turned off, and the bed is allowed to cool. After the bed is cooled, a timer will switch the left chamber to regeneration and the right chamber to air-drying.

Additional Filtering

Secondary or additional filtering equipment is usually located near the actual component or piece of equipment that is being operated by the air system. The preliminary filtering that has already taken place has removed a large portion of the dirt and contaminants present. However, the preliminary filtering does not clean the air to the standards required by certain pneumatic components. Additional filters and strainers are installed to produce the extremely clean air required by many pneumatic valves and actuators.

Filtering Contaminants

The big difference between strainers and filters is the size of the particles that they can remove. Particle sizes are measured in microns. One micron is equal to 0.000039 inches or, for simplicity, approximately 0.00004 inches.

To better understand the size of a micron, remember that there are about 25 microns in a thousandth of an inch (0.001). Individual classes of particles are based on size. Particles that are 10 microns or larger are classified as dust. The next smaller class of particles is known as cloud particles. These range from 0.1 - 10 microns in size. Smoke particles range from 0.01 to 1.0 microns in size. The most difficult particles to remove from a compressed air system are aerosols. The aerosol particles present are very fine oil particles that range in size from 0.01 to 0.8 microns. These particles cannot be removed from compressed air systems by aftercooling or ordinary separators. Most filters used will not remove these either.

Separators, strainers, and filters are identified by the size of the particle that they can remove. This is in contrast to classification according to the size of the opening. Most strainers and filters will remove 98% of the impurities up to or larger than their rated size. The particles that pass through are generally long and slim, well-rounded, or shaped in some other manner such that they are not easily removed.

Filter Class and Ratings

Filters that are designed to remove solid particles from a compressed air system are either surface filters or depth filters.

Figure 34 is a simplified representation of each of these types. As can be seen, the major difference is that a surface filter will only collect particles on a single surface, while a depth filter collects particles on several different layers. The surface filter openings are normally all the same size and arranged in a regular pattern. Depth filter openings vary in both size and geometric arrangement. Typically, the larger openings are on the outside, and they get progressively smaller on the inside. Other applications reverse this arrangement and place the largest openings on the inside. Regardless of the type of arrangement used, the filter will be installed so that the air always flows through the larger openings first.

In addition to the two types of filers, the different filters in each type can be rated in a number of different areas.

Nominal Rating

The nominal rating of a filter is given as a percentage of the number of particles of a given size that it can stop. For example, a 50-micron filter rated to 95 percent will remove 95 percent of the contaminants which are 50 microns or larger. Five percent of the 50-micron or larger particles will still pass through.

Absolute Rating

The absolute rating of a filter gives the smallest particle that the filter can completely remove. Filter ratings are given for new, clean filters with a specific airflow. Most manufacturers will provide a chart or table that specifies the nominal and absolute filter ratings at different airflow rates.

Dirt Capacity

The ability of a filter to remove and retain contaminants is known as its dirt capacity. This capacity varies with the amount, size, type, and concentration of solid particles in the air being treated. The airflow velocity and the maximum allowable pressure drop also affect the capacity across the filter. For example, a heavy layer of large, coarse particles can build up without causing a high filter pressure drop. In contrast, a thin layer of small, fine, or sticky particles pack together and cause a large pressure drop. Also, higher concentrations cause a filter to load up faster.

Filter capacity is also related to the size of the openings compared to the size of the particles being filtered. Filters with large openings should only be used to filter large particles, and those with small openings, small particles. If a single filter is used to remove all particle sizes, it will load up quickly and require cleaning or replacement at shorter intervals. For this reason, single filters are not very efficient.

The preceding discussion has dealt with a filters ability to stop particles. The remaining ratings and terms deal with the filters ability to retain the particles that it removes. The ability of a filter to retain contaminants can be directly related to the pressure drop across the filter. The maximum allowable pressure drop for a given filter is the pressure difference that would allow the collected particles to pass through the filter. This process is called contaminant migration. The maximum allowable pressure drop for a given filter will depend on the manufacturers rating (particle size) and the filter medium used. The filter that is selected for a particular application should have a rating that is slightly higher than the maximum allowable pressure drop that the system can tolerate. Relative capacities of surface and depth filters are shown in Figure 35.

Abrasion migration is another process that can occur, which allows previously stopped particles to pass between the filtering medium and filter housing and then out into the system. This is the result of the filter becoming loose in its housing. The common cause is too much vibration.

The last process that can occur is medium migration. This occurs when part of the filter medium breaks off and is carried into the system along with the air that it is filtering. Contaminant release takes place when the trapped particles or part of the filter move around on the surfaces of the filter medium.

The performance of any filter depends on its shape, size, and the arrangement of the openings. Surface filters generally have a narrow range of performance due to the regularly spaced openings. A depth filter will perform over a much broader range of conditions because of the irregular and varied openings. Depth filters can remove both small and large particles because of the different-sized openings.

Surface Filters

Pneumatic systems are generally equipped with a wire mesh surface filter or strainer. They can be located to remove the larger particles from either the free air or the compressed air in the lines to the equipment. Most of these are constructed of stainless steel wire that is woven to provide a specified number of openings per square inch of area. This weave is called the mesh of the cloth or screen.

The following materials are also used to make surface filters:

  • Sintered (Slag) metal powders
  • Thin-felted fibers
  • Thin membranes
  • Single, layer-woven cloth

The wire diameter can vary slightly with different meshes to obtain the required strength and opening size. Strainers are economical filtering devices, but they are not suitable for all applications because of the limited sizes of particles they can remove. Some representative mesh sizes and the corresponding size of particles they can remove are listed in Table 1.


Particle Size in Microns









Depth Filters

There are a number of different types of depth filters used in pneumatic systems. The type of filter material chosen will vary depending on the individual application. Some of the common depth filter applications include:

  • Compressor intake filters (Preliminary)
  • Separators to collect moisture, oil, and solid particles
  • Air line preparation stations as final filtering prior to entering the pneumatic equipment

Depth filters are available in three major types:

  • Dry
  • Wetted
  • Oil Bath

Dry-type filters rely on the filter medium itself to remove the particles from the air. Wetted filters use a coating of oil to help collect and hold the contaminated particles.

Dry-type filters have a lower initial cost then the wet-types, but most of the dry-types use cartridge-type elements that must be replaced. The wet-type filters can all be cleaned so that over a period of time, they may cost less. The disadvantage of the wet-type filters is that the air that leaves will always contain a small amount of oil.

Another type of wetted filter is the oil-bath type. Figure 36 shows an example of the oil bath type of filter.

Figure 36: Oil Bath Filter

This type of filter operates by deflecting the incoming air through the top of an oil bath before directing it through a filter medium. The filter medium becomes coated with oil and holds the contaminated particles. This type of filter is very efficient for large airflows and can normally remove 100 percent of the particles that are 3 microns and larger. These filters can also be cleaned whenever necessary.


The final step in treating air that is to be used in many pieces of pneumatic equipment is lubrication. Most pneumatic power tools, controls, and cylinders require lubricated air to reduce wear and corrosion. Since some types of pneumatic equipment cannot tolerate oil, and others require it. Lubricating devices must be installed at specific points in the air line. Most installations combine final filtration, regulation, and lubrication at a common point. Lubricators should be installed as close as possible to the device or equipment requiring lubrication with a recommended maximum distance of 15 air line feet from the furthest device requiring lubrication. This helps to ensure that adequate lubrication is supplied where lubrication is required. Vertical bends in the piping between the lubricator and the equipment being serviced should also be minimized to ensure proper lubrication is obtained.

Compressed air system lubricating devices can be separated into two basic types: heavy lubricators and fine lubricators.

Heavy Lubricators

Heavy lubricators will supply both a fine suspended oil mist and heavy droplets of oil that form an oil film inside the air lines. Some manufacturers also refer to this type of lubricator as an oil-fog or oil-mist lubricator. Heavy lubricators should only be used for continuous or high airflows. Several different types of heavy lubricators are shown in Figure 37. The first is a wick-type, and the others are Venturi-types.

Figure 37: Heavy-Type Lubricators

The wick-type lubricator contains a porous bronze wick that extends into the oil sump. The wick becomes saturated with oil due to capillary action, which draws oil out of the bowl, and into the air passage. The air that flows through the passage picks up the oil and carries it out into the main airstream.

There are several different variations of the wick lubricator available. Some models allow the oiling rate to be varied at any airflow by adjusting the airflow with a secondary bypass adjustment valve. Because all wick-type lubricators are pressurized, caution should always be used when filling, adjusting, or working on them.

