HYDRAULIC COMPONENTS I
In a hydraulic system, many components are necessary for the operation and control of the system. For each component in a system there are several different designs available. The actual component design used for a particular system depends on service use, temperature, pressure, operation, and maintenance requirements.
This article discusses the different types of components used in hydraulic systems and their particular characteristics. The topics discussed are:
Liquids are used in hydraulic systems primarily to transmit and distribute forces to the various components to be actuated. Liquids are able to transmit and distribute force because they are almost incompressible. Pascals law states that a force applied on any area of an enclosed liquid is transmitted equally and undiminished to all equal areas throughout the enclosure. Thus, if a number of passages exists in a system, pressure can be distributed through all of them by means of a liquid.
Manufacturers of hydraulic devices specify the type of liquid best suited for use with their equipment. Their recommendations are based on the working conditions, the service required, temperatures expected both inside and outside the system, pressures the liquid must withstand, and the possibility of corrosion.
If fluidity (the physical property of a substance that enables it to flow) and incompressibility were the only qualities required, any liquid that is not too thick might be used in a hydraulic system. However, a satisfactory liquid for a particular installation must possess a number of other properties. Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are discussed in this section.
Viscosity is one of the most important properties of a liquid to be used in a hydraulic system. Viscosity is "the internal resistance of a fluid which tends to prevent it from flowing." A liquid such as gasoline, which flows easily, has a low viscosity and a liquid such as tar, which flows slowly, has a high viscosity. The viscosity of a liquid is affected by changes in temperature. As the temperature of a liquid increases, its viscosity (resistance to flow) decreases (a liquid flows more easily hot than when cold). Also, the viscosity of a liquid will increase as pressure increases.
The proper liquid for a given hydraulic system must have enough body to provide a good seal at pumps, motors, valves, etc. These components depend upon close fits for creating and maintaining pressure. Internal leakage through these clearances results in loss of pressure, instantaneous control, and pump efficiency. These leakage losses are greater with lighter liquids (low viscosity).
A liquid that is too thin will lead to rapid wearing of moving parts, or of parts having heavy loads. On the other hand, if the viscosity of a liquid is too high the internal friction of the liquid will increase which, in turn, will increase the flow resistance through the clearances of closely fitted parts, lines, and passages. This may result in pressure drops throughout the system, sluggish operation of equipment, and increases in power consumption.
The viscosity of a liquid is measured with an instrument called a viscosimeter or viscometer. There are several types, but the instrument most commonly used by American engineers is the Saybolt Universal Viscosimeter. This instrument, shown in Figure 1, measures the number of seconds it takes for a fixed quantity of liquid (60 cubic centimeters) to flow through a small orifice of standard length and diameter at a specific temperature. The time of flow is taken in seconds, and the viscosity is expressed as Second, Saybolt Universal (SSU). For example, a certain liquid might have a viscosity of 80 SSU at 130oF.
Figure 1: Saybolt Viscosimeter
The Saybolt viscosimeter consists of a container for the liquid surrounded by a bath heated by heating coils to bring the liquid to the specific temperature at which the viscosity is to be measured. There is a standard viscosimeter orifice located at the bottom of the container. Passage through the orifice is blocked with a cork.
The container is filled to a marked level with the liquid to be tested. Then a small container marked at the 60cc level is placed under the orifice. When the liquid is at the desired temperature, the cork is removed. The number of seconds required for the liquid to reach the 60cc level gives the SSU reading.
One of the properties of an ideal hydraulic liquid is the retaining of the same viscosity under all temperature and pressure conditions to which it was subjected. Many liquids, particularly petroleum-based oils, do not have this characteristic. As the temperature increases, the oil gets thinner and viscosity decreases and vice versa. The variation is greater for some liquids than for others. Pennsylvania crude oils (paraffinic) vary comparatively little in viscosity with changes in temperature, while with the Gulf Coast crude (naphthionic and asphaltic), the variation is considerably greater.
