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| Illustration 1 | g00450769 |
View of the Right Steering Pump (2) Steering pump. (9) Pressure and flow compensator valve. | |
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| Illustration 2 | g00292936 |
(1) Drive shaft (2) Steering pump (3) Swashplate (4) Retaining plate (5) Pistons (nine) (6) Cylinder barrel (7) Spring (8) Small actuator piston (9) Pressure and flow compensator valve (10) Inlet from hydraulic tank (11) Passages for pump discharge pressure (12) Flow compensator spool (margin spool) (13) Return orifice (14) Signal pressure port (15) Springs (16) Flow compensator valve (17) Large actuator piston (18) Spring (19) Outlet for the pump discharge pressure (20) Tank return passage (21) Pressure compensator spool (cutoff spool) (22) Passage to the large actuator piston (23) Pressure compensator piston (24) Spring (25) Pressure compensator valve | |
The steering system uses two automatically controlled piston pumps (2) . The steering pumps sense pressure and flow requirements for the system.
The steering pump is an axial piston pump. The output of pump (2) is controlled by pressure and flow compensator valve (9) . The pressure and flow compensator valve senses pressure and flow requirements for the system. The steering pumps provide the high pressure oil for the steering system. The steering pumps also provide oil to the pressure reducing valves for the pilot system. The right pump supplies oil to the pressure switch for the steering system.
While the engine is running, drive shaft (1) turns. The drive shaft causes cylinder barrel (6) to also turn. Nine pistons (5) are held against swashplate (3) by retaining plate (4) . The swashplate does not rotate.
At the maximum swashplate angle, some of pistons (5) are pulled out of cylinder barrel (6) . At the same time, some of pistons (5) are pushed into the cylinder barrel. The rotation of the cylinder barrel causes the pistons to move in and out of the cylinder barrel.
As a piston moves out of the barrel, the piston draws oil into the pump. When the piston moves back into the barrel, the piston will force oil out of the pump.
The angle of swashplate (3) determines the amount of oil that is drawn into the piston bores for each revolution of the drive shaft. Therefore, the angle of swashplate (3) determines the amount of oil that is pushed out of the piston bores for each revolution of the drive shaft.
Illustration 2 shows swashplate (3) at the maximum angle.
As the steering system load changes, the swashplate angle fluctuates between neutral and the maximum angle. When the swashplate approaches the maximum angle, the pump approaches maximum output.
When the swashplate angle is zero, the pistons do not move in and out of the rotating cylinder barrel. Therefore, no oil is drawn into the pump or out of the pump. In this condition, the pump is at zero displacement.
Because the pump is not displacing any oil, the pump is producing neither flow nor pressure. The pump is in the neutral condition when the system pressure suddenly drops to zero. This causes the swashplate to move to the ZERO ANGLE position.
Pressure and flow compensator valve (9) keeps the pump pressure at a level that is needed in order to fulfill the needs of the steering system. The compensator valve also keeps the flow at a similar level. The pressure and flow compensator valve contains two control pistons (12) and (21) . The two pistons work together in order to adjust the angle of the pump's swashplate.
Small actuator piston (8) causes swashplate (3) to upstroke the pump. Spring (7) combines with the pump discharge pressure in order to move the swashplate to the maximum angle. This increases pump output.
Large actuator piston (17) causes the swashplate to destroke the pump. Flow compensator spool (12) and/or pressure compensator spool (21) changes the pump displacement by regulating the amount of pump discharge pressure that is acting on the large actuator piston.
When oil is acting on large actuator piston (17) , the piston overcomes the force of both small actuator piston (8) and of spring (7) . Piston (17) can now move to the left. The swashplate will rotate clockwise when the piston moves to the left. This destrokes the pump.
Pressure compensator spool (21) prevents overloads of the pump and of the system. When the pump output pressure exceeds approximately 29000 kPa (4200 psi), pressure compensator spool (21) will override flow compensator spool (12) . This causes the output of the pump to lower until the system pressure is maintained at approximately 29000 kPa (4200 psi) .
Pump outlet pressure is maintained at 2100 ± 105 kPa (305 ± 15 psi) above the signal oil pressure by flow compensator spool (12) .
