 | |
| Illustration 1 | g00471618 |
(1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (8) Drive shaft (9) Swashplate (10) Cylinder barrel assembly (11) Pump inlet (12) Pump outlet (LL) Sump oil | |
The implement hydraulic pump (1) has the following characteristics: variable displacement, load sensing, compensation for pressure and compensation for flow. This piston type pump has variable flow and pressure. The flow and pressure are dependent on the system demands that are sensed by the compensator valve (3). The implement hydraulic pump (1) is mounted to the rear of the pump drive. The pump drive is located on the right side of the machine.
When the engine is OFF, there is no load sensing signal (4) to the pressure and flow compensator valve (3). The margin spring (2) pushes the flow compensator spool (5) completely to the right. Any pressure that is behind the actuator piston (6) is vented to the case drain across the flow compensator spool (5) .
When there is no pressure behind the actuator piston (6), the bias spring (7) is able to hold swashplate (9) at the maximum angle.
When the engine is started, drive shaft (8) starts to rotate. Oil flows into the piston bore from the pump inlet (11). Oil is forced out of the pump outlet (12) and into the system as the cylinder barrel assembly (10) rotates.
Low Pressure Standby
 | |
| Illustration 2 | g00471746 |
Operation of the pump and pressure and flow compensator valve (1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (9) Swashplate (12) Pump outlet (AA) Supply oil (BB) Reduced supply oil (EE) Oil from the large section of the tandem charge pump (LL) Sump oil | |
When implement hydraulic pump (1) produces flow, the system pressure begins to increase because the oil flow is blocked at the closed center implement control valves.
The blocked oil creates pressure which is felt at the end of flow compensator spool (5). The flow compensator spool moves left against margin spring (2). This permits system oil to flow to actuator piston (6) .
When the pressure on actuator piston (6) increases, the force of bias spring (7) is overcome and swashplate (9) is moved to a slight angle.
The implement hydraulic pump (1) produces enough flow in order to compensate for normal system leakage when swashplate (9) is at a slight angle. Also, the implement hydraulic pump has sufficient pressure in order to provide instantaneous response when an implement is activated.
If there is no flow demand from an implement hydraulic circuit, no load sensing signal (4) is generated.
The pressure at the pump outlet must only overcome the force of the margin spring (2). Pressure at the pump outlet is called "low pressure standby". Low pressure standby is approximately 2070 kPa (300 psi).
Low pressure standby is higher than margin pressure. This is due to a higher back pressure that is created by the oil which is blocked at the closed center valves when the valves are in the HOLD position.
When implement hydraulic pump (1) is at low pressure standby, the supply oil from pump outlet (12) pushes flow compensator spool (5) to the left. This will further compress margin spring (2). An increased amount of supply oil from the pump outlet (12) will flow to actuator piston (6). This will slightly destroke implement hydraulic pump (1) .
Note: The amount of system leakage and the adjustments to flow compensator spool (5) can cause the margin pressure to equal the low pressure standby. Margin pressure can never be higher than low pressure standby.
Low pressure standby is not adjustable. Low pressure standby can vary from one machine to another machine. Also, low pressure standby can vary in the same pump as system leakage increases.
Upstroking
 | |
| Illustration 3 | g00472061 |
Operation of the pump and pressure and flow compensator valve (1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (9) Swashplate (12) Pump outlet (AA) Supply oil (DD) Pilot signal oil (EE) Oil from the large section of the tandem charge pump (LL) Sump oil | |
When an implement hydraulic circuit requires flow, the pressure from pump outlet (12) is reduced. The reduced pump output pressure causes the force of margin spring (2) and load sensing signal (4) to increase. This force is on the left side of flow compensator spool (5). The force is greater than the pump supply pressure at the right of the spool.
The spool moves to the right which blocks the flow of oil to actuator piston (6). Oil that is in the chamber for actuator piston (6) is vented to the case drain across flow compensator spool (5). This allows bias spring (7) to move swashplate (9) to a greater angle.