To obtain a numerical indication of the degree to which viscosity changes with change in temperature, these two oils are taken as a basis for a scale. The change in viscosity of a specific paraffinic oil at temperatures between 100 and 210 F is assigned a viscosity index (VI) value of 100. The change in viscosity of a specific naphthionic oil over the same temperature range is assigned a value of 0. Other liquids are then assigned a viscosity index in terms of the degree to which their viscosity changes over this temperature range, as compared to the standard oils.
The greater the variation in viscosity with changes in temperatures, the lower the VI. The VI figures may range above 100 or below zero, if the liquids being measured vary to a lesser or greater degree in viscosity than the standard oils. For example, a liquid with a viscosity index of -10 would indicate a variation in viscosity over the standard temperature range to a greater degree than naphthionic oils, while an oil with a VI of 120 would show less change in viscosity with changes in temperature than paraffinic oils.
Since some hydraulic systems must operate satisfactorily under wide temperature extremes, the liquids used should have as high a viscosity index as possible while remaining consistent with the other properties the liquid must possess. The viscosity index of a liquid can often be increased through the use of chemical additives.
If motion takes place between surfaces in contact, friction tends to oppose the motion. When pressure forces the liquid of a hydraulic system between the surfaces of moving parts, the liquid spreads out in a thin film which enables the parts to move more freely.
Different liquids, including oils, vary greatly not only in their lubricating ability but also in film strength, which is the capability of a liquid to resist being wiped or squeezed out from between the surfaces when spread out in an extremely thin layer. A liquid will no longer lubricate if the film breaks down, since the motion of part against part wipes the metal clean of liquid.
Lubricating power varies with temperature changes; therefore, the climatic and working conditions must enter into the determination of the lubricating qualities of a liquid. Unlike viscosity, which is a physical property, the lubricating and film strength of a liquid is directly related to its chemical nature. Lubricating qualities and film strength can be improved by the addition of certain chemical agents.
Many different liquids have been tested for use in hydraulic systems. The liquids that are presently in use include mineral oil, vegetable oil, water, phosphate esters, ethylene glycol compounds, and oil in water. Hydraulic liquids are usually classified according to their type of base. The three most common types of hydraulic liquids are:
Water was used as a fluid medium in the first hydraulic systems. It is still suitable for certain large hydraulic installations that require high pressure and low operating speeds, but it does not meet all the requirements for general hydraulic equipment use. As a hydraulic liquid, water presents many problems. It is limited to temperatures that are above freezing and below boiling points. It promotes corrosion and rusting of metal parts and provides no lubrication of moving parts. Additionally, the hazard of foreign matter in the water can cause an abrasive action on the smooth surfaces of system components. All these factors are detrimental to the operating efficiency and long service life of the equipment.
One major advantage of water is its fire-resistant qualities. When water is used as a hydraulic liquid, it is usually combined with certain oils, ethylene glycol, and other substances. When combined with oil the combination is relatively fire resistant, however, high temperatures may cause the water to evaporate and then the oil might burn. Other combinations can eliminate ignition problems but may have mechanical or economic limitations.
One of the first oils used as a hydraulic liquid was a petroleum base automotive brake fluid. At that time, natural rubber was used in the construction of packing and gaskets. Since natural rubber is not compatible with petroleum-based liquids, the use of this type of liquid as a hydraulic medium was limited. As a result, a vegetable-based oil containing 50 percent castor oil and 50 percent alcohol was used in some applications.
This solved the problem with packing and gaskets, as natural rubber is compatible with vegetable-based oils. However, this liquid was unsatisfactory due to oxidation of the castor oil and, because liquid is an excellent conductor of electricity, this solution resulted in a high degree of electrolysis. Additionally, vegetable oils tend to break down under extreme temperature changes.
