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| Illustration 1 | g03627262 |
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(1) Signal oil
(2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Spring (flow compensator) (6) Oil flow to the pump case (7) Oil flow to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) | |
The pump control valve contains a pressure compensator and a flow compensator. The pump control valve keeps the pump pressure and the pump flow at the level that is required of the hydraulic and steering system. When the hydraulic circuits are not active, the pump is at low-pressure standby. However, if one or more circuits are active, the load signal resolver compares the signal pressure of the hydraulic system. The highest resolved signal pressure is then routed to the pressure and flow compensator valve. The pump control valve adjusts the swashplate angle of the pump in order to maintain flow and pressure requirements. The margin pressure is defined as the difference between the pump pressure and signal oil (1) with the force of spring (5).
The pump control valve limits the pressure in order to prevent overloads of the hydraulic system.
When the system pressure exceeds the setting of the pump control valve, the pressure compensator will override the flow compensator. The output flow of the pump will be lowered. Lowering the output flow will protect the hydraulic system from damage due to high pressure.
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| Illustration 2 | g03635465 |
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Typical example of a piston pump at low-pressure standby (1) Signal oil (4) Spring (pressure compensator) (5) Spring (flow compensator) (7) Oil passage to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) (11) Control piston (12) Swashplate (13) Pump drive shaft (14) Barrel (15) Bias spring (16) Bias piston (17) Pistons | |
When the engine is off, bias spring (15) holds swashplate (12) at the maximum angle. When the engine is started, pump drive shaft (13) begins to rotate. Oil is drawn into the bore of pistons (17). Barrel (14) starts to rotate and pistons (17) stroke. This forces hydraulic oil into the hydraulic system.
The pump is in low-pressure standby when the following conditions are met:
- The machine is operating.
- The implements are in the HOLD position.
- There is no demand on the steering.
As the pump produces flow, the system pressure begins to increase. The system pressure overcomes the spring force of spring (5) and signal oil (1).
Spool (9) moves up and oil flows into passage (7) to control piston (11). The oil pressure inside control piston (11) overcomes the spring force of bias spring (15) and the system pressure inside bias piston (16). Control piston (11) moves the swashplate to the minimum angle. When the swashplate is moved to the minimum angle, the oil flows through the cross-drilled passage to the pump case. The system pressure is now at low-pressure standby.
When the pump is at low-pressure standby, the pump produces enough flow in order to compensate for internal leakage. Also, the pump produces enough flow in order to maintain sufficient system pressure. Low-pressure standby is maintained in order to ensure instantaneous response under one of the following conditions:
- The steering is activated.
- An implement is activated.
Low-pressure standby is boosted by pilot pressure which is sent back into the signal network. The pump supply oil moves spool (9) upward. This compresses spring (5). Since spool (9) is moved upward, more of the pump supply oil is allowed to flow through passage (7). The oil will flow through passage (7) and flow out of the cross-drilled passage to the pump case.
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| Illustration 3 | g03635677 |
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Typical example of a piston pump during upstroke (1) Signal oil (4) Spring (pressure compensator) (5) Spring (flow compensator) (7) Oil passage to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) (11) Control piston (12) Swashplate (13) Pump drive shaft (14) Barrel (15) Bias spring (16) Bias piston (17) Pistons | |
When more oil flow is needed, the hydraulic pump upstrokes. Signal oil is sent to the pressure and flow compensator valve when increased oil flow is required by the steering system. Signal oil is sent to the pressure and flow compensator valve when increased oil flow is required by the implement control valves. Both signal oil (1) and the force of spring (5) cause spool (9) to block the oil flow into passage (7). With no oil flow to control piston (11), bias spring (15) is now allowed to increase the swashplate angle. The hydraulic pump will produce more oil flow.
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| Illustration 4 | g03635689 |
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Typical example of a piston pump during constant flow (1) Signal oil (4) Spring (pressure compensator) (5) Spring (flow compensator) (7) Oil passage to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) (11) Control piston (12) Swashplate (13) Pump drive shaft (14) Barrel (15) Bias spring (16) Bias piston (17) Pistons | |
As the pump flow increases, the pump supply pressure increases. Spool (9) moves to a metering position. Spool (9) moves when the pump supply pressure increases to the point of equaling the sum of signal pressure (1) and spring (5). The difference between signal oil (1) and the pump supply pressure is the value of spring (5).
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| Illustration 5 | g03635717 |
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Typical example of a piston pump during destroke (1) Signal oil (4) Spring (pressure compensator) (5) Spring (flow compensator) (7) Oil passage to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) (11) Control piston (12) Swashplate (13) Pump drive shaft (14) Barrel (15) Bias spring (16) Bias piston (17) Pistons | |
When less oil flow is required, the hydraulic pump destrokes. The pump destrokes when the force on the bottom of spool (9) is greater than the force of spring (5) and signal oil (1) combined. Spool (9) moves upward. More oil is allowed to flow to control piston (11). With increased oil pressure on control piston (11), the swashplate angle decreases. The hydraulic pump will produce less oil.
The following conditions will cause the pump to destroke:
- All implement control valves are moved to the HOLD position. The pump returns to low-pressure standby.
- The directional stem on the control valve is moved in order to reduce flow.
- If the engine rpm increases, the pump speed increases. The pump will destroke in order to maintain the flow requirements of the system.
- No steering demand
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| Illustration 6 | g03635724 |
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Typical example of a piston pump at high-pressure stall (1) Signal oil (4) Spring (pressure compensator) (5) Spring (flow compensator) (7) Oil passage to the control piston (8) Oil flow from the output port of the pump (9) Spool (flow compensator) (10) Spool (pressure compensator) (11) Control piston (12) Swashplate (13) Pump drive shaft (14) Barrel (15) Bias spring (16) Bias piston (17) Pistons | |
Note: The following description is for a single circuit that is in operation.
When signal pressure (11) and the spring force from spring (5) are equal to the output pressure in line (8), spool (9) moves downward. This blocks the pressure oil from control piston (11). The angle of swashplate (12) increases. When the implement is stalled, the pressure in line (8) increases to the setting of spring (4). This causes spool (10) to move upward. The oil in the inlet passage now flows through passage (7) into control piston (11). The flow of oil from passage (7) moves control piston (11). Control piston (11) moves swashplate (12) toward the minimum angle. The pump output is decreased. The pump produces enough flow in order to compensate for internal leakage. Also, the pump produces enough flow in order to maintain system pressure.
When the system pressure decreases to a pressure that is less than the setting of spring (4), spool (10) moves downward. Spool (9) now controls the flow from the pump.
When several circuits are actuated in a stall condition, the pump will not destroke. The angle of swashplate (12) will decrease enough to supply oil to the remaining circuits that are not stalled.
The following conditions will cause the pump to stall at high pressure:
- Load sense relief too high.
- Not functioning properly
- Margin pressure is set too high.