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| Illustration 1 | g00738720 |
(1) Shaft (2) Bias spring (3) Swashplate (4) Piston (5) Actuator piston (6) Pressure and flow compensator (7) Spring (8) Signal pressure line (9) Flow compensator spool (10) Pressure compensator spool (11) Orifice (12) Port plate (13) Cylinder barrel (14) Piston shoe (15) Retraction plate | |
Nine pistons (4) are mounted in cylinder barrel (13). Cylinder barrel (13) rotates with drive shaft (1). Piston shoe (14) is attached to each piston (4). The shoes are held against swashplate (3) by retraction plate (15). Swashplate (3) does not rotate. When swashplate (3) is in an angled position, the pistons move from one end of cylinder barrel (13) to the other end as the barrel rotates.
As pistons (4) move to the left, oil is drawn through port plate (12). The oil is drawn from the inlet port into the piston bore in cylinder barrel (13). As cylinder barrel (13) rotates, the pistons move to the right. The oil that is in the piston bore is forced through port plate (12) to the outlet port.
The angle of the swashplate determines the stroke of the pistons. The stroke of the pistons determines the quantity of oil that is delivered during each revolution of the drive shaft. The angle of swashplate (3) can be adjusted in order to provide a variable flow rate. The maximum angle of swashplate (3) provides the maximum flow rate. The minimum angle of the swashplate provides the minimum flow rate.
Bias spring (2) forces swashplate (3) to the maximum angle. When pressure is applied to actuator piston (5), the piston forces bias spring (2) to compress. This causes the swashplate to move toward the minimum angle. As the pressure behind the actuator piston decreases, the bias spring forces the swashplate to the maximum angle.
Pressure and flow compensator valve (6) controls the angle of the swashplate. Flow compensator spool (9) and/or pressure compensator spool (10) changes pump output by regulating the pressure that is acting on actuator piston (5). The signal pressure from line (8) and the supply pressure from the pump help determine the amount of oil that flows past the valve spools (9) and (10) to the actuator piston.
The difference between the signal pressure from the bank valve and the supply pressure from the pump is called margin pressure. The force of spring (7) determines the margin pressure. The supply pressure pushes on spool (9). The signal pressure and spring (7) push on the other side of the spool.
Note: Margin pressure is set at 2780 ± 100 kPa (400 ± 15 psi).
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| Illustration 2 | g00744659 |
With spool (9) in position (A), pump pressure is greater than the combined force of spring (7) and the signal pressure. This forces the spool to move upward.
When spool (9) moves upward, pressure oil flows past the spool. The oil then flows past compensator spool (10) and into the passage that leads to actuator piston (5) .
As the angle of swashplate (3) decreases, pump output also decreases. When the pump pressure is low enough, the combined force of the signal pressure and spring (7) causes spool (9) to move downward into position (B) .
When spool (9) moves down, the flow of pump output oil to the actuator piston is blocked. Oil flows from actuator piston (5) past the spools. The oil then returns to the case drain.
Note: The compensator valve has an orifice (11) in the signal passage that leads to the actuator piston. The orifice is used to regulate the rate of response from the actuator piston. The orifice regulates the response rate by creating a constant path for leakage to the drain.
Pressure compensator spool (10) and the spring that acts on the spool serves as a backup for the signal relief valve. If the signal relief valve is faulty or if the relief valve is adjusted incorrectly, the compensator spool would limit maximum system pressure.
Low Pressure Standby
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| Illustration 3 | g00738724 |
(2) Bias spring (3) Swashplate (5) Actuator piston (7) Spring (8) Line for signal pressure (9) Flow compensator spool (10) Pressure compensator spool (16) Land (17) Passage (18) Inlet to pressure and flow compensator valve (T) Low pressure oil to tank (LS) Minimal signal pressure (R) Reduced supply pressure from pump | |
Low pressure standby results from the following conditions: a running engine, inactive steering and inactive implement circuits. When the pump is in low pressure standby, the only flow requirement is compensation for system leakage. Therefore, there is no signal pressure in line (8) except for the slight signal from the dynamic signal orifice in the manifold valve. The orifice maintains a slight signal to the pump which results in a higher low pressure standby. Dynamic steering improves steering response time.
Before the engine is started, bias spring (2) holds swashplate (3) at the maximum angle. As the pump begins to turn, oil begins to flow and pressure increases in the system. Pressure increases in the system because of the closed centered control valves.
The pressure in inlet (18) is felt at the bottom of the flow compensator spool (9) and at the bottom of pressure compensator spool (10). As this pressure increases, the pressure pushes the flow compensator spool against spring (7). Passage (17) is opened and pressure is transmitted to actuator piston (5) .
The pressure causes the actuator piston to compress bias spring (2). As the piston compresses the spring, the swashplate moves toward the minimum angle. Piston travel is limited by land (16) .
At the minimum angle, the pump produces a small amount of flow. The flow compensates for system leakage. The small amount of flow also maintains a sufficient amount of supply pressure. This will provide an instantaneous response when an implement control lever is activated.