During the middle 1930s, a light petroleum-based oil was developed and, when used with asbestos seals, proved quite successful. By the late 1930s, advancement in packing materials permitted extended use of petroleum-based liquids in hydraulic systems. As a result, a petroleum-based oil was developed about 1940 that proved very satisfactory and, with certain refinements, is in use today. This liquid was improved through the use of a number of additives, such as oxidation and corrosion inhibitors, viscosity index improvers, and pour depressants. These additives not only permitted liquid operation at greater temperature extremes, but also increased their lubricating qualities and life characteristics.
Petroleum-based oils contain most of the desired properties required of a hydraulic liquid. However, they are flammable under normal conditions and can become dangerously explosive when subjected to high pressures and a source of flame or high temperatures. Water-based liquids are relatively fire-resistant, but do not have the high lubricity of petroleum-based oils. Some water-based liquids cause corrosion of components in a hydraulic system.
In recent years, non-flammable synthetic liquids have been developed for use in hydraulic systems where fire hazards exist. A synthetic material is a complex chemical compound that has been artificially formed by the combination of two or more simpler compounds or elements.
Some of the synthetic liquids currently used are phosphate esters, chlorinated biphenyls, or blends of each. Certain synthetic liquids have been found to chemically attack packing used in hydraulic systems, so special packing is normally required when these fluids are used.
The function of the piping in a hydraulic system is to act as a leak-proof carrier of the fluid. The piping in a fluid power system may be compared to the water piping in a home: one section provides water to the bathroom; a second section provides water to the kitchen sink; and still another section provides the water for an automatic washer, a dishwasher, or a lavatory.
The piping or plumbing in a fluid power system is too often an after thought, with the result that it is often a source of trouble. It is important that the piping in any fluid power system should be properly arranged to provide maximum efficiency and trouble free service.
Piping may be divided into three classes: rigid, semi-rigid (or tubing), and flexible (or hose).
Rigid steel pipe is available in four weights as follows:
Pipe sizes are specified by the nominal inside diameter as 1/4, 1/2, 3/4, 1, and 11/4 inches. All weights of one size are of the same outside diameter, but the actual inside diameter varies, depending on the wall thickness.
Steel, aluminum, and copper seamless tubing are all used for oil and air systems. These can be grouped as:
Fluid power systems are designed as compactly as practicable, to keep the connecting lines short. Every section of line should be anchored securely in one or more places so that neither the weight of the line nor the effects of vibration are carried on the joints. The aim should be to minimize stress throughout.
Lines should normally be kept as short and free of bends as possible. However, tubing should not be assembled in a straight line; because a bend tends to eliminate strain by absorbing vibration and also compensates for thermal expansion and contraction. Bends are preferred to elbows, because bends cause less of a head loss. A few of the correct and incorrect methods of installing tubing are shown in Figure 2.
Bends are described in terms of the ratio of the radius of the bend to the inside diameter of the tubing or pipe. The ideal bend radius is 2 to 3 times the inside diameter, as shown in Figure 3. For example, if the inside diameter of a line is 2 inches, the radius of the bend should be between 5 and 6 inches. Additionally, the bend should not decrease the inside diameter of the tubing by more than 15 percent.
Figure 3: Ideal Bend Radius
While friction head increases markedly for sharper curves than 2 to 3 times, it also tends to increase up to a certain point for gentler curves. Increases in friction in a bend with a radius of more than about 3 pipe diameters result from increased turbulence near the outside edges of the flow. Particles of fluid must travel a longer distance in making the change in direction. When the radius of the bend is less than about 2 times the pipe diameter, the increased pressure loss is due to the abrupt change in the direction of flow, especially for particles near the inside edge of the flow.
Flexible hose is available for many types and classes of work. Hose is usually specified by the inside and outside diameters. The so called tube is the lining or part that comes into actual contact with the fluid or material being handled. The carcass is the supporting structure of the hose, and it lies between the tube and the cover. The carcass material may be cotton, synthetic fiber, asbestos, or wire, and it may be woven, braided, wrapped or wound spirally. The cover is the outside covering element of the hose. The purpose of the cover is to protect the carcass from abrasion or other destructive forces, pulsating pressures, falling objects, sun rays, weather, oils, greases, acids, and chemicals.
