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| Illustration 1 | g01045010 |
Variable Displacement Piston Pump and Compensator Valve (1) Pump control valve (2) Actuator piston (3) Piston shoe (4) Swashplate (5) Drive shaft (6) Shoe plate (7) Piston (8) Bias piston (9) Bias spring (10) Cylinder barrel (11) Port plate (12) Pressure compensator spool (13) Flow compensator spool (14) Torque Limiter | |
The piston pump for the hydraulic system is controlled by the load signal from the implements. The piston pump senses both pressure and flow needs. The piston pump also senses hydraulic power. When none of the hydraulic circuits are being used, the pump is at low pressure standby.
If one or more circuits are being used, the resolver network compares the control valve work port pressures. The single highest pressure felt flows to flow compensator spool (13). The flow compensator (13) keeps the pump output at a level that is needed in order to fulfill the flow and pressure requirements of the system.
The system pressure will be greater than the requirements for the highest work port pressure unless the pump is at full stroke. Margin pressure is the difference between the pressure that is needed at the work port and the supply pressure that is higher.
The pressure compensator valve limits maximum system pressure. This protects the hydraulic system from damaging high pressures.
The control valve also controls the maximum output pressure of the pump. When the work port pressure rises above, pressure compensator spool (12) overrides flow compensator spool (13). This causes the pump to destroke. This occurs at approximately 690 kPa (100 psi) below the maximum pressure setting. If another circuit requires flow, the flow compensator valve will allow the pump to upstroke for the circuit while system pressure is limited.
The pump has two control pistons, bias piston (8) and actuator piston (2). The bias spring (9) in the bias piston causes swashplate (4) to move. The movement of the swashplate will cause the pump to upstroke or the movement will cause the pump to destroke.
Actuator piston (2) has a larger area than the bias piston. Actuator piston (2) causes the swashplate to destroke the pump. Flow compensator spool (13) and pressure compensator spool (12) changes pump output by regulating the discharge pressure of the pump that is acting on actuator piston (2) .
The torque limiter (12) is a power limiter that maintains the displacement of the pump in inverse proportion to the discharge pressure of the pump. When the hydraulic power demand reaches a set value, the torque limiter will destroke the pump. If the pump does not destroke the engine will stall.
Low Pressure Standby
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| Illustration 2 | g01054711 |
Pump and Compensator Operation (2) Torque limiter (3) Spring (4) Line to control valve (5) Hole (6) Actuator piston (7) Swashplate (8) Bias piston (9) Bias spring (10) Case drain (11) Passage (12) Passage (13) Passage (14) Passage (15) Pressure compensator spool (16) Passage (17) Flow compensator spool (18) Cavity (19) Signal line from control valve (20) Spring | |
Start the machine.
The pump is at low pressure standby, when the implement controls are in the HOLD position and the steering is inactive.
There are no flow or pressure demands from the implements on the pump but there is signal pressure in line (19) due to the bleed orifice in the priority valve.
Before the engine is started, bias spring (9) holds swashplate (7) at a maximum angle. As the pump begins to turn, this causes oil to flow. The flow of oil builds up pressure in the system because of the closed centers of the implement valves.
The pressure in passage (13) is felt at the bottom of the spool for flow compensator (17) and the spool for pressure compensator (15). As this pressure increases, the pressure pushes the spool for the flow compensator against spring (20). Spool (17) moves up. The movement of the spool opens passage (13) to passage (16) in order to allow the pressure oil to flow to actuator piston (6) .
The oil acts against the actuator piston. The oil pressure overcomes the force of bias spring (9) and supply pressure in bias piston (8). This causes the piston to move to the right. The piston moves the swashplate toward the minimum angle. The actuator piston continues to move to the right. The actuator piston uncovers hole (5). Uncovering the hole allows oil to drain into the case.
Hole (5) limits the maximum travel of the actuator piston (6) to the right. The pump will produce enough flow in order to account for the system leakage. The pump will also produce enough flow in order to account for the leakage to the pump case.
Upstroking
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| Illustration 3 | g01054732 |
Pump and Compensator Operation (2) Torque limiter (3) Spring (4) Line to control valve (5) Actuator piston (6) Swashplate (7) Bias piston (8) Bias spring (9) Case drain (10) Passage (11) Passage (12) Passage (13) Passage (14) Pressure compensator spool (15) Passage (16) Flow compensator spool (17) Cavity (18) Signal line from control valve (19) Spring | |
When the pump increases the output due to the demand for the increased flow of oil, the pump is upstroking. Because of the increased flow demand, the signal pressure combines with the force of the spring (19) in cavity (17) .
