Pontiac FIREBIRD 1972 Service Manual page 1168

STEERING 9-15
the power steering pump through the open position of the
rotary valve (Fig. 9-1 1) and back to the power steering
pump reservoir without circulating in the power cylinder
in which the rack-piston is located. Since all passages are
open, flow resistance is low in the neutral position at all
times except when steering in turns, the power required to
operate the pump is at the minimum.
The power cylinder is full of oil at all times, although in
the straight-ahead position the pressure on both sides of
the rack-piston is equal and very low.
Oil from the steering gear pump flows through a passage
in the gear housing to an annular groove around the valve
body. Four holes evenly spaced around the valve are
drilled from the bottom of this groove inside surface of the
valve body. Eight pressure holes evenly spaced around the
valve body are also drilled through the valve body wall but
these are through a land portion on the inside surface of
the valve body with one hole on each side of the four inlet
pressures holes.
When no twisting force is applied to the steering gear stub
shaft from the steering wheel there is sufficient clearance
between the land groove alignment of the valves to permit
oil to flow between the valves. Oil flows back to the pump
via four drilled holes through the valve spool wall that
align with a groove on the stub shaft assembly. From here
oil flows around the stub shaft to an area between the
rotary valve and adjuster plug, through the return port to'
the pump.
OIL FLOW-RIGHT TURN POSITION
When a right turn is executed, oil from the power steering
pump flows through the rotary valve, through the steering
gear housing to an area between the housing end plug and
the rack-piston nut to assist in forcing the rack to turn the
pitman shaft and steering leakage for assist in the turn.
When the steering wheel is turned to the right, resistance
to turning is encountered between the front wheels and the
roadbed, tending to twist the stub shaft. Since the stub
shaft is pin-locked to the torsion bar at one end and the
opposite end indexes the valve spool by a pin on the stub
shaft, the twisting action moves the valve spool to the right
in relation to the valve body. This slight movement causes
the land in the valve spool to restrict the right side opening
between the valve spool land and valve body lands and
opens the clearance on the left side of the spool lands (Fig.
9- 12).
The right openings being restricted permit oil to flow
through the unrestricted passages to the left (Fig. 9-12) to
an annular groove around the valve body which aligns
with an oil passage in the gear housing.
Oil is then directed to flow between the housing end plug
and the rack-piston nut to force the rack upward permit-
ting the steering worm to screw into the rack-piston nut.
This forces the pitman shaft to turn and reduces driver
turning effort in executing the right turn. The oil in the
upper end of the cylinder is simultaneously forced out
through the rotary valve and back to the pump reservoir.
The higher the resistance to turning between the roadbed
and the front wheels, the more the valve spool is displaced,
and the higher the oil pressure on the lower end of the
rack-piston nut. Since the amount of valve displacement
and, consequently, the amount of hydraulic pressure built
in the cylinder is dependent upon the resistance to turning,
the driver is assured of the proper amount of smooth
hydraulic assistance at all times.
As the driver stops applying steering effort to the steering
wheel and then relaxes the wheel, the spool valve is forced
back into its neutral position by the untwisting of the
torsion bar. The spool valve lands and grooves align them-
selves with the grooves and lands in the valve body, prov-
iding a balanced clearance between the land-groove
alignment. When this happens, the oil pressure is again
equal on both sides of the rack-piston nut and the steering
geometry of the car causes the wheels to return to the
straight-ahead position.
OIL FLOW-LEFT TURN POSITION
Executing a left turn causes oil to flow from the power
steering pump through the rotary valve and to the area
between the rotary valve and the rack-piston nut rack via
a drilled passage in the steering gear housing. This is to
assist in forcing the rack nut piston to turn the pitman
shaft and linkage.
When the steering wheel is turned to the left, resistance to
turning is encountered between the front wheels and the
roadbed tending to twist the stub shaft. Since the stub
shaft is pin-locked to the torsion bar at one end and the
opposite end indexes the valve spool by means of a pin the
the stub shaft, the twisting action moves the valve spool
to the left in relation to the valve body. This slight move-
ment causes the land on the valve spool to restrict the left
side opening between valve spool lands and the valve body
lands and opens the clearance on the right side of spool
lands (Fig. 9-13).
The left openings being restricted permit oil to flow
through the unrestricted passages to the right to an annu-
lar groove around the valve body which aligns with an oil
passage in the gear housing.
Oil is then directed to flow between the rotary valve and
the rack-piston nut via a drilled passage in the steering
gear housing to force the rack- piston nut downward. This
forces the pitman shaft to turn and reduces driver turning
effort in executing the left turn. The oil in the lower end
of the housing is simultaneously forced out through the
rotary valve from a drilled passage in the housing and
back to the pump reservoir.