Several classes of pressure are used in classifying the types of hose:
The hose material should be oil resistant. This requirement is clear for all hydraulic systems. For air or pneumatic systems, it should be noted that oil vapor from the compressor are introduced into the lines that are connected to pneumatic tools. The carcass of the hose should be strong enough for the intended service, and the cover should be rugged enough to withstand hard abrasive wear. The hose should be flexible and easy to handle.
Flexible hose must not be twisted on installation, since this reduces the life of the hose considerably and may cause the fittings to loosen as well. Twisting of the hose can be determined from the identification stripe running along its length. This stripe should not spiral around the hose.
The ideal bend radius for flexible hose varies according to size and construction of the hose and the pressure under which the system operates. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value.
Flexible hose should be installed so that it will be subjected to a minimum of flexing during operation. Support clamps are not necessary with short installations; but with hose of considerable length (48 inches for example), clamps should be placed not more than 24 inches apart. Closer supports are desirable and in some cases required.
A flexible hose must never be stretched tight between two fittings. About 5 to 8 percent of the total length must be allowed as slack to provide freedom of movement under pressure. When under pressure, flexible hose contracts in length and expands in diameter. Examples of correct and incorrect installations of flexible hose are shown in Figure 4.
Figure 4: Flexible Hose Installation
Flared connectors are commonly used in fluid power systems containing lines made of tubing. These connectors provide safe, strong, dependable connections without the necessity of threading, welding, or soldering the tubing. The connector consists of a fitting, sleeve, and nut, as shown in Figure 5.
Figure 5: Flared Tube Connector
The fittings are made of steel, aluminum alloy, or bronze. The fittings should be of the same material as that of the sleeve, nut, and tubing. For example, use steel connectors with steel tubing and aluminum alloy connectors with aluminum alloy tubing. Fittings are made in unions, 45 degree and 90 degree elbows, tees, and various other shapes.
For connecting to tubing, the ends of the fittings are threaded with straight machine threads to correspond with the female threads of the nut. In some cases, however, one end of the fitting may be threaded with tapered pipe threads to fit threaded ports in pumps, valves, and other components. For example, unions have straight machine threads on both ends, while elbows have straight machine threads on one end, but may have either tapered pipe threads or straight machine threads on the other end. Tees and crosses also are available in several different combinations.
Tubing used with this type connector must be flared prior to assembly. The nut fits over the sleeve and, when tightened, draws the sleeve and tubing flare tightly against the male fitting to form a seal.
The male fitting has a cone shaped surface with the same angle as the inside of the flare. The sleeve supports the tube so that vibration does not concentrate at the edge of the flare, and distributes the shearing action over a wider area for added strength.
Bite-type connectors are commonly referred to as flareless tube connectors (Figure 6). This type of connector eliminates all tube flaring, yet provides a safe, strong, and dependable tube connection. This connector consists of a fitting, a sleeve or ferrule, and a nut.
Figure 6: Flareless Tube Connector
Flareless tube fittings are available in many of the same shapes and thread combinations as flared-tube fittings. The fitting has a counterbore shoulder for the end of the tubing to rest against. The angle of the counterbore causes the cutting edge of the sleeve or ferrule to cut into the outside surface of the tube when the two are assembled together.
The nut presses on the bevel of the sleeve and causes it to clamp tightly to the tube. Resistance to vibration is concentrated at this point rather than at the sleeve cut. When fully tightened, the sleeve or ferrule is bowed slightly at the midsection and acts as a spring. The spring action of the sleeve or ferrule maintains a constant tension between the body and the nut and thus prevents the nut from loosening.
