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| Illustration 1 | g03713672 |
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Engine Off (1) Cylinder barrel assembly (2) Hydraulic pump (3) Case drain passage (4) Case drain filter (5) Pressure and flow compensator valve (6) Margin spring (7) Load sensing signal (8) Flow compensator spool (9) Load sensing relief valve (10) Signal passage to actuator piston (11) Actuator piston (12) Swashplate (13) Bias spring (14) Drive shaft (15) Pump inlet (16) Pump outlet | |
Hydraulic pump (2) has the following characteristics:
- variable displacement
- load sensing
- compensation for pressure
- compensation for flow
This piston pump has variable flow and pressure. The flow and pressure are dependent on the system demands that are sensed by pressure and flow compensator valve (8).
The hydraulic pump has the following components:
Cylinder barrel assembly (1) - The cylinder barrel contains nine pistons. The cylinder barrel assembly rotates whenever the engine is running. The pistons move oil into the barrel and out of the barrel.
Drive shaft (14) - The rotation of the pump is counterclockwise when the pump is viewed from the drive end. The cylinder barrel assembly is splined to the drive shaft.
Bias spring (13) - If there is no pressure behind the actuator piston, the bias spring will hold the swashplate at the maximum angle.
Swashplate (12) - The displacement of the pump is controlled by the angle of the swashplate. Angling of the swashplate causes the pistons to move in and out of the rotating barrel.
Actuator piston (11) - When oil pressure increases behind the actuator piston, the piston will overcome the force of the bias spring. This causes the angle of the swashplate to be reduced.
Pressure and flow compensator valve (5) - The pressure and flow compensator valve controls the delivery of oil and the return of oil to the actuator piston.
When, the engine is OFF, pressure and flow compensator valve (5) does not receive load sensing signal (7). Margin spring (6) pushes flow compensator spool (8) to the bottom. Any pressure that is behind actuator piston (11) is vented to the case drain across flow compensator spool (8).
When there is no pressure behind actuator piston (11), bias spring (13) is able to hold swashplate (12) at the maximum angle.
When the engine is started, drive shaft (14) starts to rotate. Oil flows into the piston bore from pump inlet (15). Oil is forced out of pump outlet (16) and into the system as cylinder barrel assembly (1) rotates.
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| Illustration 2 | g03713676 |
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Engine off (5) Pressure and flow compensator valve (6) Margin spring (7) Load sensing signal (8) Flow compensator spool (10) Signal passage to actuator piston (17) Adjustment screw (18) Orifice (19) Pressure compensator spool | |
Pressure and flow compensator valve (5) is bolted to the hydraulic pump. Pressure and flow compensator valve (5) contains two spools. Flow compensator spool (8) regulates the pump output flow in response to load sensing signal (7).
Load sensing signal (7) is sent from the Proportional Priority Pressure Compensated System (PPPC). This signal is the highest signal that is commanded by any of the control valves. The signal is not the sum of the signals that are commanded by all of the control valves. This signal represents the single greatest load that is being placed on the hydraulic system.
The flow that is supplied by the hydraulic pump is the amount that is required to keep the supply pressure above the pressure of load sensing signal (7). This difference is called the margin pressure. Flow compensator spool (8) controls the margin pressure.
Margin pressure is equal to the spring force value of margin spring (6).
Margin pressure is adjusted by turning adjustment screw (17) on flow compensator spool (8). See the Testing and Adjusting section in this manual for the correct procedure.
Pressure compensator spool (19) limits the maximum system pressure.
Pressure and flow compensator valve (5) has orifice (18) in the signal passage to actuator piston (10). Orifice (18) is used to regulate the response rate of the actuator piston by creating a consistent leak path.
Note: Orifice (18) must be installed with the slot for the screwdriver parallel to the spool bores.
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| Illustration 3 | g03713682 |
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(2) Hydraulic pump
(6) Margin spring (7) Load sensing signal (8) Flow compensator spool (11) Actuator piston (12) Swashplate (13) Bias spring (16) Pump outlet | |
Low-pressure standby occurs when the engine is running and the control levers are in HOLD. There are no flow demands or pressure demands on the system. Bias spring (13) holds swashplate (12) at the maximum angle before the engine is started. The pressure in pump outlet (16) is felt at the bottoms of both spools. As this pressure increases, flow compensator spool (8) is pushed against margin spring (6).
When the system pressure becomes greater than margin pressure, flow compensator spool (8) will move up. Pressure oil will get to the back of actuator piston (11). This causes the actuator piston to move to the left. Bias spring (13) is compressed and swashplate (12) moves toward the minimum angle until equilibrium is reached.
The pump is at low-pressure standby. The flow compensator spool must remain open. This will provide oil to the back side of the actuator piston. The flow must be enough to maintain the pressure required at the back of the piston to overcome the bias spring.
Note: Low-pressure standby is a function of margin pressure. The standby flow is not adjustable but the pressure is adjustable. Low-pressure standby is equal to the margin pressure and the load sensing boost pressure. Low-pressure standby will also vary in the same pump as leakage increases. As leakage increases, the pump will upstroke slightly to compensate for the leakage.
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| Illustration 4 | g03713683 |
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(6) Margin spring
(7) Load sensing signal (8) Flow compensator spool (11) Actuator piston (12) Swashplate (13) Bias spring (16) Pump outlet | |
When an implement hydraulic circuit requires flow, the pressure from pump outlet (16) is reduced. The reduced pump output pressure causes the force of margin spring (6) and load sensing signal (7) to be greater than pump outlet pressure (16). This force pushes flow compensator spool (8) downward.
