Wednesday, December 5, 2012

770G and 772G Off-Highway Truck:Piston Pump (Steering, Fan)

(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. 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.

(1) Piston Pump
(2) Pressure and flow compensator valve
(3) Control Cylinder
(4) Springs
(5) Springs
(6) Spool
(7) Spool
(8) Control piston
(9) Swashplate
(B) Angle of swashplate (9) to the vertical
(C) Pump Pressure
(D) Return to tank
(E) Oil passage to the control piston(F) Load sensing line

The piston pump is a variable displacement pump. The output of the piston pump is determined by the angle of swashplate (9) . A greater swashplate angle (B) produces a greater output as the stroke of the pistons is increased. Load sensing pressure and pressure within the pump output line are transmitted to pressure and flow compensator valve (2) . The pressure and flow compensator valve controls the angle of the swashplate.

The position of spool (6) determines whether the angle of the swashplate will increase or whether the angle of the swashplate will decrease. Load sensing pressure and springs (5) act on one side of the spool. Pressure within the output line from the piston pump acts on the other side of the spool.

When the machine makes a turn, increased load sensing pressure moves spool (6) downward. Pressure is relieved from control piston (8) and the piston pump upstrokes until the requirements of the system are satisfied. Angle (B) of the swashplate is increased by spring force and the force due to the oil pressure within control cylinder (3) . The control cylinder contains the same pressure as the pump output line.

When the steering wheel is stationary, the requirements of the system are reduced. Pump pressure moves spool (6) upward against reduced load sensing pressure and springs (5) . Pressure is transmitted to control piston (8) and the piston pump destrokes until the reduced system requirements are satisfied. Angle (B) is at a minimum.

When the requirements of the system are satisfied, spool (6) meters oil flow to control piston (8) . The angle of the swashplate is maintained and the piston pump supplies the necessary oil flow to the system.

Spool (7) limits the maximum pressure which is generated within the system. Excessive pressure moves the spool upward against springs (4) . The spool meters pressure to control piston (8) and the pump destrokes maintaining full system pressure and the system requirements.

MARYGAR