Friday, November 30, 2012

769D Off-Highway Truck: Piston Pump (Steering)

(1) Steering piston pump(2) Pressure and flow compensator valve

The steering piston pump is a pressure compensated variable displacement axial piston pump. The steering piston pump is fastened to the rear of the hoist and brake gear pump. The hoist and brake gear pump is mounted to the left side of the flywheel housing. Steering piston pump (1) only operates when the engine is running. If the truck is moving and the engine stops, the secondary steering electric drive motor will automatically activate. When the steering wheel is moved, the secondary steering section of the electric drive pump for the secondary steering and brake will send a limited amount of oil to the steering cylinders in order to turn the truck.

(3) Pump drive shaft
(4) Swashplate
(5) Swashplate piston
(6) Pistons
(7) Barrel(8) Actuator piston

(2) Pressure and flow compensator valve
(9) Flow compensator valve(10) High pressure cutoff valve

When the engine is started, the hoist and brake gear pump 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 hydraulic tank through the inlet opening 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 check and relief valve receives the pressure oil from the steering piston pump (1). The pressure oil flows through the steering check and relief valve to the steering metering pump. When the steering wheel is turned and the steering cylinders need oil, the steering metering pump sends pressure oil through a load sensing line to pressure and flow compensator valve (9). The oil in the load sensing line for pressure and flow compensator valve (9) acts on flow compensator valve (9). The spool of flow compensator valve (9) has pressure at both ends. The load sensing pressure on this spool shifts the spool in order to allow the oil on actuator piston (8) to drain to the tank. The spring that is on swashplate piston (5) holds swashplate (4) at the maximum position. The steering piston pump is now at full flow.

If the pressure of the steering system reaches the pressure setting of high pressure cutoff valve (10), the oil pressure will cause the spool to shift. When the spool shifts, oil pressure flows to actuator piston (8). 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 is equal to the pressure setting of high pressure cutoff valve (10) .

In the NO STEER position, oil in the load sensing line from the steering metering pump is at low pressure. The oil pressure from the steering piston pump that is acting on the spool of flow compensator valve (9) will shift the spool. When the spool of flow compensator valve (9) shifts, oil pressure flows to actuator piston (8). 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 is equal to the pressure setting of flow compensator valve (9) .

In the NO STEER position, there is no demand for oil from the steering cylinders. The pump operates at a minimum swashplate angle in order to supply oil for lubrication and for leakage in the steering piston pump. This leakage is normal. This leakage from the case drain flows to the steering hydraulic tank. At the steering hydraulic tank, the oil flows through a screen that is in a fitting on the tank.

MARYGAR