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| Illustration 1 | g00279980 |
Variable Displacement Piston Pump The maximum swashplate angle is shown. (1) Spring. (2) Swashplate. (3) Actuator piston. (4) Port plate. (5) Pressure and flow compensator valve. (6) Drive shaft. (7) Shoe plate. (8) Piston shoe. (9) Piston. (10) Cylinder barrel. | |
The angle of the swashplate determines the amount of oil that is drawn into each piston bore. The angle determines the amount of oil that is pumped out of each piston bore for each revolution of the drive shaft.
Infinite swashplate angle positions exist between neutral and the maximum angle. As the swashplate angle increases, the amount of oil that is pulled into the pump increases. As the swashplate angle increases, the amount of oil that is discharged through port plate (2) to the output port increases.
When swashplate (2) is at the minimum angle (0 degrees), pistons (9) do not move in and out of the rotating cylinder barrel. No oil is drawn into the pump and no oil is pumped out of the pump. The pump is not generating oil flow, except for a small amout of oil leakage.
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| Illustration 2 | g00283181 |
Pressure and flow compensator valve (11) Plug. (12) Seat. (13) Pressure compensator spool. (14) Signal passage. (15) Seat. (16) Chamber for load pressure. (17) Drain passage to pump case. (18) Flow compensator spool. (19) Passage to swashplate control piston. (20) Passage from pump outlet. | |
The pump has pressure and flow compensator valve (5) . The pressure and flow compensator valve maintains the pump pressure and the flow level that is demanded by the steering system. The pressure and flow compensator valve maintains the load and the flow demand in two ways. One way is by allowing pump oil to actuator piston (3) . The other way is by draining oil from the actuator piston.
The actuator piston works with swashplate control spring (1) in order to continually adjust the swashplate angle. Pump outlet pressure is maintained at 2100 kPa (305 psi) above the work port pressure requirements.
The compensator valve also has a pressure limiting ability which prevents overloads of the pump and of the system. When the work port pressure increases above 22800 kPa (3305 psi), pressure compensator spool (13) will override flow compensator spool (18) and pump output will be lowered. This action begins when the pressure is 2100 kPa (305 psi) below the maximum pressure setting.
The following schematics show the actions of the steering/pilot pump and of the pressure and flow compensator valve during different conditions in the hydraulic steering system.
Upstroking
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| Illustration 3 | g00280123 |
Operation of the steering/pilot pump and the pressure and flow compensator valve (1) Pressure compensator spring. (2) Flow compensator spring. (3) Cavity. (4) Signal line. (5) Flow compensator spool. (6) Check valve. (7) Passage. (8) Actuator piston. (9) Signal line from metering pump. (10) Pressure line to metering pump. (11) Case drain. (12) Check valve. (13) Passage. (14) Bias spring. (15) Swashplate. (16) Plug. (17) Pressure compensator cavity. (18) Pressure compensator spool. (19) Passage. (20) Passage. (21) Line. (22) Passage. (A) High pressure signal oil. (B) Low pressure oil. (C) Return oil. | |
Upstroking is the increased displacement by the pump in response to an increase in flow demand. Due to increased flow demand, the signal pressure plus the force of spring (2) in cavity (3) is greater than the pump discharge pressure. When this occurs, spool (5) moves downward. This blocks the flow of supply oil to actuator piston (8) .
When flow compensator spool (5) is positioned downward, the oil that is in passage (13) can drain to passage (19) . The oil then continues to flow past flow compensator spool (5) and past pressure compensator spool (18) . The oil then flows through passage (7) to case drain (11) .
The force of bias spring (14) can now move the swashplate toward the maximum angle. This increases pump flow. As the flow requirements are achieved, the pump output pressure increases until the pressure in passage (20) moves spool (5) upward to the metering position.
The inlet check valve (6) prevents steering kickback from external forces. In the steering metering pump control section, there is another check valve between the pump supply and the return oil ports. The check valve provides steering capability when the engine is not running. This is done by allowing oil to recirculate between the metering pump and the steering cylinder.
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| Illustration 4 | g00277934 |
Metering Position | |
In the metering position, pump pressure is greater than the combined force of spring (2) and the signal pressure in cavity (3) . Spool (5) moves up.
Pressure is now sent to actuator piston (8) . This pressure is greater than the force that is moving swashplate (15) toward the maximum angle. The angle of swashplate (15) decreases and pump output decreases.
When the pump pressure is reduced enough, the combined force of the signal pressure and of the spring in cavity (3) will cause spool (5) to move down. The oil pressure behind actuator piston (8) flows to the case drain.
Slight up and down spool movement is called metering. Metering maintains equal force on both ends of spool (5) . The pressure of spring (2) is equal to 2100 kPa (305 psi). The pump pressure is 2100 kPa (305 psi) greater than the signal pressure. The difference is called the margin pressure.
