THEORY OF OPERATION - CONTINUED
HYDRAULIC SYSTEM - CONTINUED
Low Pressure Standby.
When pilot system and steering system piston pump produces flow, system pressure begins to increase
because flow of oil is blocked at steering metering pump. Pilot system does not contain a load signal cir-
Blocked oil creates pressure which is felt at end of flow compensator spool. Flow compensator spool
moves left against margin spring. This permits system oil to flow to actuator piston.
When pressure on actuator piston increases, force of bias spring is overcome and swashplate is moved to a
Piston pump produces enough flow to compensate for normal system leakage when swashplate is at a
slight angle. Also, piston pump has sufficient pressure to provide instantaneous response when steering
metering pump is activated.
If there is no flow demand from a steering metering pump hydraulic circuit, no load sensing signal is gen-
Pressure at pump outlet must only overcome force of margin spring. Pressure at pump outlet is called "low
pressure standby". For pilot system and steering system piston pump, low pressure standby is approxi-
mately 590 ± 30 psi (4,068 ± 207 kPa).
Low pressure standby is higher than margin pressure due to a higher back pressure created by oil which is
blocked at closed center valves when valves are in HOLD position.
N OT E
Amount of system leakage and adjustments to the flow compensator spool can cause margin pressure to
equal low pressure standby. Margin pressure can never be higher than low pressure standby.
When piston pump is at low pressure standby, supply oil from pump outlet pushes flow compensator spool
to left. This will further compress margin spring. An increased amount of supply oil from pump outlet will
flow to actuator piston. This will slightly destroke piston pump.