The Venturi-type lubricators operate differently. With this type, there is no separate air bypass. The air that enters the lubricator flows through a reduced-diameter section known as a Venturi. The properties of a Venturi cause the speed of the air to increase while the pressure decreases. The low pressure area created by the Venturi draws oil into the measuring chamber. The oil drops and enters the airstream, where they are atomized and carried to the tools being operated. The amount of oil flow is regulated by a needle valve. This type of lubricator does not normally have a pressurized bowl, so it can be filled while in operation.

The reservoirs for all heavy lubricators, as well as all fine lubricators, should be capable of providing a minimum of 200 hours of lubrication at the recommended setting required for the application.

Fine Lubricators

Fine lubricators will provide only a fine suspended mist of oil in the airstream. This type of lubricator provides more positive lubrication at greater distances from the lubricator. This type of unit is especially effective with complicated piping systems and for intermittent equipment operation. These units are also suitable for lubricating more than one piece of equipment from a single air station.

Many fine lubricators are variations or improved types of heavy lubricators. Figure 38 shows a typical fine-type lubricator. When the lubricator is operating, metered oil from the reservoir is drawn into the mixing or measuring chamber. Here, a small portion of the air from the airstream atomizes it. The difference is that the air is directed against a deflector shield instead of out the discharge as in a heavy lubricator. The deflector separates oil droplets greater than 2 microns in size from the finer particles. The heavier oil then drops back into the reservoir, and the oil mist leaves the lubricator. The oil mist mixes with the main airstream as it leaves. There are usually flow guides or vanes provided to vary the amount of lubricating oil supplied as the airflow through the lubricator changes.

Pneumatic Cylinders

Pneumatic cylinders are used to convert the pressure and movement of compressed air into straight-line mechanical force and motion.

The compressed air entering one end of a cylinder causes the piston inside to move. This movement is then transmitted through the piston rod and becomes a mechanical force. The higher the air pressure on the piston, the higher the output mechanical forces. The movement and force of the piston combine to do work. The flow rate (cfm) of the fluid (air) determines the piston speed and also the pneumatic output in horsepower. Pneumatic cylinders are almost identical to hydraulic rams and cylinders.

Many standard and special pneumatic cylinders are manufactured for a variety of applications. These include single- and double-acting, double piston, and double-end rod cylinders. There are also modifications of the standard cylinders, each having its own special name. However, all cylinders use similar components and function in a similar manner.

Most pneumatic cylinders are constructed of steel or brass tubing that has been machined on the inside to a smooth finish. They may also be chrome-plated on the inside to protect them from wear and corrosion. The rod and cap ends are also made of steel or brass and are held in place by threaded rods or bolts, or they are welded to the cylinder. The pistons are usually made of high-grade cast iron or steel. They may also be chrome-plated for corrosion resistance and to improve operating life. Piston rings can be made of formed rubber-like materials, O-rings, or leather. The piston rods are made of plain or stainless steel. Plain steel rods are usually plated. Lubrication of all the metal and nonmetal surfaces in a pneumatic cylinder is normally required to improve service life.

Single-Acting Cylinder

A single-acting pneumatic cylinder has a power stroke in one direction only. The power stroke is usually on the out, or extending, stroke because the piston has a larger surface area on the cap end and can exert more force.

Figure 39 shows several examples of single-acting cylinders.

The load attached to the cylinder or some other external force is required to return the piston to its original position after the work is completed. A small air vent should be installed in the dead side of the cylinder to allow air at atmospheric pressure to fill and escape from the space as the cylinder operates. This prevents an air lock from occurring.

If compressed air is directed to the rod end of the piston instead of the cap or blank end, the in, or retracting, stroke becomes the power stroke. In this type of application, the cylinder is called a pull-type cylinder.

A modified single-acting cylinder is equipped with an internal spring. The spring is located within the cylinder and is used to return the piston to its original position after the power stroke is completed and the pressurized line is vented. The spring is only strong enough to overcome internal friction and exhaust the air from the cylinder. The spring is normally not strong enough to return a heavy load.

The cylinder body of a spring-return cylinder is longer than a double-acting cylinder that has the same stroke. The additional length is needed to accommodate the return spring. The type and strength of spring required for the application is usually selected by the manufacturer. On occasion, an external spring may be used in place of an internal one.

Many single- and double-acting cylinders are equipped with an exhaust flow control metering valve. The valve restricts the amount of exhaust air that can pass from the cylinder through the valve and regulates the speed at which the cylinder can return. In addition, the restriction prevents the cylinder from experiencing any shock caused by a quick return. Many times, this metering out principle causes the cylinder to act as a shock absorber, although it is not considered as such.

Double-Acting Cylinders

Figure 40 shows the main parts of a typical double-acting cylinder. The shape of these individual parts and the methods used to assemble them may vary slightly with different manufacturers. Special modifications are also added to many cylinders to improve their performance in different applications. Although pneumatic cylinders resemble hydraulic cylinders, there is one major difference: pneumatic cylinders do not require lines to return air to the receiver.

The double-acting cylinder shown in Figure 40 can exert a pneumatic force in either direction. When air is directed into port A, which is on the rod end, the piston moves to the right. Air in the cap end of the cylinder is pushed out port B and exhausted to the atmosphere. When air is directed into port B, the piston moves to the left, and air in the rod end is exhausted through port A to the atmosphere. Depending on the application, the exhaust port may be located on the cylinder or in the directional control valve.

Because the rod takes up some of the surface area of the piston for a given pressure, less force can be exerted on the rod side than on the cap side of the piston. Therefore, the thrust is greater when the piston is moved to the left than when it is moved to the right. Most standard cylinders are double-acting. All double-acting cylinders operate smoothly at 10 psi or less under no load conditions.

A modified double-acting cylinder may have a piston rod extending out of both ends. Figure 41 shows this application. This is called a double-rod or double-end rod cylinder. In this cylinder, the surface area of the piston is the same on each side, and the thrust is equal in both directions.

Figure 41: Double-End Cylinder

A double-end rod cylinder is used when motion is required in two directions, or on each end of the cylinder, and when there is only a limited amount of space available. In this type of cylinder, each cylinder end requires a rod bearing and packing.

Reciprocating motion of a part can be created by using a double-end cylinder, as shown in Figure 41. The extra rod extension can also be used to mount cam valves or limit switches that cannot be mounted on a single-rod cylinder.

Two-Piston Cylinder

A two-piston cylinder is similar to a double-end cylinder. However, instead of having one internal piston that reciprocates, there are two independent pistons. The pistons can move in unison with one another or independently, depending on the application. Figure 42 shows a schematic of a two-piston cylinder.

As shown in Figure 42, when compressed air enters port A, both pistons will move apart if ports B and C are vented to the atmosphere. If compressed air is applied through ports B and C, the pistons will move together if port A is vented to the atmosphere. Only venting one of the outside ports to atmosphere can accomplish independent movement. For example, if compressed air is applied to port A with port C closed and port B vented, only the right piston will move.

The pistons can also be set up to operate in a timed sequence using a directional control valve. Compressed air will be applied to port A, and then ports B and C will be vented in a timed sequence so that one piston moves before the other.

Cushioning Devices

Almost all cylinders are available with a cushioning device. This is a way of slowing down the motion of the piston and any attached loads as the cylinder approaches the end of its stroke. Slowing down the piston reduces the possibility of a mechanical or pneumatic shock that would occur if the piston stopped suddenly. Cushioning devices are usually referred to as spears or sleeves. Figure 43 shows an example of a cushioning device.

Figure 43: Cushioning Device

As the piston rod approaches the end of its stroke, the cushion spear or sleeve enters the cushion port. This reduces the size of the opening through which the air leaves the cylinder. The shape of the spear determines how quickly the flow passage is closed and the rate at which piston speed is reduced. When the passage is closed, any air remaining in the cylinder is forced out through a needle or check valve restriction in the cap. The selection of the shape of the spear and the size of the needle valve restriction are based on the applications requirements. Cushioning information is available from pneumatic cylinder manufacturers.

Various other devices can be used to decelerate or slow the speed of a moving cylinder piston. The devices commonly used include metering or needle valves, fixed orifices, check valves, pilot-operated flow control, fast-acting exhaust, and relief valves. Control is usually accomplished in the discharge lines and is referred to as meter-out control.

Pneumatic Control Valves

The various control valves used in pneumatic systems are similar in operation to the directional control valves used in hydraulic systems.

The primary purpose of these valves is to direct the flow of air from one portion of a pneumatic system to another, permitting the actuating devices to perform work. Pneumatic system valves and their respective manifolds and subplates should have a minimum service rating of 150 psi, and the valves should be capable of shifting at 40 psi or less.

