Vehicle Systems
Hydraulic System Operation
37
Oil Flow
The oil from the priority valve flows through inlet (4).
When the steering wheel is stationary (NEUTRAL),
the oil is stopped by spool (1). The oil can not flow
through the steering unit to the steering cylinder until
the steering wheel is turned.
The steering wheel is connected to spool (1) by a
shaft assembly and splines. When the steering
wheel is turned, spool (1) turns a small amount until
springs (7) are compressed. Then, sleeve (2) starts
to turn.
As long as the steering wheel is turned, the spool
and sleeve both turn as a unit, but they turn a few
degrees apart.
When the spool and sleeve are a few degrees apart,
oil passages are opened between them. This lets
the pump oil from inlet (4) flow through passages in
body (11) to the metering section.
When the steering wheel is turned, pin (8) turns with
the sleeve and causes drive (12) to turn also. The
drive causes a rotation of gear (5) inside gear (6).
This rotation of the gear sends a controlled
(metered) flow of pilot oil back through body (11).
This oil flows to port (9) or (10) and then to the
steering cylinder.
Port (9) or (10), that is not used for pressure oil to
the steering cylinder, is used for return oil from the
other end of the steering cylinder
Pump Gears In Metering Section
(5) Internal pump gear. (6) External pump gear. (8) Pin. (12) Drive.
If the steering wheel rotation is stopped, springs (7)
will move sleeve (2) back in alignment with spool (1)
(NEUTRAL position). This will close passages
between the metering section and control section
and the steering unit will be in the NEUTRAL
position.
When the engine is off, the steering unit can be
manually operated. The control section will work as
a pump. The oil that is returned from the steering
cylinder is not returned to the tank. The suction of
the control section will open an internal check valve
and let return oil from the steering cylinder go to the
inlet side of the control section. During power
operation, supply pressure keeps the check valve
closed.
