Chapter 9 Steering Systems 191
before reaching the pinion gear. The spool valve is con-
nected to the valve body through a torsion bar that allows
the valve to rotate slightly inside the valve body before
transferring motion to the valve body. Figure 9-54 is a cross
section of a typical control valve. Note the relationship of
the fluid passages in the spool valve and valve body. Fluid
flow through the rotary valve depends on how the passages
in the spool valve and the valve body are aligned.
When the steering wheel is in the straight ahead, or
central, position, pressure from the pump circulates
through the valve assembly and returns to the reservoir.
When the steering shaft turns the valve assembly, the spool
valve moves in relation to the valve body before the
turning effort passes through the valve body to the pinion
gear. Note in Figure 9-55A the spool valve has turned
slightly to the left. Fluid passages in the spool valve are
aligned with passages in the valve body that direct fluid to
the side of the power piston that assists a left turn.
Pressurized fluid enters the left turn side of the power
piston, assisting the driver with a left turn. Fluid on the right
turn side of the power piston exhausts through the valve
and valve body passages to the pump reservoir.
In Figure 9-55B the spool valve has turned slightly to
the right. Fluid passages in the spool valve are aligned with
holes in the valve body that direct fluid to the right turn
side of the power piston. Pressurized fluid enters the right
turn side of the power piston, assisting the driver with a
right turn. Fluid on the left turn side of the power piston
returns to the pump reservoir through the valve passages.
When the driver returns the steering wheel to the
straight ahead position, the spool valve returns to its
central position inside of the valve body and fluid pressure
is exhausted from the power piston.
Conventional Linkage Power Steering
Two kinds of power steering are used on conventional
linkage steering systems. The self-contained steering gear
design is in almost universal use. Linkage types are found
on a few vehicles.
Tie rod
Boot Power
piston
Rack
assembly
Rack housing
Tube
Power piston Rack housing
Tie rod
Power piston Rack housing
Boot
Figure 9-53. A—Power piston installed inside the rack housing
cylinder. B—Power piston movement during a left turn.
C—Power piston movement during a right turn. (Hunter)
A
B
C
Torsion bar
Dust cover
Seal
Tubing
connections
Bearing
Cap
Spring
Rack guide
Rack guide seat
Rack housing cap
Nut
Bearing
Gear
housing
O-ring
Control
valve
Bearing
Figure 9-54. A cross-sectional view of a typical control valve
and valve body. (Lexus)