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| Illustration 1 | g01105912 |
Piston Pump (1) Drive shaft (2) Swashplate (3) Piston assembly (4) Cylinder barrel (5) Pump inlet (6) Pump head (7) Bias spring (8) Slipper (9) Actuator piston (10) Valve plate (11) Pump pressure passage (12) Pump control valve | |
The brake and the hydraulic fan pump is a variable displacement piston pump. The axial piston type pump is used to supply oil flow to the hydraulic fan motor. The movement of piston assembly (3) in the pump creates an area of low pressure. Atmospheric pressures will push oil from the hydraulic tank into the pump. The oil is pumped to the fan motor in order to rotate the fan.
When the engine is in operation and drive shaft (1) is driven by the gears that are in the transmission, the components that rotate are cylinder barrel (4), piston assemblies (3), and slipper (8). There are nine piston assemblies in cylinder barrel (4) .
Oil from the hydraulic tank flows into pump head (6) through pump inlet (5). The oil then flows from pump inlet (5) through inlet passages in valve plate (10). When drive shaft (1) rotates, the openings of cylinder barrel (4) move toward the inlet passages of valve plate (10) .
Each piston assembly (3) inside cylinder barrel (4) is held against swashplate (2) by slipper (8). Swashplate (2) can be at any angle between the maximum angle and the neutral angle.
The angle of swashplate (2) determines the amount of oil that is pushed out of each cylinder barrel (4). Swashplate (2) can be at any angle between the neutral angle and the maximum angle. The neutral angle is perpendicular with drive shaft (1). When swashplate (2) is at the neutral angle, piston assemblies (3) do not move in and out of rotating cylinder barrel (4). Therefore, no oil is drawn into the brake and the hydraulic fan pump and no oil is pushed out of the pump. The brake and the hydraulic fan pump has zero displacement and zero flow.
When swashplate (2) is at the maximum angle, piston assemblies (3) move in and out of cylinder barrel (4). The movement of piston assemblies (3) will allow the maximum amount of oil to be drawn in to cylinder barrel (4). The pump will produce the maximum displacement.
The swashplate angle is controlled by actuator piston (9). The movement of the actuator piston (9) is regulated by oil pressure from pump control valve (12).
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| Illustration 2 | g01332901 |
Pressure and Flow Compensator Valve (11) Pump pressure passage (12) Pump control valve (13) Cutoff valve adjustment screw (14) Cutoff spring (15) Cutoff spool (16) Margin spool adjustment (17) Margin spring (18) Load sensing port (19) Margin spool (20) Case drain passage (21) Signal passage to the actuator piston | |
Pump control valve (12) is mounted to the brake and the hydraulic fan pump. Pump control valve (12) contains two spools. Margin spool (19) regulates the pump output flow in response to load sensing port (18). Cutoff spool (15) protects the pump for high pressures.
Pump control valve (12) maintains the correct pressure and correct oil flow. The pump is controlled by either sending pump oil to actuator piston (9) or by draining the pump oil from actuator piston (9) .
Actuator piston (9) and bias spring (7) work together in order to adjust the angle of swashplate (2). Pump control valve (12) prevents overloading of the pump.
Pump Destroke
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| Illustration 3 | g01332919 |
Piston Pump in Destroke (2) Swashplate (7) Bias spring (9) Actuator piston (11) Pump pressure passage (15) Cutoff spool (17) Margin spring (18) Load sensing port (19) Margin spool (20) Case drain passage (21) Signal passage to the actuator piston | |
When the hydraulic system requires a reduction in flow, a reduced signal is sent to load sensing port (18).
High pressure oil flows to margin spool (19) through pump pressure passage (11). When the pressure against spool (19) overcomes the force of both signal (18) and margin spring (17), spool (19) will begin to move.
The oil will flow around the spool (19). Oil will flow to actuator piston (9) .
When the oil pressure behind actuator piston (9) is sufficient, the piston will start to move. The piston rod will push swashplate (2) against bias spring (7). Swashplate (2) will overcome bias spring (7). The swashplate (2) will move to a minimal angle. The piston pump will be destroked.
Pump Upstroke
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| Illustration 4 | g01332923 |
Pump Upstroke (2) Swashplate (7) Bias spring (9) Actuator piston (11) Pump pressure passage (15) Cutoff spool (17) Margin spring (18) Load sensing port (19) Margin spool (20) Case drain passage (21) Signal passage to actuator piston | |
When the hydraulic system requires an increase in oil flow, a signal is sent to load sensing port (18). The pressure at the load sensing port (18) is increased. The combination of the force of margin spring (17) and the increase of pressure at load sensing port (18) will shift spool (19) downward. This will allow the oil that is behind piston (9) to flow through signal passage (21). The oil will flow around spool (15). The oil will flow around the upper section of spool (19). The oil will flow through passage (20) to the case drain.
As the oil is metered out of the piston (9), the force of bias spring (7) overcomes the pressure behind piston (9). The angle of swashplate (2) begins to increase. As the angle of swashplate (2) increases, the piston pump will upstroke.
Constant Flow
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| Illustration 5 | g01332925 |
Pump at Constant Flow Pump Upstroke (2) Swashplate (7) Bias spring (9) Actuator piston (11) Pump pressure passage (15) Cutoff spool (17) Margin spring (18) Load sensing port (19) Margin spool (20) Case drain passage (21) Signal passage to actuator piston | |
The force from margin spring (17) and pressure from load sensing port (18) act on the end of spool (19) .
The pump pressure in passage (11) will act on the opposite end of spool (19) .
When the pressures that are acting on the ends of margin spool (19) equalize, margin spool (19) will meter oil to actuator piston (9) through signal passage (21). The force behind actuator piston (9) will equalize with the force of bias spring (7). Swashplate (2) 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.
When a constant flow of hydraulic oil is demanded by the hydraulic system, the supply oil pressure in the pump pressure passage (11) will be constant. The supply of oil will flow at a constant rate until the pressure at load sensing port (18) changes.
High Pressure Cutoff
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| Illustration 6 | g01332927 |
Pump at High Pressure Cutoff Pump Upstroke (2) Swashplate (7) Bias spring (9) Actuator piston (11) Pump pressure passage (14) Cutoff spring (15) Cutoff spool (17) Margin spring (18) Load sensing port (19) Margin spool (20) Case drain passage (21) Signal passage to actuator piston | |
High pressure cutoff in a piston pump limits the maximum pressure at any pump displacement. During normal operation of a piston pump, cutoff spool (15) is forced down by spring (14) .
As the pressure in passage (11) increases, oil forces cutoff spool (15) against spring (14). As the pressure that is acting on spool (15) becomes greater than the force of spring (14), spool (15) overrides spring (14) and the spool moves up. As cutoff spool (15) moves up, oil flows around spool (15). The oil will flow through pilot passage (21). This will extend piston (9). The angle of swashplate (2) will be decreased. This will cause the pump to maintain the cutoff pressure.