The spool moves to the bottom which blocks the flow of oil to actuator piston (11). Oil that is in the chamber for actuator piston (11) is vented to the case drain across flow compensator spool (8). This allows bias spring (13) to move swashplate (12) 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.
- If a main control spool in a hydraulic control valve is in a given position, the pump will slightly upstroke to compensate for system leakage.
- The hydraulic pump will upstroke when the demand increases from changing the position of the main control spool in a hydraulic 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 hydraulic pump will upstroke when the engine speed decreases.
Note: The load sensing signal pressure does not need to increase to upstroke the hydraulic pump.
For example, if one implement hydraulic circuit is activated at an operating pressure of
If another implement hydraulic circuit is activated at an initial operating pressure of
The pressure of load sensing signal (7) plus the force of margin spring (6) is now higher than the output pressure of the pump at the bottom end of the spool. Flow compensator spool (8) is pushed to the bottom. This allows oil that is behind actuator piston (11) to be vented to the case drain. The angle of swashplate (12) now increases and the hydraulic pump provides more flow.
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| Illustration 5 | g03713685 |
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(6) Margin spring
(7) Load sensing signal (8) Flow compensator spool (11) Actuator piston (12) Swashplate (16) Pump outlet | |
When a constant flow of oil is demanded by an implement hydraulic circuit, the supply oil pressure from pump outlet (16) will increase at flow compensator spool (8).
Force from margin spring (6) and pressure from load sensing signal (7) will act on the top end of flow compensator spool (8).
Pump outlet pressure (16) will act on the bottom of flow compensator spool (8).
Once the forces become equal on each end of the spool, flow compensator spool (8) will meter oil to actuator piston (11) and the system will stabilize.
Swashplate (12) is held at a relative constant angle to maintain the required flow.
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| Illustration 6 | g03713688 |
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(2) Hydraulic pump
(6) Margin spring (7) Load sensing signal (8) Flow compensator spool (11) Actuator piston (12) Swashplate (13) Bias spring (16) Pump outlet | |
When less flow is required, hydraulic pump (2) destrokes. Hydraulic pump (2) destrokes when the pump outlet pressure (16) becomes greater than the pressure of load sensing signal (7) and margin spring (6).
Flow compensator spool (8) moves toward the top which allows more oil flow to actuator piston (11). Pressure on actuator piston (11) is now increased.
The increased pressure overcomes the force of bias spring (13) which moves swashplate (12) to a reduced angle. When the pump outlet pressure (16) matches the combined pressure of load sensing signal (7) and margin spring (6), the flow compensator spool returns to a metering position. Hydraulic pump (2) will return to a constant flow.
The following conditions result in destroking the hydraulic pump:
- When a main control spool for a hydraulic control valve is moved to the HOLD position, the hydraulic pump will destroke.
- If the main control spool for a hydraulic control valve is moved to a position that requires less flow, the hydraulic pump will destroke.
- If multiple hydraulic control valves are being used, the hydraulic pump will destroke when there is a reduction in demand from any one of the hydraulic control valves.
- If there is a slight reduction in the highest operating pressure, or there is a reduction in the built-in system leakage, the hydraulic pump will destroke.
- If the engine speed is increased, the hydraulic pump destrokes.
The force on the top of flow compensator spool (8) is the sum of margin spring (6) and load sensing signal (7).
The force on the bottom of flow compensator spool (8) is pump outlet pressure (16).
Once the pressures become equal on each end of the spool, flow compensator spool (8) will meter oil to actuator piston (11) and the system will stabilize.
Note: Load sensing signal pressure (7) does not need to decrease to destroke the hydraulic pump.
For example, if two implement hydraulic circuits are activated at operating pressures of
If the implement hydraulic circuit which is activated at
Pump outlet pressure (16) moves flow compensator spool (8) to the top which allows more oil flow behind actuator piston (11). The angle of swashplate (12) now decreases and the hydraulic pump provides less flow.
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| Illustration 7 | g03713692 |
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(5) Pressure and flow compensator valve
(6) Margin spring (7) Load sensing signal (8) Flow compensator spool (9) Load sensing relief valve (11) Actuator piston (12) Swashplate (13) Bias spring (16) Pump outlet (19) Pressure compensator spool (20) Cutoff spring | |
Note: This condition will only occur when the load sensing relief valve is set below the value of the high-pressure cutoff.
If piston pump (2) is at a high-pressure stall or maximum system pressure, load sensing signal (7) and margin spring (6) are equal to the supply pressure at pump outlet (16).
Load sensing relief valve (9) limits the maximum system pressure at any pump displacement.
If the load sensing relief valve (9) is not adjusted correctly, pressure compensator spool (19) serves as a backup relief to protect the hydraulic system. The pressure compensator spool (19) also controls pressure spikes within the hydraulic system.
At the high-pressure stall, the piston pump is at minimum flow and the supply oil at pump outlet (16) is at maximum pressure. These conditions are maintained for a single implement in a stall condition.
If multiple implement hydraulic circuits are activated and one circuit is at a stall, the piston pump (2) will upstroke to meet the increased flow demands. This flow meets the needs of the other circuits that are operating at a lower work port pressure.