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| Illustration 1 | g00471417 |
Hydraulic pump group (1) Main hydraulic pump (2) Auxiliary hydraulic pump | |
The hydraulic pump group for the pipelayer hydraulic system supplies hydraulic oil to the winches, the pilot control valves and the counterweight.
Main hydraulic pump (1) is an piston type pump that is automatically controlled. Main hydraulic pump (1) senses both the pressure and the flow needs of the system. Main hydraulic pump (1) supplies oil to the winches and the pilot control valves.
Auxiliary pump (2) is a gear type pump. This pump supplies oil to the counterweight group.
Main Hydraulic Pump
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| Illustration 2 | g00471418 |
Main hydraulic pump (1) Compensator valve (2) Main drive shaft (3) Swashplate (4) Retaining plate (5) Cylinder barrel (6) Piston (7) Actuator piston (8) Auxiliary drive shaft (9) Bias piston (10) Bias spring | |
The pump for the hydraulic system is a load sensing variable displacement axial piston pump. The pump has two control pistons: bias piston (11) and actuator piston (10) . The pump also has compensator valve (3) which limits system pressure. The compensator valve senses both the pressure and flow needs of the system.
When drive shaft (4) is rotated, cylinder barrel (7) also turns. Nine pistons (8) are held in cylinder barrel (9) and the pistons turn with the cylinder barrel. Each piston is held against nonrotating swashplate (5) by retaining plate (6) . At the maximum swashplate angle, the pistons are pulled out of cylinder barrel (7) . This movement pulls oil from the inlet port into the piston bore in cylinder barrel (7) . As cylinder barrel (7) rotates, the angled swashplate pushes the pistons back into the cylinder barrel. This movement pushes the oil out of the piston bore into the output port.
The angled swashplate determines the amount of oil that is drawn into each piston bore. Therefore, the angle determines the amount of oil that is pumped out of the pistons per revolution of the drive shaft. The pump produces a larger output with a larger angle.
When the angle of swashplate (5) is at the minimum, pistons (8) do not move in the rotating cylinder barrel. Therefore, no oil is drawn into the pump and no oil pressure is produced. The pump is in this position under the following conditions:
- The system pressure is at zero.
- The implement controls are in the NEUTRAL position.
- The signal oil is drained to the tank.
The pump has compensator valve (3) that keeps the pump pressure and the pump flow at a level needed to fulfill the system requirements. The compensator valve operates by controlling the oil pressure in actuator piston (9) . This piston works with bias piston (11) and spring (12) in order to continually adjust the angle of the swashplate. Pump outlet pressure is kept at approximately 2000 kPa (290 psi) above the system requirements. The compensator valve also limits the system pressure in order to prevent overloads of the pump and the system. When the work port pressure approaches 28300 kPa (4100 psi), the pressure compensator spool will move to the left. This movement allows oil to charge the actuator piston and the swashplate angle is lowered. The pump output is reduced while the system pressure is held at 28300 kPa (4100 psi) .
The following schematics display the pump and the compensator valve in different conditions in the hydraulic system.
Upstroking
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| Illustration 3 | g00471614 |
Pump and compensator valve operation (upstroking) (AA) Pump pressure oil (low pressure) (EE) Signal oil (high pressure) (LL) Return oil (1) Flow compensator spool (2) Signal line from resolver network (3) Chamber (4) Output to control valves (5) Passage (6) Bias piston (7) Suction line (8) Flow compensator spring (9) Passage (10) Passage (11) Passage (12) Passage (13) Passage (14) Swashplate (15) Case drain (16) Actuator piston (17) Passage (18) Piston (19) Pressure compensator spool (20) Pressure compensator spring | |
Upstroking is the condition when the pump is increasing the displacement (output). This condition occurs when the signal pressure in line (2) increases due to a high load at a low pump pressure. The highest resolved signal pressure goes through line (2) and the oil fills chamber (3) . The signal pressure plus the force of springs (8) move spool (1) to the left. This movement blocks the flow of supply oil to actuator piston (16) . With spool (1) in this position, oil that is under the actuator piston drains past the pressure compensator spring cavity to case drain (15) . Now supply oil flows through passage (5) to bias piston (6) . This oil combines with the bias spring in order to move swashplate (14) toward the maximum angle. This produces an increase in pump output.
