Piston Pump
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| Illustration 1 | g01022614 |
Steering and Implement Pump (Engine Off) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
Piston pump (1) has the following characteristics:
- variable displacement
- load sensing
- compensation for pressure
- 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 pressure and flow compensator valve (2) .
Note: Load sensing signal pressure is sometimes referred to as signal oil. The two terms have the same meaning throughout this story.
The piston pump has the following components:
Piston and barrel assembly (3) - 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.
Shaft (4) - The rotation of the pump is counterclockwise when the pump is viewed from the drive end. The piston and barrel assembly is splined to the drive shaft.
Bias spring (5) - If there is no pressure on the right side of the actuator piston, the bias spring will hold the swashplate at the maximum angle.
Swashplate (6) - The displacement of the pump is controlled by the angle of the swashplate. The angle of the swashplate causes the pistons to move in and out of the rotating barrel.
Actuator piston (7) - 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 (2) - 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 (2) does not receive load sensing signal pressure (8) or supply pressure from pump outlet (9). Margin spring (10) pushes flow compensator spool (11) completely downward. Any pressure that is on the right side of actuator piston (7) is vented to the case drain across flow compensator spool (11) .
When there is no pressure behind actuator piston (7), bias spring (5) is able to hold swashplate (6) at the maximum angle.
When the engine is started, shaft (4) starts to rotate. Oil flows into the piston bore from pump inlet (12). Oil is forced out of pump outlet (9) and into the system as the piston and barrel assembly (3) rotates.
Compensator Valve
Pressure and flow compensator valve (2) is bolted to the top of the piston pump. Pressure and flow compensator valve (2) compensates for pressure variations and flow variations in the implement hydraulic system in order to meet the system demands.
Flow compensator spool (11) regulates the pump output flow in response to the following oil pressures:
- Load sensing signal pressure (8)
- Supply pressure from pump outlet (9)
The load sensing signal is received through the load sensing passage for the pump.
The flow that is supplied by the hydraulic pump is the amount that is required in order to keep the supply pressure 2200 ± 100 kPa (320 ± 15 psi) above the load sensing signal pressure (8). The difference between load sensing signal pressure (8) from the main hydraulic control valve and the supply pressure from pump outlet (9) is called the margin pressure. Flow compensator spool (11) controls the margin pressure.
Margin pressure is equal to the spring force value of margin spring (10) .
The margin pressure is adjusted by turning adjusting screw (13) on flow compensator spool (11) .
Note: The maximum system pressure is controlled by the relief valve (load sensing signal) and the margin pressure. The relief valve (load sensing signal) is set at 22800 kPa (3300 psi) and the margin pressure is set at 2200 kPa (320 psi). The maximum system pressure is 25000 ± 500 kPa (3625 ± 75 psi).
Pressure compensator spool (14) is used as a backup to limit the maximum system pressure to 26000 ± 500 kPa (3770 ± 73 psi) if the load sensing signal relief valve fails.
Pressure and flow compensator valve (2) has orifice (15) in the load sensing signal passage to actuator piston (16). Orifice (15) is used in order to regulate the response rate of the actuator piston by creating a consistent leak path.
Note: If the relief valve (load sensing signal) is operating correctly, the relief valve (load sensing signal) controls the maximum system pressure. Maximum system pressure should be the pressure of the relief valve (load sensing signal) plus the margin pressure. If the relief valve (load sensing signal) is not adjusted correctly the pressure compensator acts as a backup.
Low Pressure Standby
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| Illustration 2 | g01022627 |
Steering and Implement Pump (Low pressure Standby) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
When piston pump (1) produces flow, the system pressure begins to increase because the oil flow from pump outlet (9) is blocked at the closed center implement control valves.
The supply pressure from pump outlet (9) is felt at the bottom end of flow compensator spool (11). The supply pressure from pump outlet (9) is greater than the combined pressure of load sensing signal pressure (8) and margin spring (10). The flow compensator spool moves upward against the margin spring. This permits system oil to flow to actuator piston (7) .
When the pressure on actuator piston (7) increases, the force of bias spring (5) is overcome and swashplate (6) is moved to a slight angle.
Piston pump (1) produces enough flow in order to compensate for normal system leakage when swashplate (6) is at a slight angle. Also, the piston pump has sufficient pressure in order to provide instantaneous response when an implement is activated.
Even when no implements are being used, the steering system maintains a load sensing oil to the pump.
The pressure at pump outlet (9) must overcome the force of margin spring (10) and the load sensing signal pressure (8). Pressure at pump outlet (9) is called "low pressure standby". Low pressure standby is approximately 4500 kPa (650 psi).
The low pressure standby is higher than the margin pressure. The higher pressure is needed to maintain the pilot pressure.
When piston pump (1) is at low pressure standby, the supply pressure from pump outlet (9) raises flow compensator spool (11) higher. This will further compress margin spring (10). An increased amount of supply oil from pump outlet (9) will flow to actuator piston (7). This will slightly destroke the hydraulic pump.
The low pressure standby can be adjusted by changing the setting of the margin spring.
Upstroke
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| Illustration 3 | g01022648 |
Steering and Implement Pump (Upstroke) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
When an implement hydraulic circuit requires flow, the pressure from pump outlet (9) is reduced. The force of margin spring (10) and load sensing signal pressure (8) is greater than supply pressure from pump outlet (9). The overall force is on the top end of flow compensator spool (11) .
The spool moves down. The spool blocks the flow of oil to actuator piston (7). Oil that is in the chamber for actuator piston (7) is vented to the case drain across flow compensator spool (11). This allows bias spring (5) to move swashplate (6) 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 a hydraulic control valve that demands the oil.
