VALVE ASSY, EGR 7095488C1 - Caterpillar

Illustration 1 g00926091
Schematic of the Steering Metering System for the 554 Forwarder (1) Steering cylinders (2) Implement control valve (3) Steering metering pump (4) Spool (5) Check valve (6) Priority valve (A) Load sensing port "LSS" (B) Signal line to pump (C) Pump pressure lineWhen the engine is running, the implement pump supplies pressure oil to priority valve (6) through pressure line (C). When the steering wheel is not being turned, the valve spool in the priority valve moves to the RIGHT position.A small amount of oil flows through the priority valve to spool (4). This ensures that the supply of oil to the metering pump is always at operating temperature.The steering column is attached to the steering metering pump by a spline coupling. When the operator turns the steering wheel, the spline coupling turns. Spool (4) moves from the HOLD position. The oil pressure at the outlet of the priority valve decreases. The decreased oil pressure causes the valve spool to shift to the LEFT position.With valve spool (6) in the LEFT position, an increased amount of oil flows to the steering metering pump. Pressure oil then flows through the spool to the steering cylinders.As the operator turns the steering wheel faster, the spool moves farther from the HOLD position. The rate of oil flow to the steering cylinders increases and the speed of turning increases. Spool (4) is a modulating spool. As spool (4) moves farther from the HOLD position, more oil flows through spool (4). The modulating spool is represented in the schematic by the adjustable orifice symbol.When the operator turns the steering wheel, signal pressure oil flows from the steering metering pump to priority valve (6) .When the steering system reaches the end of range of articulation or an external force acts on the machine, the oil pressure in the signal circuit increases. The increased pressure in the signal circuit keeps the valve spool in the priority valve shifted to the LEFT position. Pressure oil will continue to be available for flow to the steering metering pump.The maximum pressure in the signal circuit is limited by a pressure reducing valve located on the combination valve. Limiting the signal pressure limits the maximum oil flow to the steering metering pump by controlling the maximum upstroke of the implement pump.If the pressure in the steering circuit suddenly decreases during a turn, check valve (5) opens. The check valve prevents cavitation in the steering metering pump and steering cylinders. Oil from the return lines can flow into the circuit.