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| Illustration 4 | g00451189 |
Metering positions | |
The pump output pressure increases until the pressure in passage (9) moves spool (1) to the right to the metering position. Initially, when spool (1) is in position (A) , pump pressure is greater than the combined force of springs (8) and the signal pressure in chamber (3) . Spool (1) moves to the right. Pressure is sent to actuator piston (16) .
Since the area of actuator piston (16) is greater than the area of bias piston (6) , there is a larger force under actuator piston (16) . This force moves actuator piston (16) toward the right and the swashplate angle is decreased. Pump output decreases. When the pump output decreases, the force of the signal oil in chamber (3) and the force of springs (8) move spool (1) to the left to position (B) . The oil pressure behind actuator piston (4) is allowed to drain. Bias piston (6) and the bias spring force swashplate (14) to increase the angle. This slight oscillation of spool (1) is called metering. Metering keeps the pressure on both ends of spool (1) equal. Spring (8) is equal to 2410 kPa (350 psi). Therefore, the pump pressure is 2410 kPa (350 psi) above the signal pressure from the control valves. This difference is called margin pressure.
Destroking
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| Illustration 5 | g00471678 |
Pump and compensator valve (destroking) (AA) Pump pressure oil (high pressure) (EE) Signal oil (low pressure) (LL) Return oil (1) Flow compensator spool (2) Signal line from resolver network (3) Chamber (4) Output to control valves (5) Passage (6) Bias piston (7) Suction line (8) Flow compensator spring (9) Passage (10) Passage (11) Passage (12) Passage (13) Passage (14) Swashplate (15) Case drain (16) Actuator piston (17) Passage (18) Piston (19) Pressure compensator spool (20) Pressure compensator spring | |
Destroking means that the pump is decreasing the displacement (output). This occurs when the signal pressure from the control valves decreases due to a low load. The lower signal pressure goes through line (2) and the oil fills chamber (3) . The signal oil pressure plus the spring force in chamber (3) is lower than the pump pressure in passage (9) . Spool (1) is pushed to the right. Pump oil is now allowed to flow through passages (11) and (17) to actuator piston (6) . Pump pressure behind the actuator piston increases until the pressure is greater than the force of the bias spring and bias piston (6) . The angle of swashplate (14) is decreased and the pump output is decreased.
The system pressure lowers to the margin pressure of 2000 kPa (290 psi) if there is a system requirement (load). If all of the implement controls are in the HOLD position, the system pressure approaches the standby pressure of 3500 kPa (508 psi). At this pressure, spool (1) moves to the metering position and the metering process begins to maintain the system pressure by making up for system leakage.
Note: See "Upstroking" for an explanation of metering.
High Pressure Stall
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| Illustration 6 | g00471729 |
Pump and compensator valve (high pressure stall) (AA) Pump pressure oil (EE) Signal oil (LL) Return oil (1) Flow compensator spool (2) Signal line from resolver network (3) Chamber (4) Output to control valves (5) Passage (6) Bias piston (7) Suction line (8) Flow compensator spring (9) Passage (10) Passage (11) Passage (12) Passage (13) Passage (14) Swashplate (15) Case drain (16) Actuator piston (17) Passage (18) Piston (19) Pressure compensator spool (20) Pressure compensator spring | |
The system enters a high pressure stall when the hydraulic system stalls under a heavy load or the cylinders reach the end of the cylinder stroke. Stall condition occurs when the pump output reaches 28300 kPa (4100 psi). The signal pressure in line (2) and chamber (3) is equal to the pump output pressure. Springs (8) keep spool (1) shifted to the left. When system pressure reaches 28300 kPa (4100 psi) in passage (12) , the force on pressure compensator spool (19) compresses spring (20) . Pressure compensator spool (19) moves to the left. Supply oil flows through passage (17) to actuator piston (16) . Pressure on the actuator piston destrokes the pump. Pump output (flow) decreases to zero while the system pressure is limited to 28300 kPa (4100 psi) (maximum system pressure).