Pneumatic control valves can be operated manually or automatically using various mechanical devices, electrical signals, compressed air, or even hydraulic power. Even though there are a number of different types of directional control valves, almost all of these are composed of basic elements.

The control valves used are identified by four different methods:

  1. Number of connecting ports - Two-, three-, four-, and five-way valves. Even though the number of ports usually identifies the valve, some four-way valves are available with five ports. The fifth port is a second exhaust port and performs no other function.
  2. Number of control positions - Two or three, depending on the application. Most two-position valves are simple on/off valves, while three-position valves are left/center/right. Usually, two- and three-way valves are two-position valves, and four- or five-way valves may be either two- or three-position. Some three-way valves have three positions.
  3. 'Type of control element - Poppet, spool, reciprocating disc, and rotary. Many two-way valves are of the poppet-type. Rotary valves can be of either the spool or plate-type.
  4. Method of operation - Manual or automatic. Automatic controls used to operate the valves can be actuated by a mechanical, electrical, pneumatic, or hydraulic power source. Many automatic valves are also equipped with a manual override.

Control Valve Elements

As mentioned previously, control valves may be identified by the internal element that controls the flow of air through the valve. The different types of elements include:

  1. Spool
  2. Poppet
  3. Reciprocating disc
  4. Rotary

Each of these valve elements is different in construction. However, except for the rotary valve, they function in much the same manner. The rotary valve controls the airflow with a rotating rather than a reciprocating or shuttle action. All control valves have some type of internal passage, or flow channel, that connects the external, or primary, working connections of the valve housing.

The spool valve element is the type most widely used in pneumatic systems. The spool valve is preferred for several reasons. It is easy to operate, even at high pressures, because the pressurized surfaces of the spool are almost equal in size and area. The pressure forces tend to balance each other out making the valve easy to shift. The spool design also permits the valve to be made with many variations that change the airflow path through the valve without extensive changes to the body of the valve. Spool valves can also be constructed to shift into more than one position. They can be returned to any one position when not functioning. Shifting is accomplished manually or automatically, depending on the application.

Rotary valves can be operated manually or automatically. Rotary spool valves are made with a round element that has passages that line up with various openings or ports in the side of the valve body. If an element has one drilled passage, it is a two-way valve. If it has two drilled passages, it can be a three-way or a four-way valve. Figure 44 shows a three-position rotary valve.

Figure 44: Three-Position Rotary Spool Valve

Rotary plate, or disc, valves look very much like rotary spool valves from the outside. However, their interior construction is quite different. A port plate, or valve disc, connects the pneumatic ports in different ways. The ports can be located in the top, bottom, or sides of the valves. The internal passages in the disc connect the ports in the body, permitting air to flow through the valve.

Two-Way Valves

Two-way flow control valves are classified as either normally open or normally closed. Figure 45 shows a normally open two-way. This valve permits the air to flow through the valve from the inlet to the outlet when the valve is not actuated. When the valve is actuated, the airflow is shut off. A normally closed valve operates just the opposite. When it is closed, the airflow is shut off. When the spool valve is actuated, air is allowed to flow through the valve. In each case, the valve is held in its normal operating position by a spring.

Figure 45: Two-Way Normally Open Spool

Two-way valves are often used in pneumatic systems to actuate cylinders. They fit into the system well because the air has to flow in only one direction. When released, the air exhausts from the cylinder to the atmosphere.

One of the disadvantages of two-way spool valves is that they are not always pressure-balanced. When they are in their normal operating position, compressed air acts equally on the exposed valve surfaces of the control element. When the control element is shifted, pressure will act on only one valve surface of the element. Additional pressure must be applied to overcome the unbalanced pressure when shifting the valve back to its normal position.

Three-Way Valves

Three-way valves have three primary or working connections on the outside of the valve body and three ports inside the valve. Pilot lines are not considered primary lines. Most three-way valves are shifted back and forth when they are actuated. Three-way valves may be used as diverters or selector valves, holding valves, and directional control valves. Figure 46 shows typical three-way valves. From the figure, it can be seen that all these valves have the same basic construction. The differences between them are the ways their connections are made. In addition to the function they perform, some three-way valves can also be classified as normally open or normally closed. This depends on the application and the position of the valve when it is in the non-actuated position.

The basic three-way control valve can be used to maintain a load in the raised or lowered position. The action it produces depends on the way it is piped into the system. In the non-actuated position shown, the control element is held open by the spring. This connects the power end of the cylinder to the exhaust port "T" in the valve. When the valve is actuated, pressurized air flows through the valve and raises the cylinder.

In some applications, it is desirable for a three-way valve to hold all of its ports blocked at certain times. This is usually referred to as a closed or blocked-center valve. When the valve control element is in the mid-position, both outlet ports are covered even though the inlet is open. When the valve is shifted to the right or left, air flows through the valve. Most valves of this type are supplied with springs in each end that cause the valve to center automatically.

When the three-way diverter valve shown in Figure 46 is in the non-actuated position, the control element is held in place by the spring. Air under pressure at P is able to flow through the valve and out port 1 to cylinder A. When an external force on the stem actuates the valve, pressurized air flows from P through port 2 to cylinder B. When the actuating force is removed, the valve shifts back to the non-actuated position. The diverting valve is useful for alternate operation of two cylinders.

Refer to the diverter and holding in Figure 46. By connecting ports 1 and 2 to the rod and cap ends of the same horizontal cylinder, the three-way diverter valve can be used as a selector valve to control the movement of the cylinder. In the non-actuated position, the control element is held in place by the spring. Pressurized air at P flows through the valve and out port 1 to the rod end of the cylinder. When the valve is actuated, air is directed to the cap end of the cylinder, shifting it forward. This can also be considered a safety circuit, because the air pressure always helps the cylinder retract when the valve is shifted to the non-actuated position.

Four-Way Valves

From the previous discussion, it should now be evident how easy it is to direct airflow in simple pneumatic circuits using two- or three-way valves. However, there are many times when pneumatic circuits become more complicated. For example, more than one two-way valve may have to be actuated at the same time to operate one or more actuators in the system effectively. In another application, dual two-way valves might control the pilot lines of more complex, pilot-operated three- or four-way valves. These, in turn, may control the airflow to an actuator. In many cases, these simple two-way valves may be replaced with a single valve that can perform more than one function at a time. Four-way valves are used in this manner to simplify pneumatic circuits. Even though they may seem more complicated, they reduce the total number of controls in a system.

Figure 47 shows that a four-way valve has four primary or working connections. The connections include a pressure line(P), an exhaust line (T), and two actuator connections ("1" and "2" or "A" and "B"). Although there are two internal exhaust passages, there is only one exhaust port. Many four-way valves have two exposed or ported valves instead of an internal passage, because used or spent air is exhausted directly to the atmosphere. However, they are still referred to as four-way valves.

Figure 47: Typical Four-Way Valve

Usually, a four-way valve supplies air to one end of a cylinder through one chamber while bleeding off air from the other side of the cylinder. When the valve position is reversed, the airflow is reversed. Like other automatic valves, pilot lines are not considered primary lines and are shown separately with their own connections.

Because four-way valves are more flexible in their operating functions, they are manufactured as multiple-position valves. This means that the valves may be shifted into more than one position during their operating cycles. Although there are some exceptions, most four-way valves have two or three operating positions, with the center position being a neutral or rest position. With two-position valves, one of the end positions is the neutral or rest point.

Figure 48 shows a schematic of a two-position, four-way valve. When the spool is in the non-actuated position (shifted left), flow from the pressure line P is directed through port 1 to the cylinder rod end. In conjunction with this, the cylinder cap end is connected through port 2 to the exhaust port. When the spool is actuated, or shifted to the right, the airflow through the valve to the cylinder is reversed.

Figure 48: Two-Position, Four-Way Valve

This type of two-position valve is called a closed crossover valve, because all of the ports are closed when the control element passes through the center position. Most two-position pneumatic valves are of the closed crossover-type. Some valves are designed to have all ports open to each other when the control element passes through the center position. This type of valve is called an open crossover.

Figure 49 shows a typical three-position, four-way valve. This valve has the same internal flow connections (porting) when it is actuated to the left or right as the two-position, four-way valve in Figure 48. The difference is the added center position of the control element. In fact, nearly all three-position, four-way valves have these same internal flow connections when actuated to the left or right. The control element in this particular valve has all of the ports closed when it is centered. The centering is usually accomplished by springs located at each end of the control element.

Figure 49: Three-Position, Four-Way Valve

With a closed-center valve, the actuator is firmly held in the position once the valve is shifted. While the valve is centered, the air pressure at P is shut off to the actuator and the atmosphere. The spring-centered, closed-center valve works extremely well with solenoid-operated controls. If the solenoid does not operate, the control element centers and shuts down the actuator.