The pump now produces more flow. This condition is known as "upstroking".
The following conditions can result in upstroking the pump:
- If an implement hydraulic circuit is initially activated from low pressure standby, the load sensing signal increases the pump output flow. This increased pump output flow is routed to the position of a main control spool of an implement control valve that demands the oil.
- If a main control spool in an implement control valve is in a given position, the pump will slightly upstroke in order to compensate for system leakage .
- The implement hydraulic pump will upstroke when the demand increases from changing the position of the main control spool in an implement control valve.
- If another implement hydraulic circuit is engaged, there is a need for increased pump flow.
- If the demand on the implement hydraulic system remains constant or the demand increases, the implement hydraulic pump will upstroke when the engine speed decreases.
Note: The pressure of the load sensing signal does not need to increase in order for the implement hydraulic pump to upstroke.
For example, if one implement hydraulic circuit is activated at an operating pressure of 13800 kPa (2000 psi), the system pressure is 15630 kPa (2265 psi). The pressure of 15630 kPa (2265 psi) is a combination of the margin pressure and the pressure of the signal oil.
If another implement hydraulic circuit is activated at an initial operating pressure of 6900 kPa (1000 psi), the maximum pressure of the load sensing signal will still be 13800 kPa (2000 psi). The supply oil will decrease momentarily in order to provide more flow that is now needed at the additional circuit.
The force at the left of flow compensator spool (5) is now higher than the force at the right of the spool. Flow compensator spool (5) is pushed to the right. This allows oil that is behind actuator piston (6) to be vented to the case drain. The angle of swashplate (9) now increases and the pump provides more flow.
Constant Flow
 | |
| Illustration 4 | g00472660 |
Operation of the pump and pressure and flow compensator valve (1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (9) Swashplate (12) Pump outlet (13) Adjustment for the flow compensator spool (AA) Supply oil (BB) Reduced supply oil (DD) Pilot signal oil (EE) Oil from the large section of the tandem charge pump (LL) Sump oil | |
When a constant flow of oil is demanded by a implement hydraulic circuit, the supply oil pressure from pump outlet (12) will increase on the right side of flow compensator spool (5). The pressure on left side of flow compensator spool (5) is the sum of the following pressures: load sensing signal (4) and margin spring (2). Flow compensator spool (5) will meter the force on both ends of the spool. Once the forces become equal on each end of the spool the system will stabilize.
Swashplate (9) is held at a relative constant angle in order to maintain the required flow.
The difference between the pressure of load sensing signal (4) and the supply oil pressure is called margin pressure. Margin pressure is the value of margin spring (2) .
Margin pressure is 1830 ± 100 kPa (265 ± 15 psi). Margin pressure is adjusted by turning adjustment screw (13) on flow compensator spool (5) .
Destroking
 | |
| Illustration 5 | g00473473 |
Operation of the pump and pressure and flow compensator valve (1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (9) Swashplate (12) Pump outlet (AA) Supply oil (BB) Reduced supply oil (DD) Pilot signal oil (EE) Oil from the large section of the tandem charge pump (LL) Sump oil | |
When less flow is required, implement hydraulic pump (1) destrokes. Implement hydraulic pump (1) destrokes when the force at the right of flow compensator spool (5) becomes greater than the force at the left.
Flow compensator spool (5) moves to the left which allows more oil flow to actuator piston (6). Pressure on actuator piston (6) is now increased.
The increased pressure overcomes the force of bias spring (7) which moves swashplate (9) to a reduced angle. When the pressure from pump outlet (12) matches the force at the left of flow compensator spool (5), the spool returns to a metering position. Implement hydraulic pump (1) will return to a constant flow.
The following conditions result in destroking the implement hydraulic pump:
- When a main control spool for an implement control valve is moved to the HOLD position the implement hydraulic pump will destroke.
- If the main control spool for an implement control valve is moved to a position that requires less flow the implement hydraulic pump will destroke.