This combination of signal pressure and spring force is greater than the pump discharge pressure. This combination of signal pressure and spring force causes the spool (16) to move down. As the spool moves down, the spool blocks the flow of the supply oil to actuator piston (5). Oil in the passage for actuator piston (13) can now drain to passage (15). This is possible due to the blockage of oil to piston (5). The oil then flows past flow compensator spool (16). The oil then flows past pressure compensator (14). The oil then flows through passage (11) into case drain (9) .
Supply oil flows through passage (10) to bias piston (7). The oil acts against piston (7) and combines with the force of bias spring (8). This causes swashplate (6) to upstroke.
This increases the pump flow. As the flow requirements are satisfied the pump output pressure increases. The pressure increases until the pressure in passage (12) moves spool (16) up to the metering position.
Metering
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| Illustration 4 | g01054746 |
(2) Torque limiter (3) Spring (4) Line to control valve (5) Actuator piston (6) Swashplate (7) Bias piston (8) Bias spring (9) Case drain (10) Passage (11) Passage (12) Passage (13) Passage (14) Pressure compensator spool (15) Passage (16) Flow compensator spool (17) Cavity (18) Signal line from control valve (19) Spring | |
In the metering position, the signal pressure in line (18) and the force of spring (19) equals the pump supply pressure below spool (16) .
The pump will maintain the flow that is required in order to maintain system pressure. The pump will stay in the metering position until the flow requirements change.
Destroking
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| Illustration 5 | g01054758 |
(2) Torque limiter (3) Spring (4) Line to control valve (5) Actuator piston (6) Swashplate (7) Bias piston (8) Bias spring (9) Case drain (10) Passage (11) Passage (12) Passage (13) Passage (14) Pressure compensator spool (15) Passage (16) Flow compensator spool (17) Cavity (18) Signal line from control valve (19) Spring | |
When the pump decreases the output due to the decreasing demand for the flow of oil, the pump is destroking. The decreasing demand for the flow of oil causes a combination of the signal pressure and the force of spring (19) in cavity (17) to be less than the pump pressure in passage (12). Spool (16) is pushed up.
Oil behind actuator piston (5) cannot flow through passage (11) to case drain (9). Pump oil now flows through passage (12), past the spool (16), through passage (13) and into actuator piston (5). Pump pressure behind actuator piston (5) is now greater than the combined force of bias piston (7) and bias spring (8). The angle of swashplate (6) decreases. This decreases pump output and system pressure decreases.
Once the lower flow requirements are met, flow compensator spool (16) moves down to the metering position. Swashplate (6) will maintain an angle that is sufficient to provide the lower required flow. If all implements are returned to hold, the pump goes to the low pressure standby.
High Pressure Stall
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| Illustration 6 | g01054768 |
(2) Torque limiter (3) Spring (4) Line to control valve (5) Actuator piston (6) Swashplate (7) Bias piston (8) Bias spring (9) Case drain (10) Passage (11) Passage (12) Passage (13) Passage (14) Pressure compensator spool (15) Passage (16) Flow compensator spool (17) Cavity (18) Signal line from control valve (19) Spring | |
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 (18) and cavity (17) becomes equal to the pump output pressure. Spring (19) keeps spool (16) shifted downward.
When the pressure in passage (12) rises, the upward force on the pressure compensator spool (14) will overcome the force of spring (3). This will cause spool (14) to move up. Supply oil flows through passage (13) to actuator piston (5) .
Pressure that is felt on the actuator piston will destroke the pump. For single valve operation, pump output decreases while the system pressure is limited. For multiple valve operation, the system pressure will be close to the maximum. The system pressure will be close to the maximum, but the pump will produce flow in order to meet the needs of the other circuits with the lower pressure requirements.
During a high pressure stall, movement of the control lever into the HOLD position causes the pump to return to the low pressure standby.
Torque Limiter
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| Illustration 7 | g01054768 |
(2) Torque limiter (3) Spring (4) Line to control valve (5) Actuator piston (6) Swashplate (7) Bias piston (8) Bias spring (9) Case drain (10) Passage (11) Passage (12) Passage (13) Passage (14) Pressure compensator spool (15) Passage (16) Flow compensator spool (17) Cavity (18) Signal line from control valve (19) Spring | |
The torque limiter is a power limiter that maintains the displacement of the pump in inverse proportion to the discharge pressure of the pump. When the hydraulic power demand reaches a set value, the torque limiter will destroke the pump. If the pump does not destroke the engine will stall.