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| Illustration 1 | g00901943 |
Variable Displacement Piston Pump (X) Pump pressure tap. | |
The variable displacement piston pump supplies oil flow for the brake system, the steering system, the decking blade circuit and the grapple hydraulic circuit. The piston pump is located under the floor of the operator's cab on the upper left side of the transmission. The pump is mounted on a pump drive which is part of the transmission.
Pressure tap (X) is used to check the pump system pressure.
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| Illustration 2 | g00901944 |
Variable Displacement Piston Pump (1) Pressure and flow compensator valve. (5) Flow compensator spool adjustment. (8) Pressure compensator adjustment. (Y) Pressure tap (load sensing signal pressure). | |
Pressure and flow compensator valve (1) uses signal pressures to control pump output. Pressure and flow compensator valve (1) contains two adjustable spools.
Flow compensator spool adjustment (5) is used to adjust the flow compensator spool. The flow compensator spool and the spring maintain pump supply pressure to 2000 ± 150 kPa (290 ± 20 psi) more than the load sensing signal pressure. The pressure difference between the pump supply pressure and the load sensing signal pressure is called "margin pressure".
Pressure compensator adjustment (8) is used to adjust the pressure compensator spool. The pressure compensator spool and the spring serve as the relief valve for the system.
Load sensing signal pressure can be measured at pressure tap (Y).
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| Illustration 3 | g00901534 |
Variable Displacement Piston Pump and Compensator Valve (1) Pressure and flow compensator valve (2) Actuator piston (3) Yoke pad (4) Swashplate (5) Flow compensator spool (margin) (6) Drive shaft (7) Bias piston (8) Pressure compensator spool (pressure cutoff) (9) Piston (10) Bias spring (A) No signal (B) Pump output | |
The hydraulic system contains an automatically controlled piston type pump. The piston pump senses both pressure and flow needs. When none of the hydraulic circuits are being used, the piston pump is at low pressure standby.
If one or more circuits are being used, the resolver network compares the control valve work port pressures. The single highest pressure felt flows to pressure compensator spool (8). The spool keeps the pump output at a level which will fulfill the system flow and pressure requirements.
The actual system pressure will be greater than the highest work port pressure requirements unless the pump is at full stroke. The difference between the requirement of the work port and the higher main system pressure is the margin pressure. The pressure compensator valve also limits maximum system pressure. This protects the hydraulic system from damage at high pressures.
The pump has two control pistons, bias piston (7), and actuator piston (2). Bias spring (10) around bias piston (7) causes swashplate (4) to move, and the pump will upstroke.
Actuator piston (2) has a larger surface area than bias piston (7), and the swashplate will cause the pump to destroke. Flow compensator spool (5) and/or pressure compensator spool (8) changes pump output. The spools regulate the discharge pressure of the pump, as the discharge pressure acts on actuator piston (2) .
Pressure and flow compensator valve (1) routes the pump discharge pressure to actuator piston (2). Actuator piston (2) is larger than bias piston (7). The oil pressure which acts against actuator piston (2) overcomes the force of bias spring (10) and the pump destrokes. Pump outlet pressure is maintained at 2000 ± 150 kPa (290 ± 20 psi) above the needs of work port pressure by flow compensator spool (5) .
The pressure and flow compensator valve also controls the maximum pressure output of the pump. When work port pressure rises above 21000 ± 350 kPa (3050 ± 50 psi), pressure compensator spool (8) overrides flow compensator spool (5) and the pump will destroke. This occurs at approximately 690 kPa (100 psi) below the maximum pressure setting.
Low Pressure Standby
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| Illustration 4 | g00901841 |
Variable Displacement Piston Pump and Compensator Valve Operation (1) Spring (2) Spring (3) Actuator piston (4) Swashplate (5) Cavity (6) Flow compensator spool (7) Passage (8) Pressure compensator spool (9) Passage (10) Passage (11) Bias piston (12) Bias spring (13) Passage (14) Passage (A) Signal (B) Pump output (C) Case drain | |
Low pressure standby occurs when the engine is running. The implements must be in hold and the steering must not be active. There are no flow or pressure demands on the pump. Therefore, there is no signal pressure in line (A) .
Before the engine is started, bias spring (12) holds swashplate (4) at a maximum angle. As the pump begins to turn, oil starts to flow. Pressure increases in the hydraulic system because of the closed center implement valves.
The pressure in passage (13) is felt at the bottom of both flow compensator spool (6) and pressure compensator spool (8). The pressure pushes flow compensator spool (6) against spring (1) as the pressure increases. Spool (6) moves up and passage (7) opens. This allows pressure oil to flow to actuator piston (3) .
The oil acts against actuator piston (3), and the pressure overcomes the force of bias spring (12). This causes actuator piston (3) to move to the right. When the piston moves to the right, the swashplate moves toward a minimum angle. Actuator piston (3) continues to move to the right and actuator (3) opens a cross-drilled hole through the stem inside the piston. This allows oil to drain to the case.
The cross-drilled hole limits the maximum travel of actuator piston (3) to the right. The pump produces enough flow in order to replace any system leakage and leakage to the pump case through the cross-drilled hole. The system will maintain main system pressure at 3600 kPa (520 psi).
Note: Low standby pressure will vary in the same pump as the system leakage or pump leakage increases. As leakage increases, the pump will upstroke slightly in order to compensate for the leakage. Actuator piston (3) will cover more of the cross-drilled hole. As this happens, low pressure standby will drop toward margin pressure. Eventually, leakage hits the point when actuator piston (3) covers the cross-drilled hole completely. Because of the increase in the swashplate angle, low pressure standby will equal margin pressure.