- If multiple implement control valves are being used, the implement hydraulic pump will destroke when there is a reduction in demand from any one of the implement control valves.
- If there is a slight reduction in the highest operating pressure, or there is a reduction in the built-in system leakage.
- If the engine speed increases, the pump destrokes.
When implement hydraulic pump (1) destrokes, supply oil pressure from pump outlet (12) decreases on the right side of flow compensator spool (5). The pressure on left side of flow compensator spool (5) is the sum of the following pressures: load sensing signal (4) and margin spring (2). Flow compensator spool (5) will meter the force on both ends of the spool. Once the forces become equal on each end of the spool the system will stabilize.
Note: The pressure of the load sensing signal (4) does not need to increase in order for the implement hydraulic pump to destroke.
For example, if two implement hydraulic circuits are activated at operating pressures of 13800 kPa (2000 psi) and 6900 kPa (1000 psi), the system pressure is 15630 kPa (2265 psi).
If the implement hydraulic circuit which is activated at 6900 kPa (1000 psi) is returned to the HOLD position, the maximum pressure of load sensing signal (4) will still be 13800 kPa (2000 psi). However, the pressure of the supply oil is momentarily increased due to the reduced oil flow that is required in the implement hydraulic circuits.
The supply oil pressure from pump outlet (12) moves flow compensator spool (5) to the left which allows more oil flow behind actuator piston (6). The angle of swashplate (9) now decreases and the implement hydraulic pump provides less flow.
High Pressure Stall
 | |
| Illustration 6 | g00604349 |
Operation of the pump and pressure and flow compensator valve (1) Implement hydraulic pump (2) Margin spring (3) Pressure and flow compensator valve (4) Passage from the implement control valves for the load sensing signal (5) Flow compensator spool (6) Actuator piston (7) Bias spring (9) Swashplate (12) Pump outlet (13) Adjustment for the flow compensator spool (14) Pressure compensator spool (15) Cutoff spring (16) Adjustment screw for the pressure compensator spool (AA) Supply oil (BB) Reduced supply oil (DD) Pilot signal oil (EE) Oil from the large section of the tandem charge pump (LL) Sump oil | |
Pressure compensator spool (14) is in parallel with flow compensator spool (5). Pressure compensator spool (14) limits the maximum system pressure at any pump displacement.
Pressure compensator spool (14) is forced to the right during normal operation by cutoff spring (15) .
If the implement hydraulic pump is at a high pressure stall or maximum system pressure, the pressure of load sensing signal (4) is equal to pressure of the supply oil from pump outlet (12) .
The combined pressure of load sensing signal (4) and margin spring (2) forces flow compensator spool (5) to the right. When flow compensator spool (5) moves to the right, a passage will normally be opened. This allows the oil that is holding actuator piston (6) to be vented to the case drain. This will cause the implement hydraulic pump to upstroke.
If the pressure of the supply oil from pump outlet (12) is high enough, pressure compensator spool (14) is forced left against cutoff spring (15) .
The movement of pressure compensator spool (14) blocks the oil from actuator piston (6) from flowing to the case drain. This allows the supply oil to flow to actuator piston (6) .
The increase in pressure on actuator piston (6) overcomes the force of bias spring (7). Swashplate (9) moves and the implement hydraulic pump destrokes.
The implement hydraulic pump is now at minimum flow and the supply oil at the pump outlet pressure is at maximum pressure. These conditions are maintained for a single implement in a stall condition. During these conditions, the need for a main system relief valve in the implement hydraulic system is eliminated.
Maximum system pressure is 18950 ± 345 kPa (2750 ± 50 psi). Maximum system pressure is adjusted by turning adjustment screw (16) for pressure compensator spool (14).
If multiple implement hydraulic circuits are activated, implement hydraulic pump (1) still produces flow while one of the circuits is at a high pressure stall. This flow meets the needs of the other circuits that are operating at a lower work port pressure. Implement hydraulic pump (1) could be producing maximum flow while the pressure of the supply oil from pump outlet (12) is at the maximum.