The following schematics illustrate the operation of the steering pumps and the pressure and flow compensator valves during different conditions.
Upstroking
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| Illustration 3 | g00293037 |
Steering Pump and Pressure and Flow Compensator Valve (Upstroking) (2) Pump (3) Swashplate (7) Spring (8) Small actuator piston (9) Pressure and flow compensator valve (12) Flow compensator spool (margin spool) (15) Springs (17) Large actuator piston (18) Spring (21) Pressure compensator spool (cutoff spool) (24) Spring (26) Hydraulic tank for the steering and brake systems (27) Return line to the hydraulic tank (28) Drain line from the large actuator piston to the hydraulic tank (29) Chamber for the large actuator piston (30) Passage to the large actuator piston (31) Connecting passage between the pressure valve and the flow valve (32) Line for the signal oil from the control valve (33) Supply oil line for the steering pump (34) Spring chamber for the small actuator piston (35) Pressure oil passage to the flow compensator valve (36) Pressure oil passage to the flow compensator spool (37) Pump case drain line (38) Connecting passage between the flow compensator spool and the pressure compensator spool (39) Pressure oil passage to the pressure compensator spool (40) Pressure oil passage to the pressure compensator valve (41) Passage to the small actuator piston (42) High pressure oil line to the steering control valve (A) Pressure oil (B) Signal oil (C) Return oil (D) Supply oil | |
Upstroking means that the pump is increasing the displacement. The displacement is due to an increased load. When the system requires more flow, signal oil from the steering control valve flows through line (32) . This oil fills the chamber of springs (15) .
The signal oil pressure combines with the force of springs (15) . This causes flow compensator spool (12) to move downward. When the flow compensator spool moves downward, the flow compensator spool blocks the flow of pump oil through passage (35) .
While the flow compensator spool is lowered, the oil in the chamber of large actuator piston (29) can flow into passage (30) . The oil flows past the cavity of spring (24) , past flow compensator spool (12) , and through line (27) . The oil then flows back to hydraulic tank (26) .
Oil from the steering pump flows through line (41) into chamber (34) . The oil in the chamber of large actuator piston (29) is now vented. The combined force of spring (7) and of the pump oil in chamber (34) causes small actuator piston (8) to move swashplate (3) toward the maximum angle.
As the swashplate moves closer to the maximum angle, the pump output increases. As pump output increases, the pressure in passages (38) and (36) also increases. Oil in passage (36) flows through an orifice and the oil acts against spool (12) .
The pressure across the orifice in passage (36) becomes greater than the force of springs (15) and of the signal oil in the chamber of spring (15) . This causes the oil in passage (36) to move the flow compensator spool upward.
When the spool moves all the way to the top, the pump oil in passage (35) can flow past the spool, through passage (31) , and then to the chamber for large actuator piston (29) .
Large actuator piston (17) is larger than small actuator piston (8) . Because of this difference in size, the oil pressure that is acting against the large actuator piston exerts a greater amount of force than the combined oil and the spring force that is acting against the small actuator piston.
The oil pressure that is acting against the large actuator piston overcomes the force of spring (18) , and the combined force of the oil and of the spring in chamber (34) . This causes large actuator piston (17) to move down.
As large actuator piston (17) moves down, swashplate (3) moves toward the minimum angle. This causes destroking of the pump. As the angle of the swashplate moves toward the minimum angle, the pump output and the pump pressure decreases.
When the pump pressure decreases, the signal pressure oil in line (32) combines with the force of springs (15) . This moves flow compensator spool (12) downward. This allows pump oil pressure in chamber (29) to vent to the hydraulic tank. This causes the pump to upstroke again.
The continuous rise of the flow compensator spool and fall of the flow compensator spool will maintain the pump pressure in passage (36) . The pressure in passage (36) is maintained at 2100 ± 105 kPa (305 ± 15 psi) above the signal pressure in chamber (15) . The force of springs (15) is equal to 2100 ± 105 kPa (305 ± 15 psi). This difference is called the margin pressure.