Before installing a new flareless tube connector, the end of the tubing must be square, concentric, and free of burrs. For the connection to be effective, the cutting edge of the sleeve or ferrule must bite into the periphery of the tube. This is accomplished by presetting the sleeve or ferrule on the tube using a presetting tool which has the same dimensions as the fitting body, and which can be obtained from the fitting manufacturer. If a presetting tool is not available, a suitable male thread fitting may be used. If a fitting must be used, a steel fitting is preferred for this operation. If an aluminum fitting is used as a preset tool, it should not be reused in the system.
After presetting, the connector is disassembled for inspection. If the sleeve or ferrule is satisfactorily installed, the connector is ready for final assembly in the system.
As discussed earlier, Pascals theorem, from which the fundamental law for the science of hydraulics evolved, was proposed in the 17th century. One stipulation that was necessary to make the law effective for practical applications was a piston that would fit the opening exactly. This was not accomplished until the next century, with the invention of the cup packing, which led to the development of the hydraulic press.
The packing was probably the most important invention in the development of hydraulics as a leading method of transmitting power. Of course, the invention and development of machines to cut and shape closely fitted parts were also very important. However, some type of packing is usually required to make the piston, and many other parts of hydraulic components, fit exactly.
Through the years many different materials and designs have been used in the development of suitable packing devices. The materials must be durable and provide effective sealing. In addition, they must be compatible with the fluid used in the system. Several different designs are necessary to satisfy the various requirements of hydraulic systems.
These packing materials are commonly referred to as seals or sealing devices. In turn, the seals used in hydraulic systems and components are divided into two general classes as follows:
A static seal, usually referred to as a gasket, is used to provide a seal between two parts where no relative motion is involved. Gaskets are used in the assembly of cover plates on reservoirs and end plates or other non-moving parts of certain types of pumps, motors, and valves. Static seals, due to their design use, should have zero leakage during equipment operation.
A dynamic seal is commonly referred to as packing. The packing is used to provide a seal between two parts that move in relation to each other. A piston that moves back and forth in a cylinder would use packing to provide a seal. A dynamic seal is allowed small amounts of leakage for lubrication purposes, but should be minimized and is compatible with the intended application.
These two classifications of seals, gaskets and packing, will apply in most cases. It should be noted that certain types of seals may be used as a gasket or packing.
Many of the seals in hydraulic systems prevent external leakage. These seals provide two purposes, to seal the fluid in the system and to keep foreign matter out of the system. Other seals simply prevent internal leakage within the system. These applications are illustrated in Figure 7.
Figure 7: Application of Seals
Gaskets are installed between the cylinder wall and the end caps, points A and B, to prevent external leakage. Packing is installed between the piston rod and one end cap, point D, which also prevents external leakage. Packing is also installed on the piston, point C, to prevent internal leakage. Although leakage of any kind results in a loss of efficiency, slight internal leakage is desired in hydraulic systems to provide lubrication of moving parts.
Several different types of material are used in the construction of seals. In the early years, seals were made of such materials as rope, sawdust, rags, etc. These materials were jammed into a stuffing box by means of a packing gland. The use of these materials led to extrusion of the material through clearance spaces, rapid wear, and continual leakage in varying amounts. Therefore, these materials were not effective.
Natural rubber has many of the characteristics required in an effective seal. However, as discussed earlier, natural rubber is not compatible with petroleum base fluids. Since this type of fluid is commonly used in hydraulic systems, rubber seals are limited in their use.
Today, seals are made of materials that have been carefully chosen or developed for specific applications. These materials include synthetic rubber, cork, and metal. Asbestos seals are sometimes used where heat is a problem.
In the late 1930s the development of suitable synthetic rubber was accomplished. Some of these compositions were resistant to petroleum-based fluids and, therefore, became the leading material for hydraulic seals. Since then, great advancements have been made in this field. New synthetics have been developed and the earlier ones improved.