If a control lever is moved to the HOLD position during a high pressure stall, then the signal pressure in line (2) and chamber (3) returns to the control valve. This oil drains to the tank. System pressure begins to bleed down. Spring (20) moves pressure compensator spool (19) to the right. The system pressure in passage (9) begins to move flow compensator spool (8) to the right.
Supply oil flows past compensator spool (1) and pressure compensator spool (19) to actuator piston (16) . Actuator piston (16) destrokes swashplate (14) until the system pressure decreases. As the system pressure approaches the low standby pressure, flow compensator spool (1) moves to the left to the metering position. Swashplate (14) maintains a slight angle in order to produce a sufficient pump output for system leakage.
Low Pressure Standby
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| Illustration 7 | g00472056 |
Pump and compensator valve operation (low pressure standby) (AA) Pump pressure oil (LL) Return oil (1) Flow compensator spool (2) Signal line from resolver network (3) Chamber (4) Output to control valves (5) Passage (6) Bias piston (7) Suction line (8) Flow compensator spring (9) Passage (10) Passage (11) Passage (12) Passage (13) Passage (14) Swashplate (15) Case drain (16) Actuator piston (17) Passage (18) Piston (19) Pressure compensator spool (20) Pressure compensator spring | |
The system is in a low pressure standby when the engine is running and the attachments are in the HOLD position. There are no flow demands or pressure demands on the pump. Therefore, there is no signal pressure in line (2) .
Before the engine is started, the bias spring holds swashplate (14) at the maximum angle. The engine is started. As the pump begins to turn, the system pressure builds up. Pressure begins to build up behind spools (1) and (19) . When the pressure reaches approximately 3000 kPa (435 psi), spool (1) moves to the right. This allows oil to flow to actuator piston (16) . Actuator piston (16) moves to the right. This causes swashplate (14) to move toward the minimum angle. The actuator piston moves to the right until the pump output is maintained at 3000 kPa (435 psi) .
Note: Low pressure standby is not the same pressure as the margin pressure. Margin pressure is 2000 kPa (290 psi). This pressure that is needed to compress springs (8) is not enough to move swashplate (14) against the bias spring and bias piston (6) . Pump output increases until the swashplate moves against the bias spring and bias piston (6) . System pressure is held at 3000 kPa (435 psi). This is the low pressure standby. Some variation in the low pressure standby can occur with changes in the rpm.
Auxiliary Hydraulic Pump
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| Illustration 8 | g00472150 |
Auxiliary hydraulic pump (1) Manifold (2) Body (3) Cover (4) Gear (5) Gear (6) Drive shaft | |
The auxiliary hydraulic pump is a single-section gear pump. The pump is driven by the main hydraulic pump. A coupling connects drive shaft (6) to the auxiliary drive shaft on the main hydraulic pump. The pump is fastened to the main hydraulic pump. The auxiliary hydraulic pump supplies oil to the counterweight hydraulic circuit.
During operation, oil from the hydraulic tank enters the pump through a port on the side of the pump. The oil fills the space between the gear teeth. The gears turn and the oil is sent out of the pump through another port in the opposite side of the pump. This oil travels to the counterweight control valve.
When the counterweight control valve is in the HOLD position, the oil passes through the valve to the hydraulic oil cooler. If the counterweight control valve is shifted to the EXTEND position or the RETRACT position, the oil is supplied to the counterweight hydraulic cylinder.
See Hydraulic Systems Operation, "Counterweight Control Valve" for more information.