Three-position, four-way valves are also manufactured with fully open or partially open centers. The outward appearance of the valve is usually the same as the closed-center valve. However, the internal ports and control elements are quite different. Most of these valves are spring-centered and pilot-operated, although manually operated valves are not uncommon.

Five-Way Valves

A five-way valve is actually nothing more than a special four-way valve with five external connections. It has been pointed out that many four-way valves have two external exhaust ports but are still classified as four-way valves. If these two exhaust passages are changed to pressure passages, the valve becomes a true five-way valve.

As shown schematically in Figure 50, the two exhaust passages are changed to HP (high-pressure) and LP (low-pressure), and the former P passage becomes the E passage. This permits air at two different pressures to operate the actuator. This could provide either a fast advance/hold on a cylinder or a fast/slow speed for a motor application.

Manually Operated Valves

There are various types of manually operated valves used in pneumatic systems. The majority of these are used to isolate, admit, or shut off air in the system. Due to the fact that pneumatic systems normally operate at lower pressures than hydraulic systems, the pressure ratings of most pipe valves make them acceptable for use in a pneumatic system. Manually operated valves are considered two-way valves because they only have two connection ports: inlet and outlet.

Figure 51 shows the commonly used, manual two-way valves. Cast bronze or malleable iron are the materials typically used. The globe valve has some flow losses and a tendency to create turbulent flow if it is not properly sized. Globe valves can be used to regulate a small amount of the airflow by opening them partially. However, they are not designed for use as throttling valves.

Figure 51: Common Manual Two-Way Valves

Another two-way valve is the gate valve. It is similar to the globe valve in service, construction, and pressure ratings. In contrast, a gate valve has low flow losses and little tendency to cause turbulent flow but should still only be used as a shutoff valve. Gate valves cannot be used to regulate airflow in a line because throttling causes the metal seat to wear quickly, causing the valve to leak by.

The two-way plug valve shown is usually made only for 50 psi air service. If it is used at pressures over 50 psi, the force required to maintain the seal between the plug and the body would wedge the plug into the body and make it hard to turn. However, this kind of valve has low flow losses and is fully opened or closed with one-quarter turn of the handle. Other plug valves with ratings up to 200 psi are available.

The plug valve can also be converted into a three- or four-way valve. This is accomplished by removing the pressure spring retainer plug in the bottom of the valve and replacing it with the proper hollow fitting. The hollow fitting provides a place for the air to enter at the bottom of the valve, and it also holds the pressure spring in place. The valve body can be drilled with two, three, or four outlets. The valve then becomes a selector valve and not just an on/off valve.

The ball valve is another two-way valve. A number of ball valves manufactured are suitable for 2,500 psi service. A precision-built ball valve is similar to a plug valve because of the very low flow losses through the valve. However, a ball valve, because of its round shape and the position of the nonmetallic seal rings, is easier to operate than a plug valve. Ball valves are used mainly as shutoff valves to start and stop compressed airflow in a line.

A needle valve is also a two-way valve and is excellent for very high-pressure service or as a throttling or flow-reducing valve. Its packing rating and the strength of the valve body determine the pressure rating of this valve. Because the valve seat is smaller than the inlet and outlet connections, this valve also has a high pressure loss. For applications where the valve will be shut off frequently, a soft seat is recommended to increase the service life. Many manufacturers make needle valves suitable for pneumatic service.

Pilot and Solenoid Valves

The most common, automatically-operated pneumatic control valve is the electrically operated solenoid. Solenoid valves may be either direct-acting or pilot-operated. Figure 52 shows a direct-acting, normally closed, two-way solenoid valve. In this valve, the plunger controls all of the airflow through the valve. When the solenoid coil is de-energized, the plunger is held against the valve seat by the plunger spring. When the electrical solenoid coil is energized, the plunger is lifted, which allows air to flow through the valve.

Due to the fact that solenoids have limited lifting strength, the opposing force of the closing spring must remain small. With a limited spring force, the spring cannot shut off high pressures without first reducing the valve seat size. Therefore, the greater the pressure, the smaller the valve seat size must be.

When large airflows at high pressures are required, a pilot-operated solenoid valve is almost always used. Figure 53 shows this type of valve. The pilot solenoid coil operates a small pilot plunger, which in turn allows air at system pressure to operate the valve poppet or piston. When the pilot plunger is closed, pressurized air flows through the control orifice in the piston and holds the piston against the valve seat. When the solenoid coil is energized, the plunger is lifted, which relieves the air pressure on the top of the piston. Compressed air in the inlet passage then lifts the piston and flows through the valve to the outlet. Although the solenoid valve shown will only permit flow in one direction, two-direction, or reverse flow, valves are also available.

Air Receivers

Pneumatic systems require an adequate supply of fluid (gas) for the efficient operation of the system. In most cases, this supply of gas must be stored under pressure. The amount of pressure and volume depend on the operating requirements of the system.

A receiver is usually part of the compressor system. The compressor forces the gas into the receiver, where it is stored at the maximum pressure required by the system. The stored, pressurized gas may then be provided directly to the pneumatic system, where it is used for system operations and then exhausted to the atmosphere. It may also be used to charge (fill) other receivers such as cylinders or bottles, which are, in turn, used to furnish gas to pneumatic systems.

In addition to its function as a storage point, a receiver may also be used to dampen pulsations and absorb shock from system sources, provide standby, temporary, shutdown or emergency power, compensate for system imbalance, reduce fluid (compressed gas) velocity, and allow contaminant removal or heat transfer to occur.

Through the storage of a volume of gas under pressure, the receiver functions to maintain the pneumatic system at a near-constant pressure and therefore reduces the frequency and length of the start-stop-start cycles of the compressor. It is important that the receiver be of the proper size and construction to function properly. When major changes are made to a pneumatic system following initial installation, receiver sizing and construction requirements should be re-evaluated to ensure continued proper operation of the system.

A typical air receiver (Figure 54) is cylindrical in shape and may be mounted horizontally or vertically, depending on the space available for installation. When vertical mounting is used, the receiver bottom should be convex-shaped to permit proper draining of accumulated moisture, oil, and foreign matter.

Figure 54: Typical Air Receiver

Each receiver should be fitted with the following accessories and connections:

  • Inlet and outlet connections
  • Drain connection and valve
  • Connection for operating line to compressor regulator
  • Pressure gauge (¼" NPT connection)
  • Relief valve
  • Manhole or handhole plate, depending on the size of the receiver

The inlet connection is located near the top of the receiver. The outlet connection is located at a point some distance above the bottom of the receiver. This helps to prevent water, oil, and other foreign matter that settles to the bottom of the receiver from entering the system. The line between the compressor and receiver should be kept as short and free of bends as possible to eliminate excessive vibration due to pulsations of air and to reduce friction caused by the flow of air through the lines.

The relief valve should be of adequate size and capacity to prevent damage to the receiver and the system as a result of a maximum pressure transient. The manhole or handhole plate should be of adequate size and properly located to allow periodic cleaning of the entire receiver inside.

A nomograph, shown in Figure 55, can be used to determine the recommended receiver size for a system. For example, a system requires 40 scfm and atmospheric for 10 minutes with an initial pressure of 100 psi and a minimum pressure of 70 psi (maximum pressure drop of 30 psi).

The first step is to draw a line from the 10-minute point, through the 40-scfm point, to the pivot line. From the pivot line, draw a line to 30 psi on the pressure drop scale. The point where this line crosses the receiver volume scale indicates the required receiver volume of 200 ft3.

Pneumatic Motors

Pneumatic, or air, motors and other rotary air actuators can be used in a variety of applications. All of these devices are used to convert energy into some form of useful work.

Pneumatic motors convert the movement (kinetic energy) and pressure (potential energy) of a stream of compressed air into a continuous rotating force or movement. In general terms, a pneumatic motor converts pressure energy directly into mechanical energy. The output of the shaft is the point where the motor is connected to the machine or device to be operated. The pressure of the compressed air admitted to the motor will determine its force or output torque. The flow rate of the air will determine the speed and the mechanical horsepower output.

The general construction of a pneumatic motor is similar to that of a hydraulic motor or pump. However, certain design changes are made so that pneumatic motors operate efficiently using compressed air. Efficiency is important because the power losses that occur in the compressor and pneumatic system influence motor operation. To make sure that the compressor in a pneumatic system delivers as much compressed air as possible, its volumetric efficiency is usually higher than its mechanical efficiency. This means the air compressing ability of the compressor is designed for maximum output but at the cost of more horsepower for operation.