Destroking
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| Illustration 4 | g00293180 |
Steering Pump and Pressure and Flow Compensator Valve (Destroking) (2) Pump (3) Swashplate (7) Spring (8) Small actuator piston (9) Pressure and flow compensator valve (12) Flow compensator spool (margin spool) (15) Springs (17) Large actuator piston (18) Spring (21) Pressure compensator spool (cutoff spool) (24) Spring (26) Hydraulic tank for the steering and brake systems (27) Return line to the hydraulic tank (28) Drain line from the large actuator piston to the hydraulic tank (29) Chamber for the large actuator piston (30) Passage to the large actuator piston (31) Connecting passage between the pressure valve and the flow valve (32) Line for the signal oil from the control valve (33) Supply oil line for the steering pump (34) Spring chamber for the small actuator piston (35) Pressure oil passage to the flow compensator valve (36) Pressure oil passage to the flow compensator spool (37) Pump case drain line (38) Connecting passage between the flow compensator spool and the pressure compensator spool (39) Pressure oil passage to the pressure compensator spool (40) Pressure oil passage to the pressure compensator valve (41) Passage to the small actuator piston (42) High pressure oil line to the steering control valve (A) Pressure oil (B) Signal oil (C) Return oil (D) Supply oil | |
Destroking is a reduction in oil pressure when pump output decreases. Destroking occurs when the signal oil pressure through line (32) decreases from lower loads. While the steering control valve is in the NEUTRAL position, destroking occurs when the signal oil pressure through line (32) decreases to 0 kPa (0 psi) .
When the pump oil pressure in passage (36) is greater than the force of signal oil pressure and of springs (15) , flow compensator spool (12) will move upward. This allows pressure oil in passage (35) to flow past spool (12) . The oil then flows through passages (30) and (31) . The oil then flows into the chamber of large actuator piston (29) .
Large actuator piston (17) is larger than small actuator piston (8) . Because of this difference in size, the oil pressure that is acting against the large actuator piston exerts a greater amount of force than the combined oil and spring force that is acting against the small actuator piston.
The oil pressure that is acting against the large actuator piston overcomes the force of spring (18) and the combined force of the oil and spring in chamber (34) . This causes large actuator piston (17) to move down.
As large actuator piston (17) moves down, swashplate (3) moves toward the minimum angle. This causes the pump to destroke. As the angle of the swashplate moves toward the minimum angle, the pump output and the pump pressure decreases.
Low Pressure Standby
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| Illustration 5 | g00293183 |
Steering Pump and Pressure and Flow Compensator Valve (Low Pressure Standby) (2) Pump (3) Swashplate (7) Spring (8) Small actuator piston (9) Pressure and flow compensator valve (12) Flow compensator spool (margin spool) (15) Springs (17) Large actuator piston (18) Spring (21) Pressure compensator spool (cutoff spool) (24) Spring (26) Hydraulic tank for the steering and brake systems (27) Return line to the hydraulic tank (28) Drain line from the large actuator piston to the hydraulic tank (29) Chamber for the large actuator piston (30) Passage to the large actuator piston (31) Connecting passage between the pressure valve and the flow valve (32) Line for the signal oil from the control valve (33) Supply oil line for the steering pump (34) Spring chamber for the small actuator piston (35) Pressure oil passage to the flow compensator valve (36) Pressure oil passage to the flow compensator spool (37) Pump case drain line (38) Connecting passage between the flow compensator spool and the pressure compensator spool (39) Pressure oil passage to the pressure compensator spool (40) Pressure oil passage to the pressure compensator valve (41) Passage to the small actuator piston (42) High pressure oil line to the steering control valve (A) Pressure oil (B) Return oil (C) Supply oil | |
While the engine is running and the steering control valve is in the NEUTRAL position, there is no signal oil pressure in line (32) . Because there is no signal oil pressure in the chamber of springs (15) , the pump oil pressure in passage (36) overcomes the force of springs (15) . The pump oil pressure moves flow compensator spool (12) upward.
Oil flows through passage (35) , past spool (12) , through passages (31) and (30) , and into the chamber of large actuator piston (29) . The oil acts against the large actuator piston. The oil overcomes spring (18) . This causes the piston to move down.
When the large actuator piston moves downward, the piston moves swashplate (3) toward the minimum angle. The piston moves the swashplate toward the minimum angle until pump output can maintain 3755 ± 1240 kPa (545 ± 180 psi) in the system.
