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| Illustration 1 | g01062857 |
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Typical Hydraulic Pump (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (5) Inlet opening from the hydraulic tank (6) Swashplate (7) Slipper (8) Retraction Plate (9) Control Piston (10) Outlet opening to the combination valve (11) Pump control valve | |
The hydraulic and steering pump is a variable displacement axial piston pump.
The hydraulic and steering pump has nine pistons (2). When drive shaft (1) turns, barrel (4), slippers (7) and retraction plate (8) turn. The piston ends connect to the slippers. Swashplate (6) does not turn. There is a bearing journal on each side of the swashplate. The two bearing journals are inside bearings which allow movement of the swashplate. Oil flow through the passages in the pistons lubricates the internal components of the pump.
When swashplate (6) is at the maximum angle and drive shaft (1) is turning, pistons (2) are moved in and out of barrel (4). As the pistons move out of the barrel, the pistons create a vacuum at inlet opening (5). The pressure in the hydraulic tank pushes the oil into the inlet opening. The oil passes through the inlet opening and into the piston bore in the barrel. As the barrel continues to turn, the pistons are pushed into the barrel as the slippers rotate up the angle of swashplate (6). The pistons push the oil from the piston bore through pump outlet opening (10) and into the combination valve.
When the engine is operating, the pump will produce flow in order to satisfy the following conditions:
- The demand of the steering system
- The pressure setting of the pressure compensator
- The demand of the hydraulic implements
- The internal lubrication of the pump components
- The margin pressure of the pump
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| Illustration 2 | g00933303 |
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(12) Line for signal oil pressure
(13) Adjustment screw for the flow compensator (14) Adjustment screw for the pressure compensator (15) Spring (pressure compensator) (16) Spring (flow compensator) (17) Oil flow to the pump case (18) Oil flow to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) | |
Pump control valve (11) contains a pressure compensator and a flow compensator. The pump control valve (11) keeps the pump pressure and the pump flow at the level that is needed to fulfill the requirements 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 resolver valve in the combination valve 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 (11) 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 the signal pressure (12) with the force of spring (16).
The pump control valve (11) limits the pressure in order to prevent overloads of the hydraulic system.
When the system pressure exceeds the setting of the pump control valve (11), the pressure compensator will override the flow compensator. This will lower the output flow of the pump. This protects the hydraulic system from damaging high pressures.
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| Illustration 3 | g00933305 |
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Typical Example of a Piston Pump at Low Pressure Standby (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (6) Swashplate (9) Control piston (12) Signal oil from the combination valve (15) Spring (Pressure compensator) (16) Spring (flow compensator) (18) Oil passage to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) (22) Bias piston | |
When the engine is off, spring (3) holds swashplate (6) at the maximum angle. When the engine is started, shaft (1) begins to rotate. Oil is drawn into the bore of pistons (2). Barrel (4) starts to rotate and pistons (2) 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.
When the steering metering pump is in the HOLD position, pump flow is blocked at the steering metering pump and no signal pressure is generated in line (12).
As the pump produces flow, the system pressure begins to increase. The system pressure overcomes the spring force of spring (16) and the signal pressure in line (12).
Spool (20) moves up and oil flows into passage (18) to piston (9). The oil pressure inside piston (9) overcomes the spring force of spring (3) and the system pressure inside piston (22). Piston (9) 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 at this point is called low pressure standby. The system pressure at this point is approximately
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 higher than margin pressure. This characteristic is due to a higher back pressure that is created by the closed center valves that are in the HOLD position. The pump supply oil moves spool (20) upward. This compresses spring (16). Since spool (20) is moved upward, more of the pump supply oil is allowed to flow through passage (18). The oil will flow through passage (18) and flow out of the cross-drilled passage to the pump case.
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| Illustration 4 | g00933307 |
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Typical Example of a Piston Pump During Upstroke (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (6) Swashplate (9) Control piston (12) Signal oil from the combination valve (15) Spring (Pressure compensator) (16) Spring (flow compensator) (18) Oil passage to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) (22) Bias piston | |
When more oil flow is needed, the hydraulic pump upstrokes. When the steering requires increased oil flow or when any implement control valves require increased oil flow, signal oil is sent from the combination valve to the pressure and flow compensator valve. The combination of the signal pressure in line (12) and the force of spring (16) cause spool (20) to block the oil flow into passage (18). With no oil flow to piston (9), spring (3) is now allowed to increase the swashplate angle. The hydraulic pump will produce more oil flow.
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| Illustration 5 | g00933308 |
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Typical Example of a Piston Pump During Constant Flow (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (6) Swashplate (9) Control piston (12) Signal oil from the combination valve (15) Spring (Pressure compensator) (16) Spring (flow compensator) (18) Oil passage to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) (22) Bias piston | |
As the pump flow increases, the pump supply pressure increases. When the pump supply pressure increases to the point of equalling the sum of the signal pressure (12) and the spring (16), spool (20) moves to a metering position. The difference between the signal pressure (12) and the pump supply pressure is the value of spring (16). The value of spring (16) is
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| Illustration 6 | g00933309 |
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Typical Example of a Piston Pump During Destroke (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (6) Swashplate (9) Control piston (12) Signal oil from the combination valve (15) Spring (Pressure compensator) (16) Spring (flow compensator) (18) Oil passage to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) (22) Bias piston | |
When less oil flow is required, the hydraulic pump destrokes. The pump destrokes when the force on the bottom of spool (20) is greater than the force of spring (16) and signal pressure (12) combined. Spool (20) moves upward. This allows more oil to flow to piston (9). With increased oil pressure on piston (9), 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 control valve's directional stem is moved in order to reduce flow.
- Any of the additional circuits are deactivated.
- If the engine rpm increases, the pump speed increases. The pump will destroke in order to maintain the system's flow requirements.
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| Illustration 7 | g00933312 |
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Typical Example of a Piston Pump at High Pressure Stall (1) Pump drive shaft (2) Pistons (3) Bias spring (4) Barrel (6) Swashplate (9) Control piston (12) Signal oil from the combination valve (15) Spring (Pressure compensator) (16) Spring (flow compensator) (18) Oil passage to the control piston (19) Oil flow from the output port of the pump (20) Spool (flow compensator) (21) Spool (pressure compensator) (22) Bias piston | |
Note: The following description is for a single circuit that is in operation.
When signal pressure (12) and the spring force from spring (16) are equal to the output pressure in line (19), spool (20) moves downward. This blocks the pressure oil from piston (9). The angle of swashplate (6) increases. When the implement is stalled, the pressure in line (19) increases to the setting of spring (15). This causes spool (21) to move upward. The oil in the inlet passage now flows through passage (18) into piston (9). The flow of oil from passage (18) moves piston (9). Piston (9) moves swashplate (6) 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 (15), spool (21) moves downward. Spool (20) 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 (6) will decrease enough to supply oil to the remaining circuits that are not stalled.