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| Illustration 1 | g01388783 |
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(1) Pump control valve
(2) Adjustment screw for margin pressure (3) Adjustment screw for high pressure cutoff (4) Spring (high pressure cutoff) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (high pressure cutoff) (9) Spool (margin pressure) (10) Spring (margin pressure) (11) Line for signal oil pressure | |
Pump control valve (1) contains a pressure compensator and a flow compensator. Control valve (1) 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 implement and steering system. The highest resolved signal pressure is then routed to the pressure and flow compensator valve. The control valve (1) 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 discharge pressure and signal pressure (11) with the force of spring (10).
The pump control valve (1) limits the pressure in order to prevent overloads of the hydraulic system.
When the system pressure exceeds the setting of the high pressure cutoff (3), the output pressure will be lowered. Lowering the output pressure will protect the hydraulic system from damage due to high pressure.
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| Illustration 2 | g01389053 |
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Typical example of a piston pump at low-pressure standby (1) Pump control valve (2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (pressure compensator) (9) Spool (flow compensator) (10) Spring (flow compensator) (11) Line for signal oil pressure (12) Control piston (13) Swashplate (14) Pump drive shaft (15) Pistons (16) Bias piston (17) Bias spring (18) Barrel | |
When the engine is off, spring (17) holds swashplate (13) at the maximum angle. When the engine is started, shaft (14) begins to rotate. Oil is drawn into the bore of pistons (15). Barrel (18) starts to rotate and pistons (15) 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 (10) and the signal pressure in line (9).
Spool (9) moves up and oil flows into passage (6) to piston (12). The oil pressure inside piston (12) overcomes the force of spring (17) and the system pressure inside piston (16). Piston (12) 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.
The pump supply oil moves spool (9) upward. This compresses spring (10). Since spool (9) is moved upward, more of the pump supply oil is allowed to flow through passage (6). The oil will flow through passage (6) and flow out of the cross-drilled passage to the pump case.
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| Illustration 3 | g01389054 |
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Typical example of a piston pump during upstroke (1) Pump control valve (2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (pressure compensator) (9) Spool (flow compensator) (10) Spring (flow compensator) (11) Line for signal oil pressure (12) Control piston (13) Swashplate (14) Pump drive shaft (15) Pistons (16) Bias piston (17) Bias spring (18) Barrel | |
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 and or implement system. Both signal pressure in line (11) and the force of spring (10) cause spool (9) to block the oil flow into passage (6). With no oil flow to piston (12), spring (17) is now allowed to increase the swashplate angle. The hydraulic pump will produce more oil flow.
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| Illustration 4 | g01389058 |
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Typical example of a piston pump during constant flow (1) Pump control valve (2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (pressure compensator) (9) Spool (flow compensator) (10) Spring (flow compensator) (11) Line for signal oil pressure (12) Control piston (13) Swashplate (14) Pump drive shaft (15) Pistons (16) Bias piston (17) Bias spring (18) Barrel | |
As the pump flow increases, the pump discharge 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 (11) and spring (10). The difference between signal pressure (11) and the pump supply pressure is the value of spring (10). The nominal setting of margin pressure is
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| Illustration 5 | g01389059 |
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Typical example of a piston pump during destroke (1) Pump control valve (2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (pressure compensator) (9) Spool (flow compensator) (10) Spring (flow compensator) (11) Line for signal oil pressure (12) Control piston (13) Swashplate (14) Pump drive shaft (15) Pistons (16) Bias piston (17) Bias spring (18) Barrel | |
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 (10) and signal pressure (11) combined. Spool (9) moves upward. More oil is allowed to flow to piston (12). With increased oil pressure on piston (12), 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.
- 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 | g01389063 |
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Typical example of a piston pump at high-pressure stall (1) Pump control valve (2) Adjustment screw for the flow compensator (3) Adjustment screw for the pressure compensator (4) Spring (pressure compensator) (5) Oil flow to the pump case (6) Oil flow to the control piston (7) Oil flow from the output port of the pump (8) Spool (pressure compensator) (9) Spool (flow compensator) (10) Spring (flow compensator) (11) Line for signal oil pressure (12) Control piston (13) Swashplate (14) Pump drive shaft (15) Pistons (16) Bias piston (17) Bias spring (18) Barrel | |
Note: The following description is for a single circuit that is in operation.
When signal pressure (11) and the spring force from spring (10) are equal to the output pressure in line (7), spool (9) moves downward. This blocks the pressure oil from piston (12). The angle of swashplate (13) increases. When the implement is stalled, the pressure in line (7) increases to the setting of spring (4). This causes spool (8) to move upward. The oil in the inlet passage now flows through passage (6) into piston (12). The flow of oil from passage (6) moves piston (12). Piston (12) moves swashplate (13) 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 (8) 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 (13) 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.