To ensure that the mechanical output of a motor can be as great as possible, motors are generally designed for greater mechanical and overall efficiency wherever possible. The mechanical efficiency of a motor is usually higher than its volumetric efficiency. This means that the motor will put out a large amount of work, or torque, but at the cost of using more air.

To operate properly, pneumatic motors require sufficient starting or breakaway force. This force is referenced to as torque, and it must be large enough to start rotation while fully loaded. This means that it must overcome the total starting friction within the motor and the connected load. The motor must also overcome inertia while it is building up speed. A motor that has only enough torque to continue to move a load after it has started moving would not necessarily be able to start the load from a dead stop.

Motor Classification

Pneumatic motors are classified by the elements that drive them and by their principle of operation. Motors classified according to the type of motor elements or mechanisms include:

  • Piston motors (rotary and axial)
  • Vane motors

Pneumatic motors are all of the positive-displacement type. This means that they have a mechanical seal somewhere between the inlet and outlet ports. The vanes in a vane motor and the valve plate in a piston motor provide the mechanical seal. The seal prevents internal air leakage and aids the motor in maintaining a steady output motion at the shaft. Non-positive energy converters, such as turbines, are seldom used as motors because they are not as efficient as positive-displacement motors.

Pneumatic motors, similar to hydraulic motors, do not deliver a smooth rotary shaft movement. Because of their construction, they have an intermittent, or pulsating, shaft output motion. However, the pulsations are small enough that the practical effect is one of relatively smooth motion and constant torque.

Positive-displacement motors are also usually of the fixed-displacement type. Figure 56; shows the graphic symbols for pneumatic motors. The differences in construction identify each type. Only changing the pressure and flow rate of the compressed air supplied to the motor can change the shaft output speed of a fixed-displacement motor.

Pneumatic Motor Construction

The construction of pneumatic motors is restricted to a few basic designs. The designs include vane, rotary piston, and axial piston. Each of these motors is a positive-displacement motor, and each one functions by changing the volume of its air chamber. As air under pressure moves through the motor, its pneumatic energy is transferred to the revolving shaft of the motor. Compressed air is prevented from leaking to the low-pressure side of the motor by using close-fitting parts that effectively seal the clearance passages between the moving parts. The ways in which the different types of motors use different arrangements to change the volume of the air chamber and reduce internal leakage or slippage vary, but all operate on the same principle.

Vane Motors

Figure 57 shows a typical vane-type air motor. This particular motor provides rotation in only one direction. The rotating element is a slotted rotor that is mounted on a drive shaft. Each slot of the rotor is fitted with a freely sliding rectangular vane. The rotor and vane are enclosed in the housing, and the inner surface is offset with the drive shaft axis. When the rotor is in motion, the vanes tend to slide outward due to centrifugal force. The shape of the rotor housing limits the distance the vanes slide.

Figure 57: Vane-Type Air Motor

This motor operates on the principle of differential areas. When compressed air is directed into the inlet port, its pressure is exerted equally in all directions. Since area A is greater than area B, the rotor will turn counter-clockwise. Each vane, in turn, assumes the No. 1 and No. 2 positions while the rotor turns continuously. The potential energy of the compressed air is converted into kinetic energy in the form of rotary motion and force. The air at reduced pressure is exhausted to the atmosphere. The shaft of the motor is connected to the unit to be actuated.

Many vane-type motors are capable of providing rotation in either direction. Figure 58 shows a motor of this design. The principle of operation is the same as that of the vane-type motor previously described. The two ports may be alternately used as inlet and outlet, thus providing rotation in either direction. Note that there are springs in the slots of the rotor. Their purpose is to hold the vanes against the housing during the initial starting of the motor, since no centrifugal force exists until the rotor begins to rotate.

Piston Motors

Figure 59; shows the action of a simple axial-piston motor. The compressed air enters the inlet port and acts on the piston. This forces the piston to the left. The action of the piston against the wobble or cam plate attached to the offset shaft causes the cam plate to rotate. This motion is then transferred through a set of gears to the output shaft. As the offset shaft rotates, the inlet port control valve also rotates, which directs compressed air to the different pistons. The force developed on the piston depends on the air pressure, and the speed of the piston depends on the rate of airflow.

Figure 59: Axial Piston Air Motor

When the piston has completed its stroke and the air pressure in the cylinder is expended, the air passes through the exhaust port and is discharged to the atmosphere. The airflow in an axial-piston motor is controlled by the inlet and outlet port configurations.

Figure 60 shows a radial-piston pneumatic motor. This motor can be constructed for non-reversible or reversible-type driving arrangements. Although it is different in construction, its operation is much the same as the hydraulic radial-piston motor. The pistons are mounted radially around an eccentric, five-sided valve block that is keyed to the output shaft. The pistons are connected to the valve block, which converts their reciprocating motion into the rotary motion of the shaft.

When a radial-piston motor is running, the compressed air enters the inlet port and is directed to the rotating valve. The valve then directs the air through the valve block to a cylinder in the motor housing. Compressed air acting on the upper portion of the piston causes it to move inward, which shifts the position of the valve block. The shifting valve block then causes the eccentric and the rotary valve to rotate. Because the eccentric is connected to the motor shaft, the motor shaft also rotates.

When the piston reaches the bottom of its stroke, the air cannot do any more work. The rotary valve allows the air from the piston to exhaust to the atmosphere. As the rotary valve continues to shift its position, the air at almost zero pressure is forced from the cylinder into the outlet line and exhausted.

Limited Rotation Actuators

Limited rotation actuators produce a rotary action similar to a rotary motor. However, they are designed to move in an arc rather than a complete revolution. Their movement typically ranges from 90° to 330°. The amount of available rotation will vary with the manufacturer and the application. Limited rotation actuators have high torque, are relatively simple in construction, and are easy to mount. They are normally used to operate levers and for partial rotation of drives. Their main source of power is through rotary vane actuators or air cylinders.

Figure 61 shows a vane-type limited rotation actuator. This actuator operates much like a swinging gate or door. The output shaft is connected directly to the vane support post. The vane and post can be moved back and forth by directing the air to one or more inlet ports. Some units are spring-loaded so that they return when the air supply is cut off. Compressed air entering the inlet port acts on the vane, causing the shaft to rotate. Some actuators may be used to change the position of a ball valve, while others may be used to operate brakes, clamps, or electric switches.

Figure 61: Vane-Type Limited Rotation Actuator

Piston-type limited rotation actuators are made by several manufacturers. Figure 62 shows a typical piston limited rotation actuator. This actuator uses a pneumatic, double-piston cylinder connected to a rod that has a tooth rack machined in it. The tooth rack meshes with a gear and shaft to change the reciprocating motion into rotary motion.

Figure 62: Piston-Type Limited Rotation Actuator

Pneumatic Tools

Pneumatic tools are air-operated machines that can be easily transported to a work area. In many instances, the tool is either held entirely by the hands or guided and controlled by hand.

The tool can also be located at a distance from the compressor. On an outdoor construction job, the compressor may be portable. In a manufacturing installation, the compressor may be stationary. Air from the compressor system is transported using distribution piping through a filter, a pressure regulator, and a lubricator. Air from the lubricator can then be fed through a flexible hose to the pneumatic tool. Pneumatic tools are generally supplied with compressed air at about 90 pounds per square inch (psi).

There are three broad classifications of pneumatic tools:

  • Rotary
  • Piston
  • Percussion

In a typical rotary pneumatic tool, the air enters the tool handle, passes through a manually operated control valve, passes through the end plates, and enters the chamber of a vane-type air motor. The air will then push the rotor from the inlet to the outlet. At the outlet, the air is discharged to the atmosphere. The typical rotary air motor will contain three to five blades, rotate at speeds of 4,000 to 25,000 rpm, and develop up to 3 horsepower. A spindle fastened to the rotor drives a set of planetary reduction gears to supply the desired output shaft speed. The output shaft can be equipped with a chuck for holding a drill, grinder, or other tool.

A piston-type air motor is used for tools when higher horsepower and slower speeds are desired. Typical examples are hoists and heavy-duty drills.

A percussion- or hammer-type air motor can be found in chippers and riveters. In this type of machine, the air will usually pass through the handle, through a control valve, and into a cylinder with a reciprocating piston. It then travels through an exhaust valve and is exhausted to the atmosphere. The piston motion is back and forth. On the driving stroke, the inertia of the piston provides a hammer blow or impact to a drill, chisel, or bit.

