THEORY OF OPERATION - CONTINUED
STEERING SYSTEM - CONTINUED
Pump upstroking occurs because of increased flow demand. Increased flow demand is met by increased
Increased flow demand causes a signal pressure in signal line from metering pump which combines with
force of pressure compensator spring in cavity, causing pump pressure to be 350 ± 15 psi (2,400 ± 100
kPa) greater than signal pressure. However, combination of signal pressure and spring force is greater than
pump discharge pressure. This difference is called margin pressure.
Pressure causes pressure compensator spool to move down, blocking flow of supply oil to actuator piston.
When oil flow to actuator piston is blocked, oil in passage from actuator piston drains to passage, then
flows past flow compensator spool, pressure compensator spool, and through passage into case drain.
Supply oil flows through passage to bias piston. Oil acts against bias piston and combines with force of
bias spring, causing swashplate to upstroke.
This also causes pump flow to increase. As flow requirements are satisfied, pump output pressure
increases until pressure in passage moves flow compensator spool up to metering position.
Slight upward and downward movement is called metering. Metering maintains equal pressure on each
end of flow compensator spool.
With flow compensator spool in position A, pump pressure is greater than combined force of spring and
signal oil pressure in pressure compensator spool cavity. This forces spool to move upward.
When flow compensator spool moves upward, pressure oil flows past spool and through passage, past flow
compensator spool, and into passage to actuator piston, causing pump to destroke.
Area of actuator piston is larger than area of bias piston. This causes force of oil acting against actuator
piston to overcome combined force of spring and oil acting against bias piston. Actuator piston moves left-
ward, causing swashplate to move toward minimum angle.