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| Illustration 1 | g03375645 |
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Piston motor (1) Bias spring (2) Plate (3) Actuator piston (4) Port plate (5) Pump outlet (6) Pressure compensator valve (7) Pump head (8) Pump inlet (9) Cylinder barrel (10) Piston assembly (11) Swashplate (12) Drive shaft | |
When the drive shaft (12) is driven, the components that rotate are cylinder barrel (9), piston assemblies (10), and plate (2). There are nine piston assemblies in cylinder barrel (9).
Oil from the hydraulic tank flows into pump head (7) through pump inlet (8). The oil then flows from pump inlet (8) through inlet passages in valve plate (4). When drive shaft (12) rotates, the openings of cylinder barrel (9) move toward the inlet passages of valve plate (4).
Each piston assembly (10) inside cylinder barrel (9) is held against swashplate (11) by plate (2). Swashplate (11) can be at any angle between the maximum angle and the neutral angle.
The angle of swashplate (11) determines the amount of oil that is pushed out of each cylinder barrel (9). Swashplate (11) can be at any angle between the neutral angle and the maximum angle. The neutral angle is perpendicular with drive shaft (12). When swashplate (11) is at the neutral angle, piston assemblies (10) do not move in and out of rotating cylinder barrel (9). Therefore, no oil is drawn into the pump and no oil is pushed out of the pump. The pump has zero displacement and a small amount of flow for leakage.
When swashplate (11) is at the maximum angle, piston assemblies (10) move in and out of cylinder barrel (9). The movement of the piston assemblies (10) allows the maximum amount of oil to be drawn into the cylinder barrel (9). The pump will produce the maximum flow.
The swashplate angle is controlled by actuator piston (3). The movement of the actuator piston (3) is regulated by oil pressure from pressure compensator valve (6).
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| Illustration 2 | g03375905 |
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Pressure compensator valve (6) Pressure compensator valve (13) Cutoff spring (14) Case drain passage (15) Orifice valve (16) Signal passage to the actuator piston (17) Cutoff spool (18) Pump pressure passage | |
Pressure compensator valve (6) is mounted to the pump. Pressure compensator valve (6) contains one spool. Pressure cutoff spool (17) regulates the pump pressure output in response to pump pressure passage (18).
Pressure compensator valve (6) maintains the correct oil pressure. The pressure of the pump is maintained by sending pump oil to control piston (3) or draining pump oil from control piston (3).
Control piston (3) and bias spring (1) work together in order to adjust the angle of swashplate (11). Pressure compensator valve (6) prevents overloading of the pump.
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| Illustration 3 | g03375917 |
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Piston pump in destroke (1) Bias spring (3) Actuator piston (5) Pump outlet port (11) Swashplate (13) Cutoff spring (14) Case drain passage (15) Orifice valve (16) Signal pressure to the actuator piston (17) Cutoff spool | |
The hydraulic system pressure is sensed by the pressure cutoff spool through passage (5). As pressure increases in passage (5), cutoff spool (17) acts on cutoff spring (13) and increases signal pressure to actuator piston (3).
When the oil pressure behind actuator piston (3) is sufficient, the piston will start to move. The piston rod will push swashplate (11) against bias spring (1). Swashplate (11) will overcome bias spring (1). The angle of swashplate (11) starts to decrease and the piston pump will destroke.
When the piston pump has destroked to the desired pressure, the pump will provide a constant pressure.
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| Illustration 4 | g03375959 |
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Pump upstroke (1) Bias spring (3) Actuator piston (6) Pressure and flow compensator valve (11) Swashplate (14) Case drain passage (16) Signal passage (17) Pressure compensator spool | |
The hydraulic system pressure is sensed by the pressure cutoff spool through passage (5). As pressure decreases in passage (5), cutoff spring (13) acts on cutoff spool (17) and decreases signal pressure to actuator piston (3).
As the oil is metered out of the piston (3), the force of bias spring (1) overcomes the pressure behind piston (3). The angle of swashplate (11) begins to increase. As the angle of swashplate (11) increases, the piston pump will upstroke.
When the piston pump has upstroked to the desired flow, the pump will provide a constant flow. Refer to section "High Pressure Cutoff" in order to view the movement within pressure compensator valve (6) at constant pressure.
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| Illustration 5 | g03375980 |
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Pump at constant flow (1) Bias spring (3) Actuator piston (5) Pump outlet (11) Swashplate (15) Orifice valve (16) Signal passage | |
When a constant flow of hydraulic oil is demanded by the hydraulic system, the supply oil pressure from pump outlet (5) will be constant. The supply of oil will flow at a constant rate until the pressure at outlet port (5) changes.
When equal pressures act on the ends of flow cutoff spool (17), spool (17) will meter oil to actuator piston (3) through signal passage (16). The force behind actuator piston (3) will equalize with the force of bias spring (1). Swashplate (11) will be held at a relative constant angle in order to maintain the required oil flow. The flow of hydraulic oil to the system will stabilize.
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| Illustration 6 | g03375989 |
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Pump at High Pressure Cutoff (1) Bias spring (3) Actuator piston (5) Pump pressure passage (11) Swashplate (13) cutoff spring (15) Orifice valve (16) Signal passage (17) Pressure compensator spool | |
High-pressure cutoff in a piston pump limits the maximum pressure at any pump displacement. During normal operation of a piston pump, pressure compensator spool (17) is forced down by cutoff spring (13).
As the pressure in the pump pressure passage increases, the oil flows through spool control orifice. The oil that flows through spool control orifice goes to cutoff spool (17). As the oil pressure increases, the force behind cutoff spool (17) also increases. As the force behind cutoff spool (17) increases, cutoff spool (17) moves against the force of cutoff spring (13). Pressure behind the cutoff spool becomes greater than the force of cutoff spring (13). Cutoff spool (17) overrides spring (13) and the spool moves up. When the pressure overrides cutoff spring (13), cutoff spool (17) will move up. As cutoff spool (17) moves up, oil flows around spool (17). The oil will flow through pilot passage (16). This oil will extend piston (3). The angle of swashplate (11) will be decreased. The swashplate angle will cause the pump to maintain the cutoff pressure.