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| Illustration 1 | g00297450 |
Variable Displacement Piston Pump and Compensator Valve (1) Drive shaft. (2) Swashplate. (3) Shoe plate. (4) Bias piston. (5) Piston. (6) Bias spring. (7) Pressure and flow compensator valve. (8) Piston shoe. (9) Cylinder barrel. (10) Actuator piston. (11) Port plate. (12) Flow compensator spool. (13) Pressure compensator spool. | |
The steering system pump is an automatic piston pump. The steering system pump senses pressure requirements and flow requirements. When the steering circuit is not being used, the pump is at low pressure standby.
The single highest pressure that is sensed flows to pressure compensator spool (13). This spool keeps the pump output at a level that is necessary to fulfill the requirements for the system flow and for the pressure.
The actual system pressure will be greater than the highest work port pressure requirements unless the pump is at full stroke. The difference between the work port requirement and the higher main system pressure is called margin pressure.
The pressure compensator valve also limits maximum system pressure. The pressure compensator valve prevents damage to the steering system components from excessive pressure.
The pump is controlled by bias piston (4) and actuator piston (10). Bias spring (6) causes swashplate (2) to move. The swashplate then causes the pump to upstroke.
Actuator piston (10) has a larger area than bias piston (4). Actuator piston (10) causes swashplate (2) to destroke the pump. Flow compensator spool (12) and/or pressure compensator spool (13) changes pump output by regulating the pump discharge pressure that is acting on actuator piston (10) .
Pressure and flow compensator valve (7) routes pump discharge pressure to actuator piston (10). Because actuator piston (10) is larger than bias piston (4), the oil pressure that is acting against piston (10) overcomes the force of bias spring (6). The oil pressure then causes the pump to destroke.
Flow compensator spool (12) maintains pump outlet pressure at 2400 ± 100 kPa (350 ± 15 psi) above the work port pressure requirements.
Pressure and flow compensator valve (7) also controls the maximum output of pump pressure. When work port pressure rises above 20000 ± 350 kPa (2900 ± 50 psi), pressure compensator spool (13) overrides flow compensator spool (12). This causes the pump to destroke. This occurs at approximately 690 kPa (100 psi) below the maximum pressure setting.
Upstroking
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| Illustration 2 | g00297451 |
Pump and Compensator Operation (1) Drive shaft. (2) Swashplate. (4) Bias piston. (6) Bias spring. (10) Actuator piston. (12) Flow compensator spool. (13) Pressure compensator spool. (14) Case drain. (15) Passage. (16) Passage. (17) Spring. (18) Spring. (19) Signal line from the metering pump. (20) Cavity. (21) Passage. (22) Passage. (23) Cavity. (A) Signal pressure oil. (B) System pressure oil. (C) Return oil. | |
Upstroking of the pump occurs because of increased flow demand. The increased flow demand is met by increased pump output.
The increased flow demand causes a signal pressure in line (19). The signal pressure in line (19) combines with the force of spring (18) in cavity (20). The force of spring (18) causes pump pressure to be 2400 ± 100 kPa (350 ± 15 psi) greater than the signal pressure. However, the combination of signal pressure and of spring force is greater than the pump discharge pressure. This difference is called margin pressure.
The pressure causes spool (13) to move down. As spool (13) moves down, the spool blocks the flow of supply oil to actuator piston (10). When the oil flow to actuator piston (10) is blocked, the oil in passage (22) drains to passage (23). The oil then flows past flow compensator spool (12). The oil then flows past pressure compensator spool (13) and through passage (16) into case drain (14) .
Supply oil flows through passage (15) to bias piston (4). The oil acts against bias piston (4). The oil combines with the force of bias spring (6). This causes swashplate (2) to upstroke.
This also causes the pump flow to increase. As flow requirements are satisfied, the pump output pressure increases. The pressure increases until the pressure in passage (15) moves spool (12) up to the metering position.
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| Illustration 3 | g00270338 |
Metering Positions (A) Spool position at high pump pressure. (B) Spool position at low pump pressure. | |
Slight upward movement and slight downward movement is called metering. Metering maintains equal pressure on each end of spool (12) .
With spool (12) in position A, pump pressure is greater than the combined force of spring (18) and of the signal oil pressure in cavity (20). This forces spool (12) to move upward.
When spool (12) moves upward, pressure oil flows past the spool. The oil then flows through passage (23). The oil flows past compensator spool (12) and into passage (22). Oil flows through passage (22) to actuator piston (10). This causes the pump to destroke.
The area of actuator piston (10) is larger than the area of bias piston (4). This causes the force of the oil that is acting against actuator piston (10) to overcome the combined force of spring (6) and of the oil that is acting against bias piston (4). Actuator piston (10) moves to the left. This causes swashplate (2) to move toward the minimum angle.
As the angle of swashplate (2) decreases, pump output also decreases. When the pump pressure is low enough, the combined force of the signal oil pressure in cavity (20) and of spring (18) causes spool (13) to move downward into position B.
When spool (13) moves down, the flow of pump output oil into passage (20) is blocked. Oil in passage (22) flows from actuator piston (10) past spool (12). The oil then returns to the case drain.
