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HYDRAULIC SYSTEMS

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Basic Hydraulic System

Figure 1 shows a typical hydraulic system. Oil from a tank or reservoir flows through a tube or pipe into a pump. An electric motor, air motor, gas turbine, or internal combustion engine can drive the pump. The pump increases the pressure of the oil; the oil pressure at the pump outlet may be 5 to 5,000 or more pounds per square inch.

High-pressure oil flows in a tube or piping through a control valve; this valve can be used to change the direction of the oil flow. A relief valve is used to protect the system against overpressure; the valve can be set at a desired safe maximum pressure. If the oil pressure in the system begins to rise above the maximum safe pressure, the relief valve opens to relieve the pressure and to prevent damage to either the system or the surroundings. The discharge is usually directed back to the vented reservoir.

The oil that enters the cylinder acts on the piston. This pressure action over the area of the piston develops a force on the piston rod. The force of the piston rod can be used to move a load or device. Oil from the cylinder returns to the reservoir. As the oil passes through the filter, dirt and foreign matter are removed from the oil. Each separate unit, such as the pump, the valve, the cylinder, or the filter is called a component of the system.

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Hydraulic Circuits

Hydraulic circuits or schematics incorporate the use of standard symbols for its particular components. With a knowledge of these symbols, the operation of a particular system can be interpreted. This section gives specific examples of circuits to be analyzed.

Example 1

A circuit designed to move an object A from position X to position Y at a nearly constant speed is shown in Figure 2.

The operator loads object A at position X and shifts the handle of the three position, open center, four way valve C. Oil, under pressure, is directed from the power unit E through the inlet port of valve C to port 1 and then to port 3 of the cylinder D. As oil pressure forces the piston forward, oil is exhausted from port 4 to port 2 of valve C and then to the reservoir of the unit E. The piston and piston rod of cylinder D cause the object to move to position Y.

When the operator shifts the handle of valve C to the extreme opposite position, oil under pressure is directed from the inlet of valve C to port 2 and then onward to port 4 of cylinder D. The piston of the cylinder begins to retract and oil is exhausted through port 3 to port 1 of valve C and through the exhaust port to the reservoir of the unit E. The piston rod of cylinder D retracts to position X. The operator shifts the handle to the neutral or center position, and the cycle is completed.

By placing the handle in neutral, oil is directed to the reservoir at no pressure during the stand-by period, this keeps heat from building up in the system. If a two position valve were used the pressurized oil would spill through to the exhaust of the relief valve and create heat.

Example 2

A circuit in which the object A is decelerated at certain points while work is performed, as the object moves from one position to another, is shown in Figure 3.

To operate the circuit, the operator places the work piece on the machine table, which is equipped with a magnetic chuck. The operator momentarily depresses pushbutton PB-1, which energizes the solenoid X of valve H. This directs pilot pressure to the pilot A of valve G. The spool shifts, allowing oil to flow to the blind end of the feed cylinder D. The feed cylinder piston rod moves the machine table forward at a rapid rate, until the cam on the feed table contacts the cam roller of the cam-operated speed control valve E.

As the cam roller is depressed, the exhaust oil flow is shut off, and it meters through the speed-control portion at a speed determined by the needle setting. When the roller rides off the cam, the cylinder operates at full speed. When the piston rod of cylinder D reaches the end of its stroke, the limit switch B is contacted, energizing the solenoid Y, shifting the spool of valve H to its original position, and directing oil to the pilot B. The spool of valve G shifts top direct oil to the rod end of cylinder D, whereupon the piston and rod retract the table at a rapid rate. The operator unloads the work piece, reloads, and is ready for the next operation.

The pilot control valve H controls the action of valve G. On equipment that provides a central station for the operator, push buttons may be used to save installation costs. Skip feed valves are often used on grinders and other large machines.

Example 3

In some systems that require emergency closing, as shown in Figure 4, a means of operation must be available if the system loses power. A gas-type accumulator can be used for this purpose. This system also incorporates the use of pressure switches for control functions. This particular system will have three separate modes of operation, which are referred to as:

  • Directional Mode
  • Failure Mode
  • Reset Mode

Directional Mode

The directional mode is the normal operating mode for the valve actuator. It can be used to open or close the valve under normal conditions.

Actuator Open

The directional control solenoid SV3 is energized and the motor/pump is turned on; SV2 is de-energized during this operation. This causes hydraulic fluid to be forced through the pump, check valve, filter, the normally open port of SV2, the normally closed port of SV3, the pilot check, and into the actuator. The pressure increases until it reaches the prescribed setting, at which time pressure switch PS3 turns off the motor starter.

