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| Illustration 1 | g03669062 |
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(1) Steering pump
(2) Pump control valve (3) Flow control adjustment (4) High-pressure cutoff adjustment | |
The steering pump (1) is a variable displacement, piston-type pump mounted to the front of the torque converter pump drive housing on the right side. The pump contains a single actuator to move the swashplate. The pump control valve (2 ) controls the pump oil flow. The pump control valve contains the flow control adjustment (3 ) and the high-pressure cutoff adjustment (4). On the 824K, oil from the steering pump flows to the secondary steering diverter valve and then to the steering control valve.
Steering Pump and Pump Control Valve
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| Illustration 2 | g03669063 |
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Low-pressure standby (1) Flow compensator spool (2) Actuator piston (3) Flow from the steering control valve (4) Bias spring (5) High-pressure cutoff spool | |
When, the engine is started and the steering pump has no demand for steering oil. The bias spring (4) moves the actuator piston (2), which holds the swashplate at maximum angle. When, the steering control valve (3) is in the "HOLD" position, pump flow is blocked at the steering control valve, and no signal pressure is generated. As the pump produces flow, the system pressure begins to increase. This pressure is felt at the left end of the flow compensator spool (1). 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 the steering control valve is moved.
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| Illustration 3 | g03669064 |
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Upstroke (1) Flow compensator spool (2) Actuator piston (3) Flow from the steering control valve (4) Bias spring (5) High-pressure cutoff spool | |
During a turn, signal pressure at the steering control valve (3 ) increases. This increased pressure causes the force (flow compensator valve spring plus oil pressure) at the right end of the flow compensator spool (1) 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 (2), 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, the bias spring (4) moves the swashplate to a greater angle, and the pump upstrokes.
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| Illustration 4 | g03670068 |
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Destroke (1) Flow compensator spool (2) Actuator piston (3) Flow from the steering control valve (4) Bias spring (5) High-pressure cutoff spool | |
When the load on the steering system decreases, signal oil pressure at the right end of the flow compensator spool (1) 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 the actuator piston (2), causing the pressure in the actuator piston to increase. The increased pressure in the actuator piston overcomes the combined force of the bias spring (4) 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 steering control valve (3 ) will be blocked.
Steering Pump - High Pressure Cutoff
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| Illustration 5 | g03669065 |
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High-pressure cutoff (1) Flow compensator spool (2) Actuator piston (3) Flow from the steering control valve (4) Bias spring (5) High-pressure cutoff spool | |
The high-pressure cutoff spool (1) 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 steering 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 the actuator piston (2) 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 the bias spring (4) 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 steering hydraulic system. Maximum system pressure is adjusted by turning the pressure compensator adjustment screw.