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| Illustration 1 | g03785827 |
| (1) Medium pressure pilot pump | |
Medium pressure pilot pump (1) is mounted the rear of the swing pump.
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| Illustration 2 | g03785574 |
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Low-pressure standby (1) Line - to medium pressure pilot pump displacement solenoid (2) Line - to pilot oil filter (3) Pressure compensator valve (4) Line - from medium pressure pilot pump displacement solenoid (5) High-pressure cutoff valve (6) Actuator piston (7) Bias spring | |
When, the engine is started and the pump has no demand. Bias spring (7) moves actuator piston (6) to the right, which holds the swashplate at maximum angle. When the pump displacement solenoid (on the pilot manifold) is not activated, pump flow is blocked and no signal pressure is generated through line (4). As the pump produces flow, the system pressure begins to increase. This pressure is felt at the left end of the flow compensator spool (3). The flow compensator spool moves against spring force and permits system oil to flow to the actuator piston.
The actuator piston moves the swashplate to the "LOW PRESSURE STANDBY" position. In "LOW PRESSURE STANDBY", the pump produces enough flow to compensate for system leakage at sufficient pressure to provide instantaneous response when a function is performed.
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| Illustration 3 | g03785896 |
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Upstroke (1) Line - to medium pressure pilot pump displacement solenoid (2) Line - to pilot oil filter (3) Pressure compensator valve (4) Line - from medium pressure pilot pump displacement solenoid (5) High-pressure cutoff valve (6) Actuator piston (7) Bias spring | |
During a function, signal pressure from the displacement solenoid through line (4) increases. This increased pressure causes the force (flow compensator valve spring plus oil pressure) at the right end of the flow compensator spool (3) to become greater than the pump supply pressure at the left end of the spool.
The increased pressure at the right end of the flow compensator spool causes the spool to shift left. The spool reduces or blocks pump output oil flow to the actuator piston (6), and opens a passage to drain. Reducing or blocking oil flow to the actuator piston reduces or eliminates the pressure acting against the actuator piston. When the pressure in the actuator piston decreases, bias spring (7) moves the swashplate to a greater angle, and the pump upstrokes.
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| Illustration 4 | g03785918 |
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Destroke (1) Line - to medium pressure pilot pump displacement solenoid (2) Line - to pilot oil filter (3) Pressure compensator valve (4) Line - from medium pressure pilot pump displacement solenoid (5) High-pressure cutoff valve (6) Actuator piston (7) Bias spring | |
When the load on the medium pressure of pilot system decreases, signal oil pressure at the right end of flow compensator spool (3) decreases. This decreased pressure causes the force (flow compensator spool spring plus oil pressure) at the right end of the flow compensator spool to decrease below the pump supply pressure at the left end of the spool. The decreased pressure at the right end of the flow compensator spool causes the spool to shift. This action allows more flow to actuator piston (6), causing the pressure in the actuator piston to increase. The increased pressure in the actuator piston overcomes the combined force of bias spring (7) and moves the swashplate to a reduced angle.
As pump flow decreases, supply pressure also decreases. When the supply pressure decreases and equals the sum of the oil pressure at the right end of the flow compensator spool and spring force, the flow compensator spool moves to a metering position and the system stabilizes. At that time, the oil from the displacement solenoid through line (4) will be blocked.
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| Illustration 5 | g03785942 |
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High-pressure cutoff (1) Line - to medium pressure pilot pump displacement solenoid (2) Line - to pilot oil filter (3) Pressure compensator valve (4) Line - from medium pressure pilot pump displacement solenoid (5) High-pressure cutoff valve (6) Actuator piston (7) Bias spring | |
High-pressure cutoff valve (5) limits the maximum system pressure for any given pump displacement. The high-pressure cutoff spool is held in the left position during normal operation by spring force.
When medium pressure or pilot hydraulic system pressure is at maximum, pump supply pressure increases and the high-pressure cutoff spool moves right against spring force. The high-pressure cutoff spool blocks oil in actuator piston (6) from returning to the tank and allows supply oil to flow to the actuator piston.
The increase in pressure allows the actuator piston to overcome bias spring (7) to destroke the pump. The pump is now at minimum flow and pump supply pressure is at maximum.
This feature eliminates the need for a main relief valve in the medium pressure or pilot hydraulic system. Maximum system pressure is adjusted by turning the pressure compensator adjustment screw.