Destroking
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| Illustration 5 | g00280127 |
Operation of the Pump and the Compensator (1) Pressure compensator spring. (2) Flow compensator spring. (3) Cavity. (4) Signal line. (5) Flow compensator spool. (6) Check valve. (7) Passage. (8) Actuator piston. (9) Signal line from metering pump. (10) Pressure line to metering pump. (11) Case drain. (12) Check valve. (13) Passage. (14) Bias spring. (15) Swashplate. (16) Plug. (17) Pressure compensator cavity. (18) Pressure compensator spool. (19) Passage. (20) Passage. (21) Line. (22) Passage. (A) Low pressure signal oil. (B) High pressure oil. (C) Return oil. | |
A decrease of displacement by the pump in response to a decrease in flow demand is called destroking. Because of the decrease in flow demand, the signal pressure and the force of spring (2) in cavity (3) is less than the pump pressure in passage (20) . Spool (5) is pushed upward.
The oil behind actuator piston (8) cannot go through passages (7) and (19) to case drain (11) . Pump oil flows through passage (20) and past spool (5) . The oil then flows through passage (13) into actuator piston (8) . The pump pressure behind actuator piston (8) is now greater than the force of actuator spring (14) . The angle of swashplate (15) decreases. The pump output decreases and the system pressure decreases.
When the lower flow requirements are achieved, flow compensator spool (5) moves downward to the metering position. Swashplate (15) will maintain an angle that is sufficient to provide the lower required flow.
Low Pressure Standby
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| Illustration 6 | g00280132 |
Operation of the Pump and the Compensator (1) Pressure compensator spring. (2) Flow compensator spring. (3) Cavity. (4) Signal line. (5) Flow compensator spool. (6) Check valve. (7) Passage. (8) Actuator piston. (9) Signal line from metering pump. (10) Pressure line to metering pump. (11) Case drain. (12) Check valve. (13) Passage. (14) Bias spring. (15) Swashplate. (16) Plug. (17) Pressure compensator cavity. (18) Pressure compensator spool. (19) Passage. (20) Passage. (21) Line. (22) Passage. (A) Pressure oil. (B) Return oil. | |
When the engine is running and the steering is not being used, a low pressure standby condition exists. There are no flow demands and no pressure demands on the pump. There is no signal pressure in line (4) .
Before the engine is started, bias spring (14) holds swashplate (15) at the maximum angle. As the pump begins to turn, oil begins to flow and pressure increases in the system because of the closed centered metering pump.
This pressure in passage (20) is felt at the bottom of pressure compensator spool (18) and of flow compensator spool (5) . As this pressure increases, the flow compensator spool is pushed upward against spring (2) .
When the system pressure increases to more than 4300 kPa (624 psi), spool (5) must move upward in order to open a passage. The passage is for the pressure oil to flow to the back of actuator piston (8) .
The actuator piston is caused to move to the right. This compresses bias spring (14) and the swashplate is moved to the minimum angle. The actuator piston continues to move to the right until the cross-drilled passage of the actuator rod is uncovered. This allows oil to drain to the case.
The cross-drilled hole limits the maximum travel of the piston to the right. At this point, enough oil flow is produced by the pump in order to make up any system leakage. Enough oil flow is also produced by the pump in order to make up any leakage to the pump case through the cross-drilled hole. This leakage is made up while a system pressure of 4300 kPa (624 psi) is maintained.
The pump is at low pressure standby. This pressure is different than margin pressure because of system leakage and because of the cross-drilled hole in the actuator piston rod.
The flow compensator spool instead of the steering metering pump oil must remain open. The flow compensator spool must move up against spring (2) in order to provide enough flow to the back side of the actuator piston. This flow makes up the leakage through the cross-drilled hole.
Note: Low pressure standby will vary in the same pump as the system leakage increases or as the pump leakage increases. As the leakage increases, the pump will upstroke slightly in order to compensate for the leakage. The actuator piston will cover more of the cross-drilled hole. As this occurs, low pressure standby will drop toward the margin pressure. The piston will completely cover the cross-drilled hole because of the increased swashplate angle that is necessary. When the leakage achieves this point, the low pressure standby will equal the margin pressure.
High Pressure Stall
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| Illustration 7 | g00280134 |
Operation of the Pump and the Compensator (1) Pressure compensator spring. (2) Flow compensator spring. (3) Cavity. (4) Signal line. (5) Flow compensator spool. (6) Check valve. (7) Passage. (8) Actuator piston. (9) Signal line from metering pump. (10) Pressure line to metering pump. (11) Case drain. (12) Check valve. (13) Passage. (14) Bias spring. (15) Swashplate. (16) Plug. (17) Pressure compensator cavity. (18) Pressure compensator spool. (19) Passage. (20) Passage. (21) Line. (22) Passage. (A) Signal oil. (B) Pressure oil. (C) Return oil. | |
When the hydraulic system stalls under load or when the cylinders reach the end of the stroke, the system pressure increases. The signal pressure in line (4) and in cavity (3) becomes equal to the pump output pressure. Spring (2) keeps spool (5) shifted downward.
When the system pressure in passage (20) is 22800 kPa (3305 psi), the upward force on compensator spool (18) will compress spring (1) . Pressure compensator spool (18) will move upward. Supply oil flows through passages (13) and (19) to actuator piston (8) .
Pressure that is felt on the actuator piston will destroke the pump. Pump output decreases while the system pressure is limited to 22800 kPa (3305psi) .
Check valve (12) prevents damage to the pump during stall conditions. Check valve (12) allows system pressure oil to bypass the margin spool. The oil then flows to actuator piston (8) .