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| Illustration 1 | g01074766 |
Variable Displacement Piston Pump and Pump Control Valve (1) Drive shaft (2) Pump. (3) Swashplate (4) Retaining plate (5) Pistons (nine) (6) Cylinder barrel (7) Spring (8) Upstroke piston (9) Pump control valve (10) Inlet from tank (11) Passages for the pump discharge pressure. (12) Flow compensator spool (13) Orifice of the return for the tank (14) Signal pressure port (15) Springs (16) Flow compensator valve (17) Destroke piston (18) Spring (19) Outlet for the pump discharge pressure (20) Tank return passage (21) Pressure cutoff spool (22) Passage to large actuator piston (23) Pressure compensator piston (24) Spring (25) Pressure compensator valve | |
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| Illustration 2 | g00118117 |
Location of Components (2) Steering pump. (9) Pump control valve. | |
The steering pump is an axial piston pump. The output of pump (2) is controlled by pump control valve (9). The compensator valve senses pressure requirements for the system and flow requirements for the system. The pump provides the high pressure oil for the steering system. The pump also provides a backup source for pilot oil. The pump is mounted to the pump drive housing.
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.
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 displaced by the pump for each revolution of the drive shaft.
When the swashplate approaches a certain angle, oil is drawn into the pump and oil is forced out of the pump.
When the swashplate angle is zero, the pistons do not move in and out. Therefore, no oil is drawn into the pump or out of the pump. In this condition, the pump is at zero displacement.
Since the pump is not discharging 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.
Pump control valve (9) keeps the pump pressure at a level that is needed in order to fulfill the needs of the steering system. The pump control valve also keeps the flow at a similar level. The pump has a control mechanism that contains two control pistons. The two pistons work together in order to adjust the angle of the pump's swashplate.
Upstroke 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.
Destroke piston (17) causes the swashplate to destroke the pump. Flow compensator spool (12) and/or pressure cutoff spool (21) changes the pump displacement by regulating the amount of pump discharge pressure that is acting on the destroke piston.
When the oil from the pump acts on the piston and on the spring, piston (17) overcomes the force of smaller upstroke piston (8) and the force of spring (7). Destroke 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 cutoff spool (21) prevents overloads of the pump and of the system. When the pressure of the pump output exceeds 31000 kPa (4500 psi), pressure cutoff spool (21) will override flow compensator spool (12) and the output of the lower pump so that the system pressure is maintained at 31,000 kPa (4500 psi). This takes place while the pump is destroked to a minimum swashplate angle.
Pump outlet pressure is maintained at 2100 ± 105 kPa (305 ± 15 psi) above the signal oil pressure by flow compensator spool (12). The pump flow depends on the external orifice that is located on the steering control valve. The flow rate from the pump to the steering cylinders produces a pressure difference at the orifice. The pressure difference is between the pump pressure and the load pressure. Flow control maintains a constant pressure difference at the orifice. Therefore constant flow is maintained.
For this pump, the differential pressure setting is 2100 ± 100 kPa (305 ± 15 psi). The preset reference pressure difference corresponds to the required flow rate. If the differential pressure rises to the preset pressure difference, the swashplate swivels back to a smaller angle. If the differential pressure drops, the swashplate swivels to a bigger angle until balance is restored within the valve.
Upstroking
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| Illustration 3 | g01065786 |
Operation of Steering Pump and Pump Control Valve (Upstroking) (2) Pump. (3) Swashplate. (7) Spring. (8) Upstroke piston. (9) Pump control valve. (12) Flow compensator spool. (15) Springs. (17) Destroke piston. (18) Spring. (19) Outlet for the pump discharge pressure. (21) Pressure cutoff spool. (24) Spring. (26) Hydraulic tank for the steering and brake system. (27) Return oil line. (28) Drain line to tank. (29) Chamber of the destroke piston. (30) Passage to destroke piston. (31) Connecting passage between the pressure valves and the flow valves. (32) Signal for the oil line of the steering control valve. (33) Pump for supply oil line. (34) Spring chamber of the upstroke piston. (35) Passage to pump control valve. (36) Passage to flow compensator spool. (37) Case drain line. (38) Passage. (39) Passage to pressure cutoff spool. (40) Passage to pressure cutoff valve. (41) Passage to upstroke piston. (42) Line to steering control valve. (43) Adjustable external orifice in the steering control valve. (A) Pressure oil. (B) Signal oil. (C) Return oil. | |
When signal oil pressure combines with the force of springs (15), flow compensator spool (12) moves downward. Then, 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 destroke 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) and into chamber (34). The oil in the chamber of destroke piston (29) is now vented. The combined force of spring (7) and of the pump oil in chamber (34) causes piston (8) to move swashplate (3) toward the maximum angle.
When the force of pressure that is acting on spool (12) becomes greater than the force of springs (15) and of the force of signal oil in the chamber of spring (15), the compensator spool moves 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 destroke piston (29) .
Destroke piston (17) is larger than upstroke piston (8). Because of this difference in size, the force of oil pressure that is acting against the destroke piston exerts a greater amount of force than the combined forces that are acting against the upstroke piston.
The oil pressure that is acting against the destroke piston overcomes the force of spring (18), and the combined force of the oil and of the spring in chamber (34). This causes destroke piston (17) to move downward.