Compressed air allows the use of tools that are relatively compact, light in weight, flexible, portable, and easy to operate. Pneumatic tools are available for use on very light assembly work and on heavy-duty construction jobs. The speed and direction of rotation of air motors can be controlled quickly and can also be adjusted to serve any speed requirements by means of a control valve. The control valve throttles the flow of compressed air to the motor. Pneumatic motors do not have a tendency to become hot when they are overloaded. They also do not present a spark hazard in explosive atmospheres and can therefore be used in oil refineries. Air motors are used under wet and humid conditions, because there is no electric shock hazard.

Abrasive Tools

Abrasive tools include grinders, buffers, and sanders. A rotary vane-type air motor is usually used for tools of this type. They are available in a wide range of speed and power.


A rotary vane-type air motor is usually used for drills, reamers, tappers, and stud setters. Drills and reamers are used for steel, wood, and other materials. Some tools are reversible. Throttling the air supply to the drill can vary drill speed.


A rotary vane-type air motor is normally used for screwdrivers, nut setters, impact wrenches, and shears. Pneumatic tools drive millions of nuts, bolts, and screws every day in large production plants that make products such as automobiles, refrigerators, radios, and appliances. Pneumatic impact wrenches are used to remove or tighten nuts by torsional or rotary impacts, and pneumatic shears are used for cutting steel.


In the pneumatic scaling or chipping hammer, a piston delivers a series of blows to a forming tool or chisel at the end of the hammer. The two types of percussion tools are the valve type and the valveless type. In the valve type, a valve arrangement is used to control the flow of air both to and from the two ends of the hammering piston. In the valveless type, the piston performs the valve action. The valve type of hammer is used for chipping and riveting, and the valveless type is used for removing scale.


Hoists operated by compressed air are used in many applications, especially in machine shops and foundries. Pneumatic hoists are used outdoors and in conditions where fumes and explosive gases are present. The air motors are used variable in speed and reversible in direction and can withstand stalling from an overload without damage.

Basic Pneumatic System

Figure 63 shows a schematic diagram of a basic pneumatic air supply system. Notice that each component in the schematic is labeled. The labels are there only as an aid in recognizing the symbols. This information isnotpart of a normal schematic, because it is understood that the person reading the schematic will already know what the component is just by looking at the symbol. Figure 63 can be used as a reference until symbol recognition becomes second nature.

Figure 63: Basic Air Supply System&

The system shown in Figure 63 operates as follows:

  • Air from the surrounding atmosphere is supplied to the suction side of the compressor through a muffler and filter arrangement.
  • The air is compressed by the fixed displacement compressor and discharged into an aftercooler.
  • The aftercooler lowers the temperature of the compressed air before it is sent to the storage reservoir or receiver.
  • From the receiver, the air flows through a separator, which is equipped with a manual drain.
  • The air leaving the separator enters a manually operated, vented cutoff valve, which is opened to connect the supply system to the air distribution header.

In many schematic diagrams, the details of the actual air supply system are not shown. This is because the air supply system itself has relatively little to do with the circuit that is actually performing the work. In these circuits, the air supply system is generally represented with a line and a triangle, as shown in Figure 64. The other application of this symbol is when a single air supply system provides the air for a number of individual circuits. In these cases, the individual circuits use the symbol to represent the supply system, and they will normally reference the reader to a different drawing to see the actual details of the supply system.

Figure 64: Air Supply System Symbols

With practice, the ability to read schematics will become easy. However, a thorough understanding of them may take a little longer. Understanding a complex circuit is generally easier if only a small segment is studied at a time. With this in mind, a few individual circuit segments will be discussed before looking at an entire system schematic.

Simple Circuits

Figure 65 is a schematic diagram of a paint shop pneumatic circuit. The air supply to the system is shown in the upper left-hand corner and is conditioned using a filter separator. Solenoid-operated, two-position, three-way valves are used to control pilot air to the control valves for machine move, gun #1 on/off, gun #2 on/off, dump valve, atomizing air, and the gun tilt cylinder. The air brake has no pilot control but uses a solenoid-operated, two-position, three-way valve to directly control the application or exhaust of air to the brake.

Figure 65: Example Pneumatic Circuit

It should be noted that the air cylinder for machine move in Figure 65 can be controlled by the solenoid-operated three-way valveormanually using a two-position, four-way valve. A time delay between the shift from extension strokes to retraction strokes and retraction strokes to extension strokes of the cylinder is also provided using a two-position, three-way, pilot-operated valve with accumulator/timers on each pilot line.

Timing Circuits

Due to the increased use of complex, automated machinery in modern industry, there are many more pneumatic timing circuits being used than in the past. These circuits can be broken down into two different functions:

  • Time On Delay
  • Time Off Delay

A time on delay valve performs its function at some finite period of time after it receives an input signal. In contrast, a time off delay valve will perform its function at some finite period of time after the input signal is removed. The duration of the signal that actuates the valve can be fixed or adjustable. Figure 66 is an example of both a fixed and adjustable timing circuit.

Figure 66: Time Delay Circuits

In both of the circuits in Figure 66, a small reservoir labeled "V" and an airflow-metering device are used to accomplish the time delay at the control valve.

The circuit operates as follows:

The control valve requires a certain, preset amount of pilot pressure to actuate. The source of the pilot pressure is the small reservoir, which is filled from the pilot pressure supply line. The incoming pilot pressure signal must flow through the restriction to fill the reservoir. Once the reservoir is full, the signal pressure actuates the control valve. The resulting time delay is how long it takes to fill the reservoir.

From this, it can be seen that there are three factors effecting the length of the time delay:

  • Size of the reservoir
  • Size of the restriction
  • Magnitude of supply pressure

In most timing circuits, the timed signal is used to operate a normally closed, normally open, or a four-way valve. The characteristics of the individual valve to be operated must also be considered when setting up a timing circuit. This is because items like the strength of the return spring and effective area of the piston will also effect timing. Some valves are available with adjustable springs that provide another means to vary the timing.

Figure 67 is an example of a timing circuit. In this circuit, the timer (A) is used to control pilot air to a two-position, five-way valve (B). In addition to the control provided by the timer, a second two-position, five-way valve (C) acts as a blocking valve between the timer and valve B. If valve C is not properly positioned by other system conditions, the timer will not affect the position of valve B. As can be seen from Figure 67, valve B is ultimately used to control the extension and retraction strokes of a cylinder (D).

Safety Circuits

Personnel and equipment safety are always factors to consider in the design and operation of a pneumatic system. Because of this, many systems must have a preset minimum and maximum operating air supply pressure. The minimum and maximum pressure can be established using a simple safety circuit. Figure 68 shows a typical pressure safety circuit. Note that the control valves used in this circuit are similar to the valves used in the timing circuit.

Figure 68: Pressure Safety Circuit

During system operation, 250 psi compressed air from the supply system is admitted to regulator one. The regulator reduces this to a nominal pressure of 100 psi. The 100 psi from the outlet of the regulator flows to valve two, which is normally closed. Tapping off this same line is a pilot signal line with pressure that is used to actuate valve two. Whenever the pressure in the pilot signal line exceeds 80 psi, the valve will shift and allow the 100-psi air to proceed through normally open valve three. The output of valve three supplies the air distribution header, which has a minimum operating pressure that is now established at 80 psi.

If the regulator fails or is improperly adjusted, pressure in the system will rise above 100 psi. If the pressure out of valve two rises to 120 psi, valve three will close. Valve three closes due to the 120-psi pressure signal felt in its pilot pressure line. When valve three closes, the air supply to the distribution lines is blocked. The actuating point of valve three now establishes the maximum system operating pressure of 120 psi.

Pneumatic Circuits

The previous section described the operation of the basic air supply system and segments of larger, more complicated systems. This section analyzes larger, more complete parts of entire systems. When analyzing the schematic for a complete system, it should be noted that the system is no more completed than the individual circuits that are in it.

Example Circuit One

Seven individual operations are performed with this pneumatic system. These operations are labeled on the drawing in Figure 69 and include pallet (rear) lift, inside lift (rear) lock, pallet (rear) stopper, pallet (front) stopper, inside lift (front) lock, pallet (front) lift, and work transit lift. Each of these operations is supplied pressure from, and exhausts to, a manifold arrangement, which includes two exhaust lines and one pressure line. Note that the outer lines (top and bottom) of this manifold are drawn as enclosure outlines and serve no functional purpose.

In comparing the circuits that perform each operation, it can be seen that there are essentially two different circuit designs used for the system. The first design is used for the lifts, and the second design is used for the lift locks and stoppers.

Directional control of a lift is accomplished with a three-position, five-way valve. The center position of this valve applies pressure to both sides of the piston in the cylinder, while the other two positions alternate the pressure and exhaust for the cylinder. Speed control is performed through the use of variable restrictions in a meter out (throttling is conducted on the exhaust air vice the pressure air) configuration. A pneumatically operated brake is installed on the piston rod and functions to physically lock the lift when air pressure is released from the brake. A pressure regulator is used on the rod end cylinder port to control force intensity when the cylinder is retracted (lift is lowered).