Note: Low pressure standby flow will increase as the system leakage increases or the pump leakage increases. The pump will upstroke slightly in order to compensate for the increase in leakage.
Upstroking
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| Illustration 4 | g00738729 |
(2) Bias spring (3) Swashplate (5) Actuator piston (7) Spring (8) Line for signal pressure (9) Flow compensator spool (11) Orifice (18) Inlet to pressure and flow compensator valve (20) Spring chamber (P) Supply pressure from pump (T) Low pressure oil to tank | |
The pump is upstroking when the angle of swashplate (3 ) is increasing in order to provide more flow.
Upstroking of the pump occurs because of increased flow demand.
When an implement is activated, the signal pressure in line (8) combines with the force of spring (7) in spring chamber (20). This combination of signal pressure and of spring force becomes greater than the pump discharge pressure. This causes spool (9) to move down. As the spool moves down, the spool blocks the flow of supply oil to actuator piston (5). The oil behind the actuator piston is open to the case drain through orifice (11) and across the spools.
The pressure behind the actuator piston is reduced or the pressure is eliminated. The reduction in pressure allows bias spring (2) to move swashplate (3) to an increased angle. This causes a larger pump displacement and the pump flow increases until the pressure in inlet (18) moves spool (9) up to the metering position.
In the metering position, the spool is balanced by the pressure that is acting on each end of the spool. This allows variations in the pump displacement to be smooth.
The following conditions can lead to the upstroking of the pump:
- activation of an implement or the steering
- increased demand from an implement
- decrease in engine speed
Note: Signal pressure does not need to increase in order for the pump to upstroke. The system may be operating with one circuit in activation. When an additional circuit is activated, the signal pressure may remain at the same level. However, supply pressure will decrease momentarily in order to compensate for the increased flow. When a decrease in supply pressure occurs, flow compensator spool (9) will move down. This causes pressure to decrease behind the actuator piston. The decrease in pressure allows the bias spring to upstroke the pump.
Destroking
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| Illustration 5 | g00738732 |
(2) Bias spring (3) Swashplate (5) Actuator piston (7) Spring (8) Line for signal pressure (9) Flow compensator spool (10) Pressure compensator spool (17) Passage (18) Inlet to pressure and flow compensator valve (20) Spring chamber (21) Outlet passage to actuator piston (P) Supply pressure from pump (T) Low pressure oil to tank (LS) Reduced signal pressure | |
Destroking of the pump occurs because of decreased flow demand. The combined signal and spring pressure in spring chamber (20) is less than the pump pressure in inlet (18). This causes spool (9) to move upward.
Pump oil now flows from inlet (18) through passage (17). The oil then flows past spool (10), through passage (21) and into actuator piston (5). The pump pressure behind actuator piston (5) is now greater than bias spring (2). The angle of the swashplate (3) decreases. This decreases the pump output and the system pressure.
When the lower flow requirements are met, the flow compensator spool (9) moves downward to the metering position. Swashplate (3) will maintain an angle that is sufficient to provide the lower required flow. If the operator is not turning the steering wheel or operating an implement, the pump will return to low pressure standby.
The following conditions can result in destroking the pump:
- controls that are moved to the HOLD position
- decreased demand from an implement
- deactivation of additional circuits
- increase in engine rpm
Note: Signal pressure does not need to decrease in order for the pump to destroke. The system may be operating with multiple circuits in activation. When deactivation of a circuit with a lower pressure occurs, the signal pressure may remain at the same level. However, supply pressure will increase momentarily because of the decreased flow. The increase in supply pressure causes the flow compensator spool to move up. This causes pressure to increase behind the actuator piston. The increase in pressure allows the bias spring to destroke the pump.
High Pressure Stall
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| Illustration 6 | g00738733 |
(5) Actuator piston (7) Spring (9) Flow compensator spool (10) Pressure compensator spool (17) Passage (18) Inlet to pressure and flow compensator valve (22) Spring (23) Passage (P) Supply pressure from pump (T) Low pressure oil to tank (LS) Signal pressure | |
When the hydraulic system stalls under a load or when the cylinders reach the end of the stroke, a signal relief valve that is located in the end section of the bank valve opens in order to limit maximum signal pressure. When the valve opens, the force of spring (7) and the force of the spring in the signal relief valve combine in order to limit supply pressure to 25000 ± 500 kPa (3626 ± 73 psi). Supply oil in inlet (18) acts upon pressure compensator spool (10) and flow compensator spool (9). The pressure then overcomes the force of spring (22) and spring (7). This causes each spool to move upward into a metering position in order to maintain the desired flow rate. Supply oil now flows past spool (10) and through passage (23). The oil then flows to actuator piston (5). Supply oil also flows past spool (9) into passage (17), but the oil is blocked by spool (10).
If a circuit stalls and activation of an additional circuit occurs, the pump will upstroke in order to meet the increased flow demands.
Note: Pressure compensator spool (10) is in parallel with flow compensator spool (9). The pressure spool limits the maximum system pressure. The spool acts as a backup to the signal relief valve that is located in the end section of the bank valve.