Chapter 9 Steering Systems 