The lift locks and stoppers each consist of a two-cylinder arrangement with a two-position, four-way valve providing control of the two cylinders. Speed control on both cylinders is accomplished with variable flow restrictions in the normal meter out configuration.

Figure 69: Example Circuit

Example Circuit Two – Stamping Press

If viewed at a glance, the pneumatic schematics associated with a press used in stamping automotive body panels might appear extremely complex to an individual who is unfamiliar with the operation of pneumatic systems. However, analyzing the schematic one section at a time reveals that there is very little which cannot be easily explained in terms of basic operation. Figure 70 shows the No. 1 Section, No. 2 Die Cushion of a 2400-ton hydraulic/pneumatic press.

The two-position, four-way valve (22) and its associated solenoids control operation of the cushion. With the valve in its illustrated position, regulated air pressure (71 psig) from the top header is supplied to the cap end of the piston-operated valve on the right and to the rod end of the piston-operated valve on the left. Airflow is metered out from the piston-operated valve on the left, controlling the speed at which the valve shifts. Tracing the flow path of air from the cushion cylinder arrangement shown at the bottom of the figure, air pressure in the cylinder is vented to atmosphere via the two silencers (31).

Shifting the four-way valve (22) to the opposite position results in regulated air pressure from the top header being supplied to the rod end of the piston-operated valve on the right and to the base end of the piston-operated valve on the left. The shift speed of the piston-operated valve on the right is controlled (meter out) in this situation. With the two piston-operated valves shifted to the position opposite of the position shown in the figure, a flow path of air from the air tank to the cushion cylinder arrangement is established. Air is supplied to the die cushion air tank by another regulated air supply.

Determining Causes of Failures

Pneumatics is defined as the science of using compressed air to perform work. The amount of work that can be performed by any given pneumatic circuit is limited only by the imagination of the system designer and the components that it contains. Although there are a large number of complex pneumatic control systems in use, the majority of pneumatic systems are made up of simple actuating and control devices.

Whether a system is simple or complex, the method used to determine the cause of a failure or breakdown is the same. The only difference is that the complex system will require more steps and time to completely check.

Understanding the System

Technicians have a thorough understanding of how a system normally operates. Whenever a piece of pneumatic equipment fails to operate, there is always a reason for the work stoppage. The cause of the problem may be in the air supply system, control valves, actuator, or in the machine being operated by the system. Regardless of the cause, being able to recognize which portions are functioning properly and which are not can only be performed by someone with prior knowledge of how pneumatic systems operate. Once this knowledge is obtained, the speed and accuracy with which the technician can locate a problem is directly related to two things:

  • System complexity
  • Troubleshooting method

A technician has no control over the complexity of a system but can directly control the method used to troubleshoot failures.

Troubleshooting Procedures

Good troubleshooting is an art based on following a logical and sequential method of locating the cause of failure in an inoperative machine. The special talent for being a good troubleshooter comes from always following the same routine procedure for locating problems instead of a hit-and-miss (Easter-egging) approach that may accomplish little.

The type and number of components determine the troubleshooting checks themselves in a system and the operations that the machine performs. The actual sequences used to troubleshoot a pneumatic system usually vary from one installation to another. The sequence in which the troubleshooting checks are performed is also influenced by the preferences of each facility's maintenance procedures.

Locating Troubles

The sequence of troubleshooting steps described in this section is based on two operations: determining if there is any air pressure in the supply system and checking for air in the pneumatic system by starting at the actuator and working back to the air supply point. In many plants, the sequence starts at the air supply and proceeds through the system to the actuator. Either way is correct and can be used effectively.

When a pneumatically operated machine fails to perform its operations, the first thing the technician needs is a copy of the pneumatic system’s schematic diagram.

This provides an overall understanding of how the system is supposed to function. Remember that schematic diagrams do not necessarily show the relative position of the system components. In instances where a control valve is operated by a cam or trip mechanism, the drawing may show a physical relationship. While looking over the drawing, try to get a general understanding of the system operation before trying to locate any specific piece of equipment. After getting a general idea of how the system operates, try to locate the specific areas that may be causing the malfunction. When tracing through the drawing, do so in a logical sequence.

Referring to the bill of materials on the schematic drawing is the most positive way to identify system components. Some valves look alike but are entirely different in function and operation. The part number and model are all-important in identifying the valving and components in a system. Most drawings have each component in the system marked with a matching item number to that given on the bill of material, making the components much easier to locate.

There are two additional guides that are helpful when troubleshooting a pneumatic system. These are the troubleshooting chart (shown at the end of this article) and the maintenance record for the system. Both are required if their greatest benefits are to be realized. The troubleshooting chart suggests several possible problem causes. The maintenance record presents a factual record of previous problems and the corrections that were made to the system to eliminate the problem. Inspection of a maintenance record will show recurring problems as well as problems that may have been caused by changes in the pneumatic component arrangement.

Operating Manuals

An operating manual should accompany every pneumatic system. These manuals are generally divided in the following five sections:

  1. General Information
  2. System Description and Operation
  3. Component Parts List
  4. Troubleshooting and Maintenance
  5. Drawings and Schematics

Operating manuals should be kept on file in a central location and be readily available to the maintenance technician. If possible, the maintenance folder should be kept in the same file. Any changes made to a system should be noted in the manual with the date and initials of the person responsible for the change.

The General Information section provides an overall picture of the system’s physical arrangement, features, and what the system is designed to accomplish.

The System Description and Operation section provides a concise description of signal, logic, and actuation functions. The airflow path through the system and the sequence of events are also explained in this section.

The Component Parts List gives the part breakdown of each component. This breakdown identifies the individual parts in the component and shows how they are assembled. Many times, the parts lists contain a description of the operation, installation, and maintenance of the individual components. Recommended spare parts to be stocked may also be provided with the component parts list.

The Troubleshooting and Maintenance section normally includes a troubleshooting chart and recommended system maintenance instructions. The troubleshooting chart serves as a guide for determining causes of failures and usually lists checkpoints that can be evaluated visually or audibly. It also lists probable causes of the malfunction and corrective steps that can be taken to eliminate it.

The Drawing and Schematics section should contain a complete system schematic as well as all subsystem schematics and detailed drawings. The item numbers that identify the component should be included as necessary. Drawings of special linkages or assemblies of one or more components are also included in this section.

To help in the understanding of system operation, the schematic diagram and bill of material should be accompanied by a written description of either the overall operation of the system or its individual components. With this aid, the flow of air through the various conditioning units, control valves, logic valves, power valves, and actuators can be traced.

Checking Air Supply Systems

One of the first things to check in an inoperative system is that a sufficient supply of air is available at the machine. Also, make sure that electrical power is available to all electrical components. Machines that have more than one air supply have to be checked more thoroughly. It may be found that while the power is actually on at the machine, a valve or a circuit breaker is not connected somewhere back in the supply system, isolating the system. These points should be checked first.

Another cause of malfunction may be a filter that is clogged to the point that the proper flow of air is not available.

A filter may allow enough air through it to indicate an adequate reading on the supply pressure gauge but not enough to allow sufficient flow when the system is operated. (A ruptured or leaking supply line will produce similar results.)

Troubleshooting the Actuator

After making sure that an inoperative system has power, disconnect the actuator (cylinder or motor) from the load connection to see if it will operate. Chips or small pieces of scrap frequently become caught in machinery and will prevent actuator operation. Disconnecting the actuator will ensure that the malfunction is not in the machine. Air-actuated compound leverage systems, such as those used on presses, sometimes travel past the center point (especially if the press is operated under no load conditions). In these instances, the linkage must be manually returned to its normal position before the press can be operated.

Cylinders that operate infrequently or have been out of service can freeze up because of dried-out lubricant, corrosion, and deterioration of rubber parts. Cylinders exposed to severe or unusual temperature conditions must also be protected against freeze-up. While the cylinder is disconnected, check for frozen pins, rod bushings, bent piston rods, and damaged cylinder tubes.

Another cause of inoperative actuators is lack of air. Leaks in the supply lines are readily detected, but a crimped supply hose or line might be hidden from view. In automated machinery, it is possible that a part or piece of scrap may fall into the machine and dent a line or block a hose. In such cases, the cylinder would probably extend or retract at a less-than-normal rate of speed. A slow-operating cylinder may also have leaking piston seals. If the actuator slowdown occurs suddenly, look for damaged lines.

