510 Auto Fundamentals
to the steering shaft and steering wheel. The
bottom of the bar is connected to the pinion on
rack-and-pinion systems, or to the worm gear
on parallelogram steering systems. The torsion
bar is also attached to the inner spool valve at
the bottom and the outer spool at the top. The
more torque the driver uses to turn the steering
wheel, the more the bar twists. When the steer-
ing wheel is turned, the torsion bar turns the
pinion shaft or worm gear. The twisting motion
of the torsion bar changes the relative posi-
tions of the inner and outer spools of the valve.
Changing the relative positions of the inner and
outer spools of the valve uncovers internal pres-
sure ports, redirecting oil pressure.
Note
What happens if the torsion bar breaks?
The torsion bar is mechanically connected
to both the steering shaft and the pinion
(rack-and-pinion systems) or worm gear
(parallelogram systems). In addition, small
projections, or tangs, connect across the
torsion bar. The tangs allow some movement
to permit torsion bar action. Even if the
torsion bar breaks, the tangs still provide
manual turning force.
Steering
shaft
Reservoir
Outer
tie rod
Power
steering
pump
Power rack-and-pinion
steering gear
Figure 25-22.
Overall view of a power rack-and-pinion steering system showing hose and tubing routing between the pump, rack-and-
pinion gear, and reservoir.
Toyota
Steering shaft
Upper
groove
in valve
housing
Stud
Spiral
groove
Outer
spool
Discharge
connection
Groove
in valve
Flange
can
Torsion
rod
Pinion
Sleeve
Lower
groove
in valve
housing
Inner
spool
Figure 25-23.
Cutaway view of a spool valve and housing used on one
type of power rack-and-pinion steering system.
Volvo