Upstroking
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| Illustration 5 | g00901676 |
Variable Displacement Piston Pump and Compensator Valve Operation (1) Spring (2) Spring (3) Actuator piston (4) Swashplate (5) Cavity (6) Flow compensator spool (7) Passage (8) Pressure compensator spool (9) Passage (10) Passage (11) Bias piston (12) Bias spring (13) Passage (14) Passage (A) Signal (B) Pump output (C) Case drain | |
Upstroking occurs when the pump is increasing output because of an increase in flow demand. Because of the increase in flow demand, signal pressure in line (A) combines with the force of spring (1) in cavity (5) .
The combination of signal pressure and spring force is greater than the pump discharge pressure. This causes spool (6) to move down, and spool (6) blocks the flow of supply oil to actuator piston (3). When oil flow to actuator piston (3) is blocked, the oil in passage (9) drains into passage (7). The oil flows past spool (6), pressure compensator (8), and out through passage (10) into case drain (C) .
Supply oil flows through passage (14) to bias piston (11). The oil acts against bias piston (11) and combines with the force of bias spring (12). This causes swashplate (4) to upstroke.
This increases pump flow. As flow requirements are satisfied, the pump output pressure increases. The pressure increases until the pressure in passage (13) moves spool (6) up to the metering position.
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| Illustration 6 | g00751345 |
Metering Position (D) Spool position at high pump pressure. (E) Spool position at low pump pressure. | |
Light up and down spool movement is called metering. Metering maintains the pressure on each end of spool (6) equally.
The force of spring (1) causes pump pressure to be 2000 ± 100 kPa (290 ± 15 psi) greater than signal pressure. This difference is called margin pressure.
With the spool in position (D), Pump pressure is greater than the combined force of spring (1) and the signal oil pressure in cavity (5). This forces spool (6) to move up.
When spool (6) moves up, pressure oil flows past the spool and through passage (7). The oil flows past spool (8) and into passage (9). Oil flows through passage (9) to actuator piston (4) .
The area of actuator piston (3) is larger than the area of bias piston (11). The force of the oil against piston (3) overcomes the combined force of spring (12) and the oil against piston (11). Actuator piston (3) moves to the right, and swashplate (4) moves toward a minimum angle.
As the angle of swashplate (4) decreases, pump output also decreases. Spool (6) moves down into position (E) when pump pressure decreases enough. The combined force of signal oil pressure in cavity (5) and spring (1) cause spool (6) to move down into position (E) .
When spool (6) moves down, the spool blocks the flow of pump output oil into passage (7). Oil in passage (9) flows from actuator piston (3), past spool (8), and back to the case drain.
Destroking
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| Illustration 7 | g00901797 |
Variable Displacement Piston Pump and Compensator Valve Operation (1) Spring (2) Spring (3) Actuator piston (4) Swashplate (5) Cavity (6) Flow compensator spool (7) Passage (8) Pressure compensator spool (9) Passage (10) Passage (11) Bias piston (12) Bias spring (13) Passage (14) Passage (A) No signal (B) Pump output (C) Case drain | |
Destroking occurs when the pump is decreasing output because of a decrease in flow demand. Because of the decrease in flow demand, signal pressure in line (A) combines with the force of spring (1) in cavity (5) and this pressure is less than the pump pressure in passage (13). Spool (6) is pushed up.
Oil behind actuator piston (3) cannot flow through passage (10) to case drain (C). Pump oil now flows through passage (13), past spool (6), through passage (9) and into actuator piston (3). Pump pressure behind actuator piston (3) is now greater than the combined force of bias piston (11) and bias spring (12). The angle of swashplate (4) decreases. This decreases pump output and system pressure decreases.
Once the lower flow requirements are met, the flow compensator spool (8) moves down to the metering position. Swashplate (4) will maintain an angle that is sufficient to provide the lower required pressure. If all implements are returned to hold, the pump goes to low pressure standby.
High Pressure Stall
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| Illustration 8 | g00901862 |
Variable Displacement Piston Pump and Compensator Valve Operation (1) Spring (2) Spring (3) Actuator piston (4) Swashplate (5) Cavity (6) Flow compensator spool (7) Passage (8) Pressure compensator spool (9) Passage (10) Passage (11) Bias piston (12) Bias spring (13) Passage (14) Passage (A) Signal at maximum pressure (B) Pump output at maximum pressure (C) Case drain | |
When the hydraulic system stalls under load or when the cylinders reach the end of the stroke, main system pressure increases. The signal pressure in line (A) and cavity (5) becomes equal to the pump output pressure. Spring (1) keeps spool (6) shifted downward.
When main system pressure reaches 21000 ± 350 kPa (3050 ± 50 psi) in passage (13), the pressure acts upon pressure compensator spool (8). The pressure overcomes the force of spring (2) and spool (8) moves upward. Supply oil now flows past spool (6) and through passage (7). Then, the oil flows past spool (8) and flows into passage (9). Then, the oil flows to actuator piston (3) .
The pump will destroke as pressure is felt on actuator piston (3). For single valve operation, pump output flow decreases while system pressure is limited to 20700 kPa (3000 psi). For multiple valve operation, main system pressure will near the maximum. However, the pump continues to produce flow in order to meet the needs of the other circuits with lower pressure requirements.
If the control lever is moved to the HOLD position during a high pressure stall, the pump returns to low pressure standby.