Note: Low pressure standby is not the same pressure as the margin pressure. Margin pressure equals 2100 ± 105 kPa (305 ± 15 psi). This is the signal pressure that is required to compress springs (15) . However, the pressure is inadequate to overcome the force of spring (7) . Spring (7) will cause small actuator piston (8) to move. Pump output increases until the signal oil pressure increases, and until the signal oil pressure causes the large actuator piston to move the swashplate. The swashplate moves until the low pressure standby pressure of 3755 ± 1240 kPa (545 ± 180 psi) is reached. Margin pressure can only be measured in a nonstall load sensing condition. Some variation in low pressure standby can occur from minimum engine rpm to maximum engine rpm. For more information, refer to the following Service Manual: 994 Wheel Loader Steering System, Systems Operation, and Testing and Adjusting, RENR2509, "Steering System Pressures - Test and Adjust" .
High Pressure Stall
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| Illustration 6 | g00519674 |
Steering Pump and Pressure and Flow Compensator Valve (High Pressure Stall) (2) Pump (3) Swashplate (7) Spring (8) Small actuator piston (9) Pressure and flow compensator valve (12) Flow compensator spool (margin spool) (15) Springs (17) Large actuator piston (18) Spring (21) Pressure compensator spool (cutoff spool) (24) Spring (26) Hydraulic tank for the steering and brake systems (27) Return line to the hydraulic tank (28) Drain line from the large actuator piston to the hydraulic tank (29) Chamber for the large actuator piston (30) Passage to the large actuator piston (31) Connecting passage between the pressure valve and the flow valve (32) Line for the signal oil from the control valve (33) Supply oil line for the steering pump (34) Spring chamber for the small actuator piston (35) Pressure oil passage to the flow compensator valve (36) Pressure oil passage to the flow compensator spool (37) Pump case drain line (38) Connecting passage between the flow compensator spool and the pressure compensator spool (39) Pressure oil passage to the pressure compensator spool (40) Pressure oil passage to the pressure compensator valve (41) Passage to the small actuator piston (42) High pressure oil line to the steering control valve (A) Pressure oil (B) Signal oil (C) Return oil (D) Supply oil | |
When the hydraulic system stalls under a load in the steering circuit, the oil pressure increases. A stall occurs when pump oil pressure reaches 29000 kPa (4200 psi). The signal oil pressure in line (32) , and in the chamber of springs (15) becomes equal to the pump output pressure.
Springs (15) and the signal oil in the spring chamber keep flow compensator spool (12) moved downward. Pressure oil in passage (38) flows through the orifice in passage (39) . The pressure oil then acts against pressure cutoff spool (21) .
As the pump oil pressure in passage (39) reaches approximately 29000 kPa (4200 psi), the pressure overcomes the force of spring (24) . This causes pressure cutoff spool (21) to move upward.
As the spool moves upward, the spool allows the pump oil to flow through passage (40) , past the spool, through passage (30) , and into the chamber for the large actuator piston (29) . The oil in chamber (29) overcomes the force of spring (18) . This causes large actuator piston (17) to move downward.
As the piston moves downward, the piston moves swashplate (3) . The swashplate moves toward the destroked position to a point when the pump output flow is enough to compensate for system leakage, and when the pump output flow is enough to maintain the system pressure at approximately 29000 kPa (4200 psi) .
If the steering system remains in a stall condition, the pump output is not enough to maintain the system pressure and the pump output is not enough to compensate for system leakage. When the load on the steering system that is causing the stall is removed, the pressure decreases below approximately 29000 kPa (4200 psi). The force of spring (24) moves pressure cutoff spool (21) downward.
When the pressure cutoff spool moves downward, the pressure cutoff spool blocks the flow of oil to large actuator piston (17) . As the pump pressure decreases, the pressure cutoff spool moves downward. This causes the pressure cutoff spool to open piston chamber (29) . The oil then flows to passage (31) and flow compensator spool (12) .
As system pressure reaches margin pressure or low pressure standby, and if there is no signal oil pressure, the flow compensator spool moves to the METERING position. The swashplate will maintain a slight angle that sufficiently compensates for system leakage. The swashplate will also maintain the lower pressure requirement.