When the valve is opening, fluid flows from the top of the cylinder through the pilot check, through SV3 and into the reservoir. When the hydraulic pressure in the actuator decays to the pressure switch, reset PS3 again. Turn on motor starter until PS3 trips open and turns the motor starter off.

Actuator Close

To close, SV3 is de-energized, the flow through the actuator reverses. When the actuator is fully closed, the pressure increases until the pressure setting of pressure switch PS1 is reached which then turns off the motor starter. When the hydraulic pressure in the actuator decays to the pressure switch, reset PS1 again. Turn on motor starter until PS1 trips open and turns the motor starter off.

Failure Mode

The redundant fail close solenoids SV1 A and/or B initiate fail close action. When SV1 A and/or B is de-energized, the accumulator pilot check valves are piloted open which causes fluid flow from the accumulator, through the pilot check and check valves and into the top cylinder port. The flow out of the cylinder flows through the accumulator pilot check into the adjustable flow orifice and into the reservoir.

The nitrogen side of the accumulator incorporates a pressure switch PS4 that is used to monitor nitrogen pressure. When the low-pressure trip setting is reached, the contacts trip a control room alarm.

During the failure mode action, the motor must be de-energized to prevent excess heat generation caused by continuous running of the pump. The failure position will override all modes of operation.

Reset Mode

Reset is accomplished by energizing the recharge solenoid valve-SV and commanding the motor/pump on. This is initiated by pressure switch PS sensing low pressure. PS will close and signal the motor starter and solenoid valve SV on.

The hydraulic fluid is then pumped from the reservoir through SV into the accumulator. PS will open when the correct pressure is reached and then commands the motor and SV to off. During the recharge, the valve is held in the fail position until the accumulator is recharged.

The accumulator will automatically recharge at any position of valve. This is accomplished by PS commanding the motor starter and SV on. The function is then performed as described above.

During the reset mode, solenoid valves SV1 A and B must be energized. If this is not done, the fluid will flow through the pilot check into the close direction relief valve and into the reservoir.

Example 4- RC-801 Automatic Pallet Entry and Discharge

Refer to Figure 5 for the following explanation.

Motor 'M' drives tandem pump 'P1-P2'.

The squarer limit switches energize hoist Squarer Interlock Valve V8. Squarers must be in the retracted position for this valve to open. Hoist cannot be raised or lowered if squarers are extended.

Large volume of tandem pump P1 supplies pressure through pressure relief valve R1, which is located in the reservoir, through pressure line filter V1 and returns.

When solenoid #130 is energized on V1, P|A and A|T, allowing hoist to lower, which is cylinder C1, the speed of the hoist drop is controlled at flow control valve A1.

Solenoid #131 releases solenoid check valve V7 when solenoid #130 is energized on valve V1.

When solenoid #161 is energized on V1, P|B and A|T, it causes the hoist to raise.

Small volume of tandem pump P2 supplies pressure to valves V2, V3, and V4 in series and returns to the reservoir. It also passes through pressure filter F2.

Valve V2 in one position allows hoist cylinder C1 to lower, with oil returning to the reservoir. When valve V2 is in the opposite position, the hoist cylinder C1 raises.

When solenoid #63 is energized on valve V6, the oil flows from hoist cylinder C1 to the reservoir. Solenoid #63 is energized until hoist has dropped a distance equal the height of the ware being palletized as determined by electric eye. The speed of the drop is controlled by fixed orifice O1.

When solenoid #38 is energized on V3, P|A and B|T, retracting plunger lift cylinder C2, lowering the clamp. The lowering speed is controlled by fixed orifice O2.

When solenoid #54 is energized on V4, P|A and B|T, extending plunger lift cylinder C2, raising the clamp. The lift speed is controlled by fixed orifice O2. Solenoid #55 is energized with solenoid #54 and opens solenoid check valve V5.

When solenoid #42 is energized on valve V4, P|A and B|T, causing oil to flow to cylinder D1, where it forces the free piston to the opposite end, after which oil must flow through fixed orifice O3. This causes the carriage to decelerate before reaching the end of its travel. The oil continues to hydraulic motor M1, which rotates to move the carriage forward. Knob A2 controls the forward speed of the carriage. While the sweep is traveling forward a small portion of the oil flow is bled off through orifice O4. This permits the sweep forward flow control to be set at a maximum with the resulting sweep forward speed still marginally slower than the mesh belt speed, which is a critical factor in the operation of the sweeping knives. Also, the sweep reverse flow control can be set at a maximum which will result in a faster return speed than the maximum forward speed.