Destroking
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| Illustration 4 | g00297452 |
Pump and Compensator Operation (1) Drive shaft. (2) Swashplate. (4) Bias piston. (6) Bias spring. (10) Actuator piston. (12) Flow compensator spool. (13) Pressure compensator spool. (14) Case drain. (15) Passage. (16) Passage. (17) Spring. (18) Spring. (19) Signal line from the metering pump. (20) Cavity. (21) Passage. (22) Passage. (23) Cavity. (A) Signal pressure oil. (B) System pressure oil. (C) Return oil. | |
The decreased flow demand causes a signal pressure in line (19). The signal pressure in line (19) combines with the force of spring (18) in cavity (20). This combination of signal pressure and of spring force is less than the pump pressure in passage (15). This causes spool (13) to move upward.
The oil behind actuator piston (10) can not flow through passage (16) to case drain (14). Pump oil now flows through passage (15). The oil then flows past spool (13), through passage (22), and then to actuator piston (10) .
The pump pressure behind actuator piston (10) is now greater than the combined force of bias piston (4) and of bias spring (6). The angle of swashplate (2) decreases. This decreases the pump output and the system pressure.
When the lower flow requirements are met, flow compensator spool (12) moves downward to the metering position. Swashplate (2) maintains an angle that is sufficient to provide the lower required pressure. If the operator does not turn the steering wheel, then the pump will return to low pressure standby.
Low Pressure Standby
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| Illustration 5 | g00297453 |
Pump and Compensator Operation (1) Drive shaft. (2) Swashplate. (4) Bias piston. (6) Bias spring. (10) Actuator piston. (12) Flow compensator spool. (13) Pressure compensator spool. (14) Case drain. (15) Passage. (16) Passage. (17) Spring. (18) Spring. (19) Signal line from the metering pump. (20) Cavity. (21) Passage. (22) Passage. (23) Cavity. (24) Cross-drilled hole. (A) Signal pressure oil. (B) System pressure oil. (C) Return oil. | |
Low pressure standby constitutes the following conditions: a running engine and inactive steering. There are no flow demands on the pump or pressure demands on the pump. Therefore, there is no signal pressure in line (19) .
Before you start the engine, bias spring (6) holds swashplate (2) at the maximum angle. As the pump begins to turn, oil begins to flow and pressure increases in the system. Pressure increases in the system because of the closed centered steering control valves.
The pressure in passage (22) is sensed at the bottom of flow compensator spool (12) and at the bottom of pressure compensator spool (13). As this pressure increases, the pressure pushes flow compensator spool (12) against spring (17). This causes flow compensator spool (12) to move upward. This opens passage (23) in order to allow pressure oil to flow to actuator piston (10) .
The oil acts against actuator piston (10) in order to overcome the force of bias spring (6). The oil causes actuator piston (10) to move to the left. When actuator piston (10) moves to the left, the piston moves swashplate (2) toward the minimum angle. Actuator piston (10) continues to move to the left until cross-drilled hole (24) is uncovered. Uncovering cross-drilled hole (24) allows the oil to drain to the case.
Cross-drilled hole (24) limits the maximum travel of actuator piston (10) to the left. The pump supplies a sufficient amount of flow that compensates for system leakage. The pump also supplies a sufficient amount of flow that compensates for leakage to the pump case. The leakage to the pump case is a result of the cross-drilled hole. The pump maintains low pressure standby. Low pressure standby pressure should not exceed 3900 kPa (565 psi).
Note: Low pressure standby will vary in the same pump as the system leakage or the pump leakage increases. The pump will upstroke slightly in order to compensate for the increasing leakage. Actuator piston (10) will cover more of the cross-drilled hole. As this happens, low pressure standby will drop toward margin pressure. The leakage will achieve the point when the piston completely covers the cross-drilled hole. This is done because of the increased swashplate angle which is required. When this happens, the low pressure standby will equal the margin pressure.
High Pressure Stall
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| Illustration 6 | g00297454 |
Pump and Compensator Operation (1) Drive shaft. (2) Swashplate. (4) Bias piston. (6) Bias spring. (10) Actuator piston. (12) Flow compensator spool. (13) Pressure compensator spool. (14) Case drain. (15) Passage. (16) Passage. (17) Spring. (18) Spring. (19) Signal line from the metering pump. (20) Cavity. (21) Passage. (22) Passage. (23) Passage. (A) Signal pressure oil. (B) System pressure oil. (C) Return oil. | |
When the hydraulic system stalls under load or when the cylinders reach the end of the stroke, the main system pressure increases. The signal pressure in line (19) and in cavity (20) becomes equal to the pump output pressure. Spring (17) keeps flow compensator spool (12) shifted downward.
When the main system pressure reaches 20000 ± 350 kPa (2900 ± 50 psi) in passage (15), the pressure acts upon pressure compensator spool (13). The pressure then overcomes the force of spring (18). The pressure then causes spool (13) to move upward. Supply oil now flows past spool (12) and through passage (23). The oil flows past spool (13) and into passage (22). The oil then flows to actuator piston (10) .
The pressure that is sensed on actuator piston (10) causes the pump to destroke. If the operator does not turn the steering wheel, then the pump will return to low pressure standby.