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| Illustration 1 | g01074766 |
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Variable displacement piston pump and steering pump control valve (1) Drive shaft (2) Steering 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 | g06155026 |
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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 (16) 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 turn. Nine pistons (5) are held against swashplate (3) by retaining plate (4). The swashplate (3) 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 (5) 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 valve 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, destroke 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
Pump outlet pressure is maintained at
For this pump, the differential pressure setting is
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| Illustration 3 | g01065786 |
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Operation of steering pump and steering 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 (12) 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 return oil line (27). The oil then flows back to hydraulic tank (26).
Oil from the steering pump flows through passage to upstroke piston (41) and into spring 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 upstroke piston (8) to move swashplate (3) toward the maximum angle.
When the force of pressure that is acting on flow compensator spool (12) becomes greater than the force of springs (15) and of the force of signal oil in the chamber of spring (15), the flow compensator spool moves upward.
When the flow compensator 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 (12) 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 steering 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 hydraulic tank. This causes the steering 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
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| Illustration 4 | g01066072 |
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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 adjustable external orifice (43) reaches the margin pressure setting. While the steering control valve is in the NEUTRAL position, destroking occurs when the adjustable external orifice (43) closes in order to compensate only for leakage.
When the steering 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 flow compensator 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 steering pump to destroke. As the angle of the swashplate moves toward the minimum angle, the steering pump output flow decreases.
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| Illustration 5 | g01455469 |
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Operation of the steering pump and steering pump control valve (high-pressure stall) (2) Steering 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
As the pump oil pressure in passage (39) reaches
As the pressure cutoff spool moves upward, the pressure cutoff 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 destroke piston moves downward, the destroke piston moves swashplate (3). The swashplate moves toward the destroked position to a point when the steering pump output flow is enough to compensate for system leakage and when the steering pump output flow is enough to maintain the system pressure at
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
When the pressure cutoff spool (21) 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, the flow compensator spool (12) moves to the METERING position. The swashplate (3) will maintain a slight angle that sufficiently compensates for system leakage. The swashplate will also maintain the lower pressure requirement.