As destroke piston (17) moves downward, 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 flow 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 the chamber of the destroke piston to vent to the 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) should equal 2100 ± 105 kPa (305 ± 15 psi). This pressure is greater than the signal pressure in the chamber of springs (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 | g01066072 |
Operation of Steering Pump and Pump Control Valve (Destroking) (2) Pump. (3) Swashplate. (7) Spring. (8) Upstroke piston. (9) Pump control valve. (12) Flow compensator spool. (15) Springs. (17) Destroke piston. (18) Spring. (19) Outlet for the pump discharge pressure. (21) Pressure cutoff spool. (24) Spring. (26) Hydraulic tank for the steering and brake systems. (27) Return oil line. (28) Drain line to the tank. (29) Chamber of the destroke piston. (30) Passage to destroke piston. (31) Connecting passage between the pressure valves and the flow valves. (32) Signal for the oil line of the steering control valve. (33) Pump for the supply oil line. (34) Spring chamber of the upstroke piston. (35) Passage to pump control valve. (36) Passage to flow compensator spool. (37) Case drain line. (38) Passage. (39) Passage to pressure cutoff spool. (40) Passage to pressure cutoff valve. (41) Passage to upstroke piston. (42) Line to steering control valve. (43) Adjustable external orifice in the steering control valve. (A) Pressure oil. (B) Signal oil. (C) Return oil. | |
Destroking occurs when the differential pressure across the orifice reaches the margin pressure setting. 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 and oil pressure in passage (35) flows past spool (12). The oil then flows through passages (30) and (31). The oil then flows into the chamber of destroke piston (29) .
Destroke piston (17) is larger than upstroke piston (8). Because of this difference in size, the oil pressure that is acting against the destroke piston exerts a greater amount of force than the combined forces that are acting against the upstroke piston.
The oil pressure that is acting against the destroke piston overcomes the force of spring (18) and the combined force of the oil and of the spring in chamber (34). This causes destroke piston (17) to move downward.
As destroke piston (17) moves downward, 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 flow decreases.
Low Pressure Standby
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| Illustration 5 | g00118120 |
Operation of the Steering Pump and Pump Control valve (Low Pressure Standby) (2) Pump. (3) Swashplate. (7) Spring. (8) Upstroke piston. (9) Pump control valve. (12) Flow compensator spool. (15) Springs. (17) Destroke piston. (18) Spring. (19) Outlet for the pump discharge pressure. (21) Pressure cutoff spool. (24) Spring. (26) Hydraulic tank for the steering and brake systems. (27) Return oil line. (28) Drain line to tank. (29) Chamber of the destroke piston. (30) Passage to destroke piston. (31) Connecting passage between the pressure and the flow valves. (32) Signal for the oil line of steering control valve. (33) Pump for the supply oil line. (34) Spring chamber of the upstroke piston. (35) Passage to pump control valve. (36) Passage to flow compensator spool. (37) Case drain line. (38) Passage. (39) Passage to pressure cutoff spool. (40) Passage to pressure cutoff valve. (41) Passage to upstroke piston. (42) Line to steering control valve. (A) Pressure oil. (B) Signal 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 destroke piston (29). The oil acts against the destroke piston. The oil overcomes spring (18). This causes the piston to move downward.
When the destroke piston moves downward, the destroke piston moves swashplate (3) toward the minimum angle. The piston moves the swashplate toward the minimum angle until pump output can maintain around 2400 kPa (350 psi) in the system.
Note: Low pressure standby is not the same pressure as the margin pressure. Margin pressure equals 2100 ± 100 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 upstroke piston (8) to move. Pump output increases until the signal oil pressure increases and until the signal oil pressure causes the destroke piston to move the swashplate. The swashplate moves until the low pressure standby pressure of about 2400 kPa (350 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. To adjust margin pressure, refer to Testing and Adjusting, SENR1373, "Steering Pump Pump Control Valve - Adjust" for your machine.
High Pressure Stall
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| Illustration 6 | g00118121 |
Operation of the Steering Pump and Pump Control Valve (High Pressure Stall) (2) Pump. (3) Swashplate. (7) Spring. (8) Upstroke piston. (9) Pump control valve. (12) Flow compensator spool. (15) Springs. (17) Destroke piston. (18) Spring. (19) Outlet for the pump discharge pressure. (21) Pressure cutoff spool. (24) Spring. (26) Hydraulic tank for the steering and brake systems. (27) Return oil line. (28) Drain line to tank. (29) Chamber of the destroke piston. (30) Passage to destroke piston. (31) Connecting passage between the pressure and the flow valves. (32) Signal for the oil line of the steering control valve. (33) Pump for the supply oil. (34) Spring chamber of the upstroke piston. (35) Passage to pump control valve. (36) Passage to flow compensator spool. (37) Case drain line. (38) Passage. (39) Passage to pressure cutoff spool. (40) Passage to pressure cutoff valve. (41) Passage to upstroke piston. (42) Line to steering control valve. (A) Pressure oil. (B) Signal oil. (C) Return 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 31000 kPa (4500 psi).
As the pump oil pressure in passage (39) reaches 31000 kPa (4500 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 destroke piston (29). The oil in chamber (29) overcomes the force of spring (18). This causes destroke 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 31000 kPa (4500 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 that is causing the stall is removed, the pressure decreases below 31000 kPa (4500 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 destroke 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 to 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.