Pontiac FIREBIRD 1972 Service Manual page 1166

STEERING 9-13
Variable ratio steering is accomplished by a pitman shaft
sector incorporating a short tooth on either side of a long
center tooth, rather than a sector with three teeth of equal
length as in a constant ratio gear (Fig. 9-9). Companion
changes are also made in the rack-piston teeth.
The rotary valve power steering gear assembly operates
entirely on displacing oil to provide hydraulic oil pressure
assists only when turning. As the entire gear assembly is
always full of oil, all internal components of the gear are
immersed in oil, making periodic lubrication unnecessary.
In addition this oil acts as a cushion to absorb road shocks
that may be transmitted to the driver.
The steering shaft, hydraulic valve, worm and the rack-
piston nut are all in line making a compact and space-
saving gear. All oil passages are internal except the
pressure and return hoses between the gear and pump.
The rotary valve feature provides a smooth transmission
through the driving range of steering wheel effort. A tor-
sion bar transmits the road feel to the driver. Response of
the steering gear to effort applied to the steering wheel is
greatly increased. This increased response gives the driver
greater control and minimizes over-steering.
The gear is designed to utilize a minimum number of parts
to reduce over-all size and weight (Fig. 9-10). The sim-
plicity of design also reduces the number of special tools
required to service the unit. Being a self-bleeding unit the
steering gear requires no external bleeding.
The rotary valve is shown schematically in Figs. 9- 11,9-12
and 9-13 and is an open-center, rotary-type valve. The
valve spool is inside the valve body and is held in a neutral
position by a torsion bar attached to one end of the valve
body through the torsion bar cap and exterids through this
valve. The other end of the torsion bar is attached to a stub
shaft which in turn is splined to the gear flange that bolts
to the steering shaft flange.
Twisting of the torsion bar allows the valve spool to dis-
place or move its position in relation to the valve body,
thereby directing oil to the proper area in the gear to
provide a hydraulic assist on turns. During the turn the
steering worm turns in the same direction as the turn. This
causes the rack-piston nut to move which in turn applies
a turning effort to the pitman shaft gear.
OPERATION
OPERATING PRESSURES
Under normal driving conditions, the hydraulic oil pres-
sure in the power cylinder should not exceed 40-100 psi.
Pressure for turning corners should be approximately 100-
600 psi. Parking pressure, the most difficult of turning
conditions, should range from 600 psi to 1350 psi, depend-
ing upon roadbed conditions and the weight of the car.
OIL FLOW-STRAIGHT-AHEAD POSITION
The rotary valve contains a valve spool which is a selective
slip fit inside the valve body and is positioned so the
grooves and lands on the outside surface of the valve spool
L
.
7
PRESSURE RETURN
Fig. 9-1 1 Oil Flow
-
Straight Ahead Position