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| Illustration 1 | g01933262 |
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Implement pumps and compensators (1) Margin pressure valve (2) Pressure cutoff valve (3) Flow compensator valve (4) Pump | |
Piston pumps (4) for the hydraulic system are controlled by the load signal from the implements. Piston pump (4) senses the pressure of the system. Then, the flow from piston pump (4) is adjusted in order to match the system pressure.
If one or more circuits are being used, the single highest pressure in the system communicates to flow compensator (3). Flow compensator (3) keeps the pump output at a level that is needed in order to fulfill the pressure requirements of the system.
The pump outlet pressure is maintained above the needed pressure of the work port by flow compensator (3).
The system pressure will be greater than the requirements for the highest work port pressure unless the pump is at full stroke. Margin pressure is the difference between the pressure that is at the load sensing port and the pressure at the pump discharge port.
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| Illustration 2 | g01376531 |
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Cross section of the implement pump and compensator (1) Margin pressure valve (2) Pressure cutoff valve (3) Flow compensator valve (4) Pump (5) Bias piston (6) Spring (7) Piston (intake) (8) Pump output (9) Piston (10) Actuator piston (11) Swashplate | |
The pumps have two actuators for control, bias piston (5) and actuator piston (10). Spring (6) in small actuator (5) causes swashplate (11) to move. The movement of the swashplate will cause the pump to upstroke or the movement will cause the pump to destroke.
Actuator piston (10) has a larger area than bias piston (5). Actuator piston (10) causes the swashplate to destroke the pump. Pressure cutoff valve (2) and margin pressure valve (1) change pump output by regulating the discharge pressure of the pump that is acting on actuator piston (10).
When none of the hydraulic circuits are being used, pumps (4) are at system standby pressure.
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| Illustration 3 | g01937154 |
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Implement pump and compensator (system standby pressure) (1) Margin pressure valve (2) Pressure cutoff valve (3) Flow compensator valve (4) Pump (5) Bias piston (6) Spring (7) Piston (intake) (8) Pump output (9) Piston (10) Actuator piston (11) Swashplate (12) Spring (13) Spring (14) Input for Load signal (15) Signal duplicator valve (16) Load sensing relief valve (17) Orifice | |
When the pump is not powered, machine engine off, there is no pressure in the pump controls. Spring (13) of pressure cutoff valve (2) pushes the pressure cutoff valve spool downward in the NEUTRAL position. Spring (12) of margin pressure valve (1) pushes the margin pressure valve spool downward in the NEUTRAL position. Bias piston (5) and actuator piston (10) are also free of pressure. The cross drilled passages of the pressure cutoff valve and the margin pressure valve allows the oil in actuator piston (10) to be drained to the pump case. Spring (6) mechanically maintains pump swashplate (11) in the MAXIMUM ANGLE position.
As soon as the pump is rotated by the engine, pistons (9) move back and forth. This generates pump flow and pressure. Also, in standby position, when no implement motions are actuated, there is no need of the system to be supplied with flow. There is also no load sensing pressure sent back to flow compensator valve (3). The pump pressure pushed bias piston (5) and flows up to flow compensator valve (3). The pump pressure also flows through a passage of pressure cutoff valve (2) up to margin pressure valve (1). Supply oil flows through an axial drilled orifice in the end of the margin pressure valve spool. When the pump pressure equals the value of spring (12), the pressure moves the margin pressure valve spool upward. This opens a small passage between the margin pressure valve spool and the flow compensator valve body. The pump output pressure can also flow through the next cross passage of pressure cutoff valve (2). The pump output pressure is then supplied to actuator piston (10). When the force of actuator piston (10) equals the force of bias piston (5) plus spring (6), actuator piston (10) moves pump swashplate (11) to the MINIMUM ANGLE position.
While the implement controls are not operated, the implement system does not demand any flow, there is also no load sensing pressure sent back to flow compensator valve (3) and the pump stays in the STANDBY position. Due to the internal leakages of the pump, the pump creates the minimum flow at the minimum pressure in order to keep acting on the margin pressure valve spool. This allows the pump output pressure to be constantly supplied to actuator piston (10). Swashplate (11) is stabilized in the MINIMUM ANGLE position. The pump output standby pressure is also approximately equal to the setting of margin pressure valve (1).