When piston seals become worn and leak, air will continually escape through the exhaust port. In addition, the cylinder will not develop enough force to do the work expected of it. This is shown in Figure 71, which shows a cylinder with worn seals. Either one or both of the piston seals may leak and produce this reaction. Piston seal wear usually occurs evenly on both seals, so air leakage will occur in both directions of piston travel.

Figure 71: Worn Piston Seals

In some instances, the piston rod breaks from fatigue at the point where the piston is attached to the rod. When this occurs, the cylinder will extend as air is admitted to the cylinder. When the airflow is reversed, the piston assembly will return, but the rod will remain extended. If the seal between the broken piston rod and the piston is not damaged, no leakage will occur, and the exhaust at the directional power valve will be normal.

Checking the Control Valve

If the actuator and mechanical linkage have free movement and are in good operating condition, loosen or disconnect the lines to the cylinder and cycle the system. If there is no indication of airflow, the problem is either in or ahead of the control valve.

The cause of trouble in a control valve can be located by performing the following tests:

  • If the valve action is sluggish, check the moving parts for insufficient lubrication or swollen seals.
  • If the valves are pilot-operated, check the pilot supply pressure. Other reasons for sluggish valve movement include heat, swollen or deteriorated packing, and binding caused by excessive operating pressure. Unbalanced poppet valves are particularly sensitive to excessive supply pressures. A clogged exhaust muffler may also result in slow actuator movement. In systems having manifold-mounted control valves, the retarded exhaust flow from one valve may cause back pressure in an exhaust passage. This may result in a malfunction or unintentional operation of other valves or actuators.

  • Worn packing, valve seats, and valve bodies can cause leakage within a valve. Abnormal internal leakage in one valve of a manifold valve can starve the other valves in the stack. If the valves are internally piloted, spring-centered or spring-returned, loss of pilot pressure may cause them to oscillate when the operating signal is received. Broken springs and valve stems are other possible causes of control valve malfunction.

Checking a Control Valve Actuator

Frequently, control valves will not shift because the actuating device will not operate. If the system malfunction is located in the control valve area, check both the valve and the actuator. If the control valve actuation is manual, it is very easy to locate any problem. Connecting pins, levers, cams, and linkages that are either worn from use or damaged in operation directly influence the operation of the control valve and actuating mechanism.

The failure of a solenoid-actuated control valve cannot be determined as easily as in a manual valve. In addition to the electrical failures, mechanical failures cause solenoid actuators to be inoperative. These include broken straps or plunger blocks, sheared clevis pins, and misalignment between the solenoid and valve caused by faulty mounting.

Electrical failures can be determined with the aid of electrical instruments. Most solenoid coils that actuate pneumatic valves are designed for continuous duty. If a solenoid fails prematurely, electrical resistance readings can be used to determine if the coil is open, shorted, or grounded. Open coils result from physical damage to the coil that severs the coil wire. An open circuit in a solenoid coil can also be caused by a short circuit, resulting in arcing and intense heat. The heat can be severe enough to melt the wire and open the circuit.

Insulation failures occur from overheating. If a solenoid coil is in an operating condition, but you do not have voltage, look for the source of trouble in the electrical supply system. Fuses may have blown or circuit breakers tripped.

Pneumatic or hydraulic pilot actuators on pneumatic control valves are usually very simple in construction. Most consist of a small piston or diaphragm located on the end of the main control valve. Most of the inlets to the pilot actuators are very small and can easily become clogged. This is especially true where pilot passages are within the valve body and do not have a direct path to the piston operator. Removing the obstruction almost always restores the valve to service.

Checking Sequence Valves

Whether or not the pneumatic system has sequence or logic valves, the troubleshooting procedures used with them apply to most systems. The sequence or logic portion of a pneumatic system can be quite complex and sophisticated. It is almost always necessary to refer to the schematic diagram and system description of operation before troubleshooting can be done efficiently.

The valves in the logic circuits are usually small (¼-inch and⅛-inch port sizes). Their function is to sense, time, sequence, interlock, and direct signals to the power valves. Components used in these circuits include shuttle valves, check valves, flow-control valves, quick-release valves, cam-limit valves, three-way and four-way sequence valves, and volumes (timing circuit reservoirs). If any one of these components fail to operate properly in a machine cycle, the net result is a false stroking or malfunction at the actuator.

There is very little that can fail in a timing volume. However, if the connection in a circuit is made at the top of the volume, moisture will probably condense in the volume. In this situation, there is no means of draining the condensate, and the volume would gradually fill with liquid. When this happens, it’s timing capacity changes. This means that an actuator would operate before its scheduled time and jam the machine.

Check to be sure that all parts of the system will drain when exhausted, especially dead-ended timing volumes.

Dirt, pipe scale, thread sealant, and other foreign materials cause most of the problems in logic circuits. Contaminants may cause check valves and shuttle valves to leak and flow-control valves to become out of adjustment. An exhaust from one input valve can usually detect a shuttle valve that leaks when another input valve is operated.

Complex systems should be piped with plugged tees at strategic points so air pressure gauges can be installed to help locate malfunctions. A pressure-indicating device that can be permanently installed is the pop-up indicator like that shown in Figure 72. These indicators operate on a very small amount of signal air and are sensitive to pressures of about 10 psig.

Figure 72: Pop-Up Pressure Indicator

Checking Master Control Valves

Master control valves are the signaling valves, or input, that initially direct or admit control air signals through the logic circuits to the operating control valves that control the actuators. In a malfunctioning system that has multiple sequence control stations, first try operating the system from another control station. If the system operates satisfactorily from the second station, the malfunction is obviously in the first control station. The proper troubleshooting approach is to isolate the portion of the system where trouble is occurring, then locate the malfunctioning component.

Pneumatic systems that repeat a cycle over and over have provisions for either manual or automatic operation. If the machine or system will not repeat as intended, be sure that the selection valve is fully in the Automatic position. Usually, automatic recycling systems are provided with an Emergency Stop. Ensure that the control is in the Run or Reset position before attempting to operate the system.

Making Final Adjustments

After the failure has been determined and the fault corrected, go back over the entire system. Operate it through the automatic and manual cycles several times to be sure that one failure has not weakened or jeopardized the normal function of another component. While operating the system, try the Reset and Emergency Stop to check their operation. Also, check the adjustment of all flow-control valves in potentially affected timing and speed circuits.

If time permits, it is a good practice to stay with the machine for a brief time to observe its performance in actual production operation. In some cases, failures are caused by some wrong maneuver by the machine operator.

Make notes about system operation in the instruction manual or in the maintenance record. Include the nature of the system failure and what steps were taken to correct it. If a failure with similar symptoms should occur again, checking for the same malfunction first can save time and trouble.

Pneumatic System Troubleshooting Chart

The following chart provides some typical problems with pneumatic systems, their causes, and solutions.

ProblemCausesSolutionsInsufficient Air VolumeSystem demands exceed delivery




Insufficient Air Volume

System demands exceed delivery

System leakage excessive

Clogged intake lines

1.1 Checkcompressorfor discharge capacity.

1.2 Check the number of components in the system.

1.3 Check air consumption of each component.

1.4 Check for piping leaks.

2.1 Check fittings and pipes.

2.2 Check actuators for leaks.

2.3 Repair all leaks.

3.1 Clean intake filter.

3.2 Check intake valves.

3.3 Check intake unloader.

3.4 Check size of inlet pipe.

Low Air Pressure

System demands exceed delivery

System leakage excessive

Low compressor discharge pressure

1.1 Check compressor discharge pressure.

1.2 Check reservoir safety valve.

1.3 Check system regulator valve.

1.4 Check system for leaks.

2.1 Check compressor discharge pressure.

2.2 Check reservoir safety valve.

2.3 Check system regulator valve.

2.4 Check system for leaks.

3.1 Check for clogged intake lines.

3.2 Check for worn intake valves.

3.3 Check for worn discharge valves.

Oil in System

System demands exceed delivery

Oil coming from the compressor

Malfunctioning oil lubricators

1.1 Check the compressor capacitor.

1.2 Check the oil separator size.

1.3 Check the air components consumption.

2.1 Check the compressor piston rings.

2.2 Check for worn intake valves.

2.3 Check for worn discharge valves.

3.1 Check the lubricator adjustment.

3.2 Check the oilviscosity.

3.3 Check air consumption of the components.

Moisture in the System

System demands exceed delivery

Moisture carryover

1.1 Check compressor capacity.

1.2 Check air consumption of the components.

2.1 Check air cooler capacity.

2.2 Check air cooler efficiency.

2.3 Check the moisture separators.

2.4 Check the system leg drains.

2.5 Check the location of the intake filter.