Many factors contribute to make synthetic rubber ideal for seals. This material is virtually impermeable in a compressed state and, therefore, requires less sealing load than many other types of seals. It is easily formed and is available in sheets or molded shapes for different applications. Some synthetic rubber seals are capable of functioning in temperature ranges as wide as -65 to 300o F. Some newer types can withstand even greater temperature ranges.
There are two classes of synthetic rubber seals. One class is made entirely of a certain synthetic rubber. The other class of seal is made by impregnating woven cotton or fine-weave asbestos with synthetic rubber. Natural rubber impregnated seals are available for some applications.
Cork has several of the required properties which make it ideally suited as a sealing material in certain applications. The compressibility of cork composition seals make them well suited for confined applications where no relief for side flow can be provided. In other words, cork can be compressed enough to provide an effective seal with only a limited spread of the material. The material itself absorbs much of the compression.
Cork can be cut to any desired thickness and shape to fit any surface and still provide an excellent seal. It can withstand sustained temperatures up to approximately 270oF.
One of the undesirable characteristics of cork is its tendency to crumble. If cork seals were used as packing or in areas where there is a high fluid pressure or high flow velocity, small particles would be cast off into the system. For this reason, cork seals have limited use in hydraulic systems.
One of the most common metals used is copper. Flat copper rings are sometimes used as gaskets to provide a seal. Molded copper rings are sometimes used as packing under high pressures. Either type is easily bent and requires careful handling. Additionally, copper becomes hard when used over long periods or is subjected to compression.
Whenever a unit is disassembled, the copper sealing rings should be replaced. However, if new rings were not available, annealing could soften the old ring. This is the process of heating a metal, then cooling, so as to make it more pliable and less brittle.
Metallic piston rings are used as packing in some actuating cylinders. These rings are similar in design to the piston rings in automobile engines. In some instances, this type of ring is made of Teflon.
Seals are usually typed in accordance with their shape or design. These types include O-ring, quad-rings, V-rings, U-rings, cup seals, and flange seals. Figure 8 shows some of these seals. A section is cut out of each seal to show the cross-sectional shape.
An O-ring, as shown in Figure 8, is circular in shape, and its cross-section is small in relation to its diameter. The cross-section is truly round and has been molded and trimmed to extremely close tolerances. The elliptical seal is also classified as an O-ring. The elliptical seal is similar to the O-ring except for its cross-sectional shape. As its name implies, its cross-section is elliptical in shape.
An O-ring is usually fitted into a rectangular groove machined into the mechanism to be sealed. O-rings may be used as gaskets or packing, and are used to prevent external or internal leakage. The O-ring forms the seal by distortion of its resilient, elastic compound, thus filling the leakage path.
Figure 9 shows the proper installation of an O-ring seal. The clearance for the seal is less than its free outer diameter, and is squeezed diametrically of round even before the application of pressure.
When pressure is applied to the O-ring, the seal moves away from the pressure into the path of possible leakage. The O-ring is designed so that the seal flows up to the passage, thus completely sealing it against leakage. The greater the pressure applied, the tighter the seal becomes. When pressure is decreased, the resiliency and elasticity of the seal results in the O-ring returning to its natural shape.
An O-ring, when set alone, is limited to systems having maximum operating pressures of 1,500 psi or less. This is particularly true when an O-ring is used as packing. In systems with operating pressures above 1,500 psi, backup washers are installed in conjunction with the seal. Backup washers are discussed in more detail later in this article.
A quad-ring is very similar to an O-ring with the major difference being that the quad-ring has a modified, square-type cross-section. Like O-rings, quad-rings are molded and trimmed to extremely close tolerances in cross-sectional area, inside diameter, and outside diameter.
This seal is relatively new and is presently used as packing for reciprocating or rotary motion, and can also be used as a static seal. Quad-rings are composed and designed so that they can be used in most applications in place of O-rings. The relatively square cross-section of the quad-ring helps eliminate the spiral twist that is sometimes encountered with the O-ring. The elimination of this twist will, in many instances, extend the life of the seal.
