THEORY OF OPERATION - CONTINUED
STEERING SYSTEM - CONTINUED
Low-pressure standby describes a running engine and inactive steering. There are no flow or pressure
demands on pump, therefore there is no signal pressure in metering pump signal line.
Before engine is started, bias spring holds swashplate at maximum angle. As pump begins to turn, oil
begins to flow and system pressure increases because of closed centered steering control valves.
Pressure in flow compensator spool passage is sensed at bottom of flow compensator spool and bottom of
pressure compensator spool. As this pressure increases, it pushes flow compensator spool against flow
compensator spring, causing spool to move upward. This opens passage to allow pressure oil to flow to
Oil acts against actuator piston to overcome force of bias spring, moving actuator piston left which moves
swashplate toward minimum angle. Actuator piston continues to move leftward until cross-drilled hole is
uncovered, allowing oil to drain to case.
N OT E
Low-pressure standby will vary in the same pump as the system leakage or pump leakage increases. The
pump will upstroke slightly to compensate for increasing leakage. Actuator piston will cover more of the
cross-drilled hole. As this happens, low-pressure standby will drop toward margin pressure. The leakage
will reach a point when the piston completely covers the cross-drilled hole. This is necessary because of the
increased swashplate angle required. When this happens, low-pressure standby will equal the margin pres-
Cross-drilled hole limits maximum leftward actuator piston travel. Pump supplies an amount of flow suffi-
cient to compensate for system leakage and leakage to pump case as a result of cross-drilled hole. Pump
maintains low-pressure standby. Low-pressure standby pressure should not exceed 580 psi (4,000 kPa).