When solenoid #46 is energized on valve V4, P|B and A|T, causing oil to flow to hydraulic motor M1, which rotates to return the carriage. The oil continues to cylinder D1, where it forces the free piston to the opposite end, after which oil must flow through fixed orifice O3. This causes the carriage to decelerate before reaching the end of its travel. Knob A3 controls the return speed of the carriage.

Pressure relief valve R1 located inside the reservoir controls pressure to valve V1 for automatic or pushbutton control of the hoist. Its setting is independent of R2, R3, or R4.

Pressure relief valve R2, located within valve V2, controls the pressure that is needed for hoist raising with manual valve V2.

Pressure relief valve R3 is used to control the pressure to valve V3, which operates the clamp bar in both directions. The setting must equal or exceed R4 and cannot exceed R2.

Pressure relief valve R4 is used to control the pressure valve V4, which powers the carriage in both directions. Its setting will not affect R3 or R2 and cannot exceed them.

  • Normal pressure setting for relief valve R1 is 800 psi
  • Normal pressure setting for relief valve R2 is 1200 psi
  • Normal pressure setting for relief valve R3 is 750 psi
  • Normal pressure setting for relief valve R4 is 600 psi

Common Causes of Failure

As with any mechanical system, failures can occur in hydraulic systems. These common causes of failure must be kept in mind when servicing systems to prevent possible damage or improper operation.

Dirt

Without a doubt, dirt causes more hydraulic components to fail than any other single cause. Dirt also includes foreign substances.

In a hydraulic system, dirt and foreign matter may cause excessive damage to the components, since the fits between the parts are held to very close limits. Dirt not only scores the parts but often causes valve spools to stick and become inoperable. Dirt sometimes becomes lodged between the piston, piston ring, and tube of the hydraulic cylinder, causing the piston ring to be broken. This in turn can cause the tube to become badly scored.

Dirt clogs small orifices in valves, causing them to malfunction. Dirt tears the rod packing and causes external leakage of the fluid. Foreign matter also causes pitting of the piston rods and valve stems. Foreign matter, such as hydrocarbons, may clog intake strainers and cause carbons or cavitation in the pump. Intake strainers have been known to collapse due to collection of foreign matter. Dirt can cause a pump to seize, and the driving means twist off the pump shaft.

Cutting oils and coolants sometimes get into the hydraulic oil, causing considerable corrosion within the system and failure of the components. Every precaution should be taken to keep these solutions out of the system.

Heat

Heat causes considerable trouble to the components of a hydraulic system. It may cause valve spools to stick, packing to deteriorate, oil to break down, deposits to cling to the finished surfaces, excessive external and internal leakage, and inaccurate feeds in the system.

Hydraulic systems must be protected from hot blasts. If heat is caused by internal conditions, install after-coolers and, if possible, correct the condition causing the heat. Some causes of high heat are ambient temperatures, restrictions in lines and components, high pressures, and high pressures being spilled through relief valves.

Misapplication

Misapplication causes many failures of hydraulic system components. The selection of the improper component as to capacity, ability to withstand shock loads, or ability to withstand certain other operating conditions may cause failure. The use of a pneumatic valve for high-pressure oil service is likely to cause trouble. The use of a cylinder with thin cast iron covers for heavy duty mill applications is another example.

Improper Fluids

Care should be used in selecting the fluid for use in a hydraulic system. Check with the pump manufacturer for a recommendation. If the oil is satisfactory for the pump, it is likely that it will be fine for other components of the system.

As discussed previously, certain hydraulic fluids have detrimental effects on seals, packing, paint, and strainers. If these fluids are used, provisions must be made accordingly. Mixing of hydraulic fluids is not recommended as one of the fluids may have a property that is detrimental to the other. Fluids that cause deposits or corrosive action should not be used in hydraulic systems.

Maintenance

Poor maintenance can often be a cause for hydraulic system failures. A regular maintenance program can reduce these failures. The oil should be changed at regular intervals. At that time the system should be cleaned and checked for leaks. Change packing and seals when necessary and never allow dirt to accumulate around the system.

Improper Design or Installation

Faulty installation or improper design may contribute to hydraulic system failure.