Quad-rings are ideally suited for both low pressures and extremely high pressures. Figure 10 shows an example of a quad-ring used as a seal between a piston and the walls of its actuating cylinder.
Several years ago, the V-ring was the predominant seal used in hydraulic systems. In recent years, it has been replaced by the O-ring in most applications; however, V-rings are still used in some applications. Unlike the O-ring, a V-ring seal will provide a seal in only one direction. Therefore, if a piston is to move in two directions under pressure, two sets of V-rings must be used. V-rings are always installed with the open end of the V facing the pressure. Male and female adapters are used in conjunction with V-rings for reinforcement.
A cup seal is sometimes used as a piston seal in hydraulic systems. A cup seal is generally made of synthetic rubber or leather. Some cup seals are made of fabricated synthetic material. As the name implies, this seal is made in the shape of a cup. Figure 11 shows a typical cup seal.
U-rings are used to prevent leakage in one direction only. Typical uses of the U-ring are in automotive brake assemblies and brake master cylinders. They are never used where high pressures will be encountered. Figure 12 shows a typical U-ring seal.
Flange seals are sometimes used as packing in some hydraulic systems. This type of packing is recommended for use only in low-pressure applications. Flange packing is the least desirable of the previously described types of seals. They are normally used only where there is insufficient space for either a V-ring packing or a U-ring packing. Figure 13 shows a typical flange packing.
Although wipers and backup washers are not classified as seals, they serve a vital role in the effectiveness and the life of seals in certain applications.
Wipers, sometimes referred to as scrapers, are used to clean and lubricate the exposed portion of piston rods. This prevents foreign matter from entering the system and scoring the internal surfaces and damaging seals. Wipers may be of the metallic or felt types, or in some applications a combination of both. The felt wiper is normally lubricated with system hydraulic fluid from a drilled passage or from an external fitting.
Metallic wipers are formed in split rings for ease of installation and are manufactured slightly undersized to ensure a tight fit. One side of the metallic wiper has a lip that should face outward upon installation. A felt wiper may be a continuous felt ring or a length of felt with sufficient material to overlap its ends. The wiper should be soft, clean, and well saturated with appropriate lubricant during installation.
One of the major problems concerning seals is the problem of extrusion. Extrusion may be defined as "distortion", under pressure, of portions of the seal into the clearances between mating metal parts." The extrusion of O-rings is shown in Figure 14.
When pressure is applied, the seals will flow into their respective clearances. When pressure is released, the O-rings will return to their original shape. However, the extrusion groove will appear as a cut beneath the surface of the O-ring. Eventually the cuts will become more severe and sections will be cut out of the O-ring. This will lead to the failure of the sealing capabilities of the O-ring.
To eliminate extrusion, you must use harder seals, reduce clearances, or use backup washers. Backup washers are commonly used for this purpose. This is a device normally used behind a seal to allow a higher pressure to be applied to the seal. A backup washer used behind an O-ring will extend the allowable seal pressure from 1,500 psi to 3,000 psi. If the O-ring is subject to pressures from alternating sides, backup washers are required on both sides of the O-ring. Figure 15 shows an installation of O-rings with backup washers.
Hydraulic systems must have a sufficient supply of uncontaminated fluid for efficient operation of the system. Although the same fluid is recirculated, a container must be provided for a supply of fluid in excess of that contained in the lines of components.
The reservoir is a basic component of any hydraulic system. In most systems, the reservoir is a separate component of the system. In other systems, for example the automatic transmission of an automobile, the reservoir also serves as the housing for the complete system. Although its primary function is to provide storage space for the fluid required by the system, a well-constructed reservoir provides several additional functions. Among these functions are:
Many factors are considered when selecting the size and configuration of a hydraulic reservoir for a particular system. It must be large enough to store more than the anticipated volume of fluid that the system will require. A reservoir capacity of at least three times the maximum rate of flow required by the system is usually sufficient. A higher ratio is desirable for fixed installation, and a somewhat lower ratio may be required for mobile equipment. Adequate space must be allowed to accommodate thermal expansion of the hydraulic fluid and changes in fluid level due to system operation.
