THEORY OF OPERATION - CONTINUED
STEERING SYSTEM - CONTINUED
When steering wheel is stationary, steering control valve is in NEUTRAL. Pressure oil from steering pump
enters steering control valve through steering pump inlet.
From steering pump inlet, pressure oil flows through flow control orifice. Flow control orifice reduces
amount of oil flow directed to metering pump. Pressure oil also flows to steering backup relief valve and
Because steering wheel is stationary, there is no flow of steering pilot oil from metering pump. However, a
small quantity of oil flows to hydraulic oil tank through orifices located in steering metering pump.
Any pilot oil that had been acting on an end of selector spool is forced across an orifice, then back to
hydraulic oil tank.
Because there is no pilot oil flow, directional spool is kept in center of steering control valve by spring.
This causes directional spool to block pressure oil flow from steering pump. Because no pressure oil is
allowed to flow through right or left turn steering cylinder passages, steering cylinders do not actuate.
Similarly, if left- or right-turn steering neutralizer valve is actuated, steering pilot oil flow to either end of
selector spool stops.
Pump standby pressure is sensed by steering backup relief valve. Pump standby pressure is connected to
steering metering pump through flow control orifice.
If an operator does not turn steering wheel, steering cylinder position will not change because oil is
blocked in left and right turn steering cylinder passages.
When steering wheel is held stationary, there is no demand for pressure oil. Low signal pressure causes
steering pump to destroke. A small amount of oil flow will still compensate for system leakage.
A hydraulic oil line connects metering pump and steering pump, allowing signal pressure oil to flow to
pressure and flow compensator valve attached to steering pump. If signal oil pressure is low, steering
pump will destroke. If signal oil pressure is high, steering pump will upstroke.
When wheels contact a stationary object, steering cylinders may experience a sudden shock load. Any
pressure generated in steering cylinders from a shock load is sensed in left and right turn steering cylinder
passages. Crossover relief valve senses oil pressure in those passages.
If pressure exceeds steering cylinder crossover relief valve settings, valve opens. This causes oil to flow to
passage with lower pressure.
Makeup check valves connect with left and right turn steering cylinder passages, allowing oil to be drawn
through hydraulic oil tank port. This prevents voiding in steering cylinder that causes crossover relief
valve to open. Makeup check valves are normally seated by spring force.