The hydraulic circuit shown in Figure 6 will give unsatisfactory service. Note the size of the piping and cylinder. Usually a cylinder with a 2-to-1 rod is able to retract the piston at twice the rate that it moves outward. With a restriction in the valve and the piping, this is impossible. To remove the oil from the rear of the cylinder, the ports and pipes must be capable of handling oil at twice the rate it enters at the rear.

This problem can be corrected by simply increasing the size of the piping to 1.25 inches while also increasing the size of the control valve used to the same dimensions.

Figure 7 shows another example of a hydraulic circuit that would eventually fail. The power unit is mounted overhead, so the exhaust oil must be forced uphill to the reservoir. This creates back pressure on all the valves, which can cause them to malfunction. Keep the unit as low as possible. If necessary, an auxiliary reservoir should be added to collect the exhaust oil and then be pumped back into the main reservoir.

Another design consideration closely related to the previous example is that of mounting the pump above the reservoir. Since most hydraulic pumps have the ability to create a vacuum condition of only approximately three PSI, the height of the pump above the minimum oil level must be considered.

Three PSI is equivalent to 7.5 feet of oil; therefore, this would be the maximum height above the oil level that the pump could be mounted. This is still not a true figure, as the oil moving through the suction line and the strainer will also cause a pressure drop.

Consider the example shown in Figure 8 for the following discussion. The pump has a 15 GPM capacity and is mounted four feet above the reservoir. The piping is 1.25" Schedule 40, having an inside diameter of 1.38".

Assuming the worst case conditions for operation, the pressure drop per foot of piping can be calculated as follows:

One other consideration is the pressure drop due to the strainer. An adequately sized strainer will cause approximately a .5 psi pressure drop. Using this, plus the pressure drop due to the piping, the maximum height can be calculated as follows:

Using the example shown in Figure 8, the pump would not adequately perform due to its elevation being greater than the calculated maximum. Most hydraulic pump manufacturers recommend that the pump never be mounted more than three feet above the minimum oil level.

Faulty installation may contribute to many hydraulic failures. Some of the more common installation problems are:

  • Flow controls reversed in system
  • Wrong connections made to directional controls
  • Installation so as to create back pressure
  • Failure to make drain connections
  • Wrong sized piping installed
  • Components not securely mounted
  • Loose pipe lines or misaligned pipe lines
  • Misalignment of piston rods and actuators
  • Leaks around connections, caused by over torquing
  • Installations allowing air to be trapped inside system

Careful planning and the ability to produce good workmanship are a prerequisite to satisfactory installation. Remember that precision equipment, often very costly for one single component, is being installed.

Hydraulic Symbols

A study of the operation of a hydraulic system is more convenient if a diagram of the system or circuit is available. The components in an installation and their connections can be determined from such a diagram. A schematic diagram indicates the functions of the various parts. In a given schematic diagram, for example, certain symbols and lines are used to represent a pump, a cylinder, and a pipe from the pump to the cylinder. The function of a pump is to increase the fluid pressure, the function of the pipeline is to transport fluid, and the function of the cylinder is to provide a means for doing work on a device or load. Thus, the schematic diagram shows the components and the connections between the components. A schematic diagram also indicates the operation of a system to a person who understands the symbols.

A schematic diagram of a hydraulic system is similar to a geographical road map. The symbols or language of the road map must be learned before the road map can be understood. Similarly, the symbols or language of a schematic diagram must be learned before the diagram can be used to trace a hydraulic system.

In the past, many different diagrams and symbols have been used, a practice which proved to be inconvenient and troublesome. A real need arose for a standard set of symbols. Accordingly, a number of conferences were held for the purpose of establishing a set of standard symbols for industrial hydraulic equipment.

A set of fluid power symbols approved by the American National Standards Institute (ANSI) is shown in Figure 9 through Figure 14. These symbols can be of inestimable value to designers, installation, and maintenance personnel. Symbols are used to show connections, flow paths, and functions of the components represented. Conditions occurring during transition from one flow-path arrangement to another can be indicated by symbols.

The locations of ports, direction of shifting spools, and the positions of the control elements on the actual component are not indicated by symbols. Symbols are not used to indicate construction, and they are not used to indicate values, such as pressure, flow rate, and other component settings. The symbols can be rotated or reversed without affecting their meaning--except in lines to reservoirs, a vented manifold, and an accumulator.

All component symbols shall be shown in the schematic diagram with the equipment at its normal, at rest, or neutral position with power on and the hydraulic unit running.