- The hydraulic pump will upstroke when the flow 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 flow demand on the implement hydraulic system remains constant or the flow demand increases, the hydraulic pump will upstroke when the engine speed decreases.
Note: The load sensing signal pressure does not need to increase in order to upstroke the hydraulic pump.
For example, if one implement hydraulic circuit is activated at an operating pressure of 13800 kPa (2000 psi), the system pressure is 16000 kPa (2320 psi). The pressure of 16000 kPa (2320 psi) is a combination of the margin pressure and the pressure of the load sensing oil 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 oil will still be 13800 kPa (2000 psi). The supply oil pressure will decrease momentarily.
The combined force of the load sensing signal pressure (8) and margin spring (10) is now higher than the supply pressure from pump outlet (9) at the bottom end of the spool. Flow compensator spool (11) is pushed to the bottom. This allows oil that is behind actuator piston (7) to be vented to the case drain. The angle of swashplate (6) now increases and the hydraulic pump provides more flow in order to meet the flow demands of both circuits.
Constant Flow
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| Illustration 4 | g01022658 |
Steering and Implement Pump (Constant Flow) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
As the pump increases flow or decreases flow in order to match the change in the flow requirements, the forces above the flow compensator spool and below the flow compensator spool will equalize.
The following pressures will act on the top end of flow compensator spool (11) :
- Margin spring (10)
- Load sensing signal pressure (8)
The following pressure will act on the bottom end of flow compensator spool (11) :
- Supply pressure from pump outlet (9)
Once the pressures become equal on each end of the spool, flow compensator spool (11) will meter oil to actuator piston (7). Bias spring (5) will be compressed and the system will stabilize.
Swashplate (6) is held at a relative constant angle in order to maintain the required flow.
Destroke
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| Illustration 5 | g01022669 |
Steering and Implement Pump (Destroke) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
When less flow is required, piston pump (1) destrokes. The piston pump destrokes when the supply pressure from pump outlet (9) becomes greater than the combined pressure of load sensing signal pressure (8) and margin spring (10) .
Flow compensator spool (11) is raised. This allows more oil flow to actuator piston (7). Pressure on actuator piston (7) is now increased.
The increased pressure overcomes the force of bias spring (5) which moves swashplate (6) to a reduced angle. When the supply pressure at pump outlet (9) matches the combined pressure of load sensing signal pressure (8) and margin spring (10), the flow compensator spool returns to a metering position. Piston pump (1) will return to a constant flow.
The following conditions result in destroking the 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 flow demand from any one of the hydraulic control valves.
- If the engine speed increases, the piston pump destrokes.
When the piston pump destrokes, supply oil pressure from pump outlet (9) decreases on the bottom side of flow compensator spool (11) .
The force on the top of flow compensator spool (11) is the sum of the following pressures:
- Margin spring (10)
- Load sensing signal pressure (8)
The following force acts on the bottom of flow compensator spool (11) :
- Supply pressure at pump outlet (9)
Once the forces become equal on each end of the spool, flow compensator spool (11) will meter oil to actuator piston (7) and the system will stabilize. The system will provide a constant flow until the flow requirements change.
Note: Load sensing signal pressure (8) does not need to decrease in order to destroke the hydraulic pump.
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 16000 kPa (2320 psi).
If the implement hydraulic circuit which is activated at 6900 kPa (1000 psi) is returned to the HOLD position, the maximum load sensing signal pressure (8) will still be 13800 kPa (2000 psi). However, the supply pressure at pump outlet (9) is momentarily increased due to the reduced oil flow that is required in the implement hydraulic circuits.
Supply pressure at pump outlet (9) raises flow compensator spool (11). This allows more oil flow behind actuator piston (7). The angle of swashplate (6) now decreases and the hydraulic pump provides less flow.
High Pressure Stall
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| Illustration 6 | g01022710 |
Steering and Implement Pump (Maximum System Pressure) (1) Piston Pump (2) Compensator Valve (3) Piston and Barrel Assembly (4) Shaft (5) Bias Spring (6) Swashplate (7) Actuator Piston (8) Load Sensing Signal Pressure (9) Pump Outlet (10) Margin Spring (11) Margin Spool (12) Pump Inlet (13) Adjustment Screw (14) Pressure Cutoff Spool (15) Orifices (16) Passage to Actuator Piston (17) Line To the Control Valve Group (18) Orifice (19) Passage | |
Note: Relief Valve (Load sensing signal) is sometimes referred to as the signal limiter valve. The two terms have the same meaning throughout this story.
Note: This condition will only occur when the relief valve (Load sensing signal) is set below the valve of the pressure compensator.
If piston pump (1) is at a high pressure stall or maximum system pressure, the combined pressure of load sensing signal pressure (8) and margin spring (10) is equal to the supply pressure at pump outlet (9) .
Relief valve (Load sensing signal) limits the maximum system pressure at any pump displacement. Relief valve (Load sensing signal) is set at 22800 kPa (3300 psi).
A margin pressure of 2200 ± 100 kPa (320 ± 15 psi) above the load sensing signal pressure is still maintained while piston pump (1) is at a high pressure stall.
If the piston pump is at a high pressure stall, the maximum system pressure will be 25000 ± 500 kPa (3625 ± 75 psi).
If the load sensing oil relief valve (20) is not adjusted correctly, pressure compensator spool (14) serves as a backup relief in order to protect the hydraulic system.
At the high pressure stall, the piston pump is at minimum flow and the supply oil at pump outlet (9) 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 (1) will upstroke in order to meet the increased flow demands. This flow meets the needs of the other circuits that are operating at a lower work port pressure.