Reservoirs are of two general types: pressurized and non-pressurized. Non-pressurized reservoirs are vented to the atmosphere. This prevents a partial vacuum from being formed as the fluid level in the reservoir is lowered. The vent also makes it possible for any air that has entered the system to find a means of escape.
Pressurized reservoirs, usually a few pounds above atmospheric pressure, are often desirable because they keep dirt out and force oil in the tank towards the pump. As the oil in the tank becomes warm and expands, it compresses the air trapped between the surface of the oil and the top of the tank. There is no vent in a pressurized reservoir.
Accumulators are incorporated in some hydraulic systems to store a volume of liquid under pressure for subsequent conversion into useful work. This source of stored energy may be the result of gravitation, the elasticity of springs, or the compressibility of gases.
In addition to a source of fluid supply, hydraulic accumulators may be used for a variety of other functions also. These include:
Accumulators may be divided into three general types:
The weight-loaded or gravity-type accumulator (Figure 16) consists of a cylinder, movable piston, ram or plunger, and a weight. The dead weight may be concrete, iron, steel, water, or other heavy material. The piston should have a precision fit inside the cylinder to reduce leakage. The inner cylinder wall should have a honed or ground finish to reduce friction and wear.
Figure 16: Weight-Loaded Accumulator
As hydraulic oil is pumped into the cylinder, the piston pushes the weight to a higher level. Thus, the potential or stored energy of the weight is increased. The energy stored in the weight is released in the downward motion as it is required by the demands of the system. The weight is adjusted so that the ram rises when the fluid pressure reaches a set level.
The travel of the ram can be controlled by an arrangement of a cam on the plunger and limit switches. The gravity force of the piston on the oil provides a nearly constant oil pressure level for the full stroke of the piston. By providing adequate piston area and ample length of piston stroke, a large volume of fluid can be supplied at high pressure. A single large accumulator may provide service for a number of different machines.
Figure 17 shows a spring-loaded type of accumulator. This device consists of a cylinder, piston, and spring. One or more springs may be used. The springs may be arranged to provide various adjustments by means of bolts.
As the oil is pumped into the accumulator, the piston or plunger compresses the spring; thus energy is stored in the spring. The energy stored in the spring is released as it is required by the demands of the system.
The pressure on the oil is not constant for all the positions of the piston, because the spring force depends on the movement of the spring. Usually, this type of accumulator delivers only a small amount of oil at low pressure.
Hydraulic fluid or oil is nearly incompressible. This means that a large increase in oil pressure results in only a small, or negligible, decrease in the volume of oil. On the other hand, a large increase in air or gas pressure results in a large decrease in the volume of the air or gas.
Relatively speaking, hydraulic oil is less elastic or spring-like than air. Oil cannot be used effectively to store energy by compressing it, whereas air or gas can be compressed to store energy. Thus, one general type of accumulator uses a gas or air, rather than a mechanical spring or a weight, to provide the spring-like action.
Air or gas-type accumulators can be divided into two types:
In the non-separator type of accumulator, the oil is in direct contact with the air or gas. In the separator type of accumulator, some type of mechanical material or device is used to separate the air or gas from the oil. In the separator type of accumulator, either a solid or a flexible barrier is placed between the oil and the air or gas to separate the two different types of fluids.
As a greater quantity of oil is pumped into the accumulator, the air or gas above the oil is compressed still further. The energy is stored in the compressed gas, and it is released as required by the demands of the system.
Figure 18: Air Or Gas Accumulator
Figure 19: Air or Gas Accumulator (Separator Type)