Upstroke or Destroke and Metering
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| Illustration 4 | g01937794 |
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Implement pump and compensator (upstroke) (2) Pressure cutoff valve (10) Actuator piston (11) Swashplate (13) Spring | |
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| Illustration 5 | g01937797 |
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Implement pump and compensator (metering) (2) Pressure cutoff valve (13) Spring | |
As soon as the implement controls are operated in order to move an implement cylinder, the cylinder load pressure or load sensing pressure is sent back to flow control valve (3). This pressure plus the force of spring (12) is generally above above the pump output standby pressure. This causes the margin pressure valve spool to move downward. This opens a passage between the margin pressure valve spool and the flow compensator valve body. This causes the pressure in actuator piston (10) to be drained through the cross passage of pressure cutoff valve (2) and the opened cross passage of margin pressure valve (1) to the pump case. The force of bias piston (5) still supplied with the pump output pressure plus spring (6) is above the force of actuator piston (10). This increases the angle of swashplate (11). The pump then UPSTROKES in order to generate more flow to the implement valve. While upstroking, the pump output pressure is equal to or below the load sensing pressure.
As soon as the pump output flow is equal to the implement control flow requirement, the pump output pressure continues to raise up. When the pump output pressure equals the load sensing pressure value plus the value of spring (12), the margin pressure valve spool moves upward. This closes the opened passage of actuator piston (10) to the pump case. This also opens the passage of the pump output pressure to actuator piston (10). Once the force of actuator piston (10) balances the force of bias piston (5) plus spring (6), the swashplate is stabilized to the adequate angle position making the pump supply the required flow to the implement control. The pump output pressure is equal to the load sensing pressure plus the setting of margin pressure valve (1). The pump will maintain the flow that is required in order to maintain system pressure. The pump will stay in the METERING position until the flow requirement changes.
As soon as the implement controls are operated in a way to reduce the speed of a cylinder, the required pump flow decreases. This makes the pump output pressure instantaneously increase due to the pump output flow being above the flow requirement of the implements. The pump output pressure that is flowing to margin pressure valve (1) and acting on the margin pressure valve spool end through the axial drilled orifice is above the load sensing pressure plus spring (12). This moves the spool upward. This also opens the passage that allows the pump output pressure to enter actuator piston (10). The force of bias piston (5) plus spring (6) is below the force of actuator piston (10). This decreases the angle of swashplate (11) so the pump DESTROKES and generates less flow to the implement valve.
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| Illustration 6 | g01937803 |
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Implement pump and compensator (high pressure stall) (16) Load sensing relief valve | |
When the implement system stalls under load or when the cylinders reach the end of stroke, the implement system pressure and the load sensing pressure increases.
When the load sensing pressure is above the setting of relief valve (16), the relief valve opens sending the load sensing signal to the tank. This also limits the load sensing signal pressure, which limits the maximum cylinder load pressure the implement system needs. The pump keeps sending out the flow in order to maintain the pump output pressure equal to the maximum load sensing pressure plus the setting of margin pressure valve (1). This limits the pump output pressure. The pump output pressure is also pushing the margin pressure valve spool upward and the pump output pressure is filling actuator piston (10). This balances bias piston (5) and spring (6). This reduces the angle of swashplate (11) to fully DESTROKES the pump.
During a high pressure stall, a movement of the lever back to the NEUTRAL position causes the pump to return to the STANDBY position.
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| Illustration 7 | g01959037 |
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Implement pump and compensator (system standby pressure) (1) Margin pressure valve (2) Pressure cutoff valve (3) Flow compensator valve (5) Bias piston (6) Spring (10) Actuator piston (11) Swashplate (13) Spring (16) Load sensing relief valve | |
The pump is designed and produced in a way that flow compensator valve (3) is always equipped with both control stages, margin pressure valve (1) and pressure cutoff valve (2). The pressure cutoff valve is basically not used in the standard operations of the implement controls. But for some failure prevention reasons, this can be actuated as a redundancy in order to fully DESTROKE the pump and protect the implement system from maximum overpressure.
In case of a failure of relief valve (16), when stalling a cylinder, the pump output pressure will raise over the setting of relief valve (16). The output pressure flowing to flow compensator valve (3), also flows through an axial drilled orifice at the end of the pressure cutoff valve spool. Once the output pressure is equal to spring (13) the spool is pushed upward. This opens a direct passage between actuator piston (10) and the pump output, bypassing margin pressure valve (1), and makes the pump output pressure directly enter actuator piston (10). This one supplied by the pump output pressure balances the force of bias piston (5) and spring (6). This causes swashplate (11) return back to the minimum angle position, also the pump is fully DESTROKED. The pump output pressure is equal to the setting of pressure cutoff valve (2).
During the stall, a movement of the lever back to the NEUTRAL position causes the pump to return to the STANDBY position.