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| Illustration 1 | g03867067 |
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Schematic of the steering pump (drive-through feature) (1) Passage (2) Flow compensator (3) Pressure Compensator (4) Load sensing port from the steering control valve (5) Passage (6) Boost pressure port (7) Oil passage (8) Passage (9) Hydraulic tank (10) Upstroke piston (11) Oil passage (12) Steering pump (13) Swash plate (14) Destroke piston | |
The steering pump is an axial piston pump. The output of pump (12) is controlled by the pump control valve. The pump control valve includes pressure and flow compensators (2) and (3). The pump control 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.
While the engine is running, the drive shaft turns. The drive shaft causes the cylinder barrel to turn. The pistons are held against swash plate (13). Swash plate (13) does not rotate.
As swash plate (13) moves past zero angle, some of pistons are pulled out of the cylinder barrel . At the same time, some of the pistons 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 swash plate (13) determines the amount of oil that is displaced through each barrel for each revolution of the drive shaft. As the steering system load changes, the swash plate angle fluctuates between zero and the maximum angle. When the swash plate increases beyond the zero angle, oil is drawn into the pump and oil is forced out of the pump.
When the swash plate 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.
The pressure compensator (3) prevents the pump pressure from exceeding
Upstroke piston (10) causes swash plate (13) to upstroke the pump. Compensator (3) spring combines with the pump discharge pressure in order to move the swash plate to the maximum angle. This increases pump output.
Destroke piston (14) causes the swash plate to destroke the pump. Pressure and flow compensator (2) and (3) change the pump displacement by regulating the amount of pump discharge pressure that is acting on the destroke piston.
Pressure and flow compensator (2) and (3) prevents overloads of the pump and the system. The pressure compensator prevents the pressure from rising above
Steering Pump in the Upstroking Position
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| Illustration 2 | g03868802 |
Upstroking is an increase of pump displacement due to an increased load. When the system requires more flow, signal oil from the steering control valve flows through port (4).
The signal oil pressure combines with the force of flow compensator (2) spring. This causes flow compensator spool (2) to move to the left. When the spool moves left, the spool blocks the flow of pump oil through passage (1).
While flow compensator spool (2) is to the left, the oil in the chamber of destroke piston (14) can flow into passage (6). The oil flows past the cavity of flow compensator (2) spring, past flow compensator spool (2), and through passage (7). The oil then flows back to hydraulic tank (9).
Oil from the steering pump flows through passage (11) and into the chamber of upstroke piston (10). The combined force of the spring in upstroke piston (10) and of the oil in upstroke piston (10) moves the swash plate toward the maximum angle that causes the pump to upstroke.
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| Illustration 3 | g03868946 |
Destroking is a reduction in pressure when pump output decreases. Destroking occurs when the signal oil pressure through line (4) decreases from lower loads. While the steering control valve is in the NEUTRAL position, and the spool is in the centered position blocking the pump to the actuators. The pump pressure increases in order to overcome the flow compensators and the spool will shift to the right. When the pump oil pressure in passage (8) is greater than the force of signal oil pressure and of pressure compensator (2) spring, flow compensator spool (2) will move to the right and oil pressure in passage (1) flows past spool (2). The oil then flows through passages (5) and (6). The oil then flows into the chamber of destroke piston (14).
Destroke piston (14) is larger than upstroke piston (10). 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 (14) overcomes the combined force of the oil and of the spring in the upstroke piston (10). This causes destroke piston (14) to move to the left.
As destroke piston (14) moves to the left, the swash plate moves toward the minimum angle. This causes the pump to destroke. As the angle of the swash plate moves toward the minimum angle, the pump output and the pump pressure decrease.
Steering Pump During High Pressure Stall
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| Illustration 4 | g03869034 |
When the hydraulic system stalls under a load in the steering circuit, the oil pressure increases. A stall occurs when pump oil pressure reaches
Flow compensator (2) spring and the signal oil in the spring chamber keep flow compensator spool (2) moved to the left. Pressure oil in passage (1) then acts against pressure compensator spool (3).
As the pump oil pressure in passage (1) reaches
As the spool moves to the right, the spool allows the pump oil to flow through passage (1), past the spool, through passage (6), and into the chamber for destroke piston (14). The oil in the chamber of destroke piston (14) overcomes the force of the spring in upstroke piston (10). This causes destroke piston (14) to move to the left.
As destroke piston (14) moves left , the piston moves the swash plate. The swash plate 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
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| Illustration 5 | g03869051 |
When the load that is causing the stall is removed, the pressure decreases below
When the pressure compensator spool moves to the left, the pressure compensator spool blocks the flow of oil to destroke piston (14). As the pump pressure decreases, the pressure compensator spool moves to the left. This causes the pressure compensator spool to open the chamber of destroke piston (14). The oil then flows to passage (1) and to flow compensator spool (2).
As system pressure reaches margin pressure, and if there is no signal oil pressure, flow compensator spool (2) moves to the METERING position. The swash plate will maintain a slight angle that sufficiently compensates for system leakage. The swash plate will also maintain the lower pressure requirement.