789D Off-Highway Truck : Piston Pump (Steering)

Inside of right hand frame rail (1) Steering piston pump(2) Pressure compensator valve

Steering piston pump (1) is a pressure compensating variable displacement axial piston pump. The steering piston pump is fastened to the pump drive. The pump drive is located on the inside right hand frame rail near the torque converter. Steering piston pump (1) only operates when the engine is running.

Section view of the steering piston pump (3) Pump drive shaft (4) Swashplate (5) Swashplate piston (6) Piston (7) Barrel (8) Inlet opening(9) Actuator piston

(2) Pressure compensator valve (10) Low pressure cut-in valve(11) High pressure cutoff valve

When the engine is started, the pump drive turns pump drive shaft (3) and barrel (7) with nine pistons (6). The ends of pistons (6) connect to a retraction plate and slippers. The retraction plate and slippers turn with barrel (7). The retraction plate and slippers are against swashplate (4). Swashplate (4) does not turn. Swashplate (4) can pivot in order to change the swashplate angle.

When pump drive shaft (3) and barrel (7) turn with swashplate (4) at the maximum swashplate angle, pistons (6) move in and out of barrel (7) as the slippers follow the angle of the swashplate. When the swashplate (4) is at the maximum angle, the steering piston pump is at maximum flow. Since the steering piston pump is driven by the engine, the engine rpm also affects pump output flow. As pistons (6) move out of barrel (7), oil is pulled from the steering tank through inlet opening (8) and into the piston cylinder in the barrel. As barrel (7) turns farther, pistons (6) are moved back into barrel (7). This pushes the oil from the piston cylinder in barrel (7). The oil flows through the outlet opening and into the steering hydraulic system.

The steering solenoid and relief valve receives the pressure oil from the steering piston pump (1). The oil then flows to the accumulators, to the accumulator charging valve, and to the steering control valve. The oil that flows to the accumulator charging valve is used as signal oil for the accumulator charging valve. While the steering accumulators are filling, the accumulator charging valve will be closed. The accumulator charging valve is also connected to the signal line for the pressure compensator valve. When the accumulator charging valve is closed, pressure will build up in the signal line for the pressure compensator valve. The oil in the signal line for the pressure compensator valve acts on low pressure cut-in valve (10). The spool of the low pressure cut-in valve has pressure at both ends. The signal pressure on this spool causes the oil in actuator piston (9) to drain to the tank. The spring that is on swashplate piston (5) holds swashplate (4) at the maximum position.

When the accumulators are full, the steering piston pump oil pressure will increase until the pressure in the signal line for the accumulator charging valve reaches the cutoff pressure. The cutoff pressure is approximately 18300 kPa (2650 psi). When this pressure is reached, the spool in the accumulator charging valve shifts. The accumulator charging valve will allow the oil in the signal line for the pressure compensator valve to drain to the steering tank. The spool in low pressure cut-in valve (10) shifts. Oil pressure is sent to actuator piston (9). The oil pressure overcomes the spring force of swashplate piston (5). This creates a minimum swashplate angle in order to destroke the steering piston pump to a low pressure standby condition. Pump output pressure is equal to the setting of low pressure cut-in valve (10). The minimum pressure of the low pressure standby is 2000 kPa (290 psi).

In the NO STEER position, demand for oil from the accumulators is low. The pump operates at a minimum swashplate angle in order to supply oil for lubrication and for steering piston pump leakage. The normal leakage of the steering metering pump is supplied by the accumulator pressure. This leakage is normal. This leakage will cause the steering piston pump to cycle due to a decrease in accumulator pressure.

Because of normal leakage in the steering system, the pressure in the accumulators will gradually decrease to approximately 16400 kPa (2378 psi). This is equivalent to 90 percent of the accumulator charging valve cut-out pressure.

When the pressure in the accumulators decreases to this setting, the accumulator charging valve will block the drain line. Pilot oil will trigger low pressure cut-in valve (10). The steering piston pump shifts back to maximum flow.

MARYGAR

789D Off-Highway Truck : Piston Pump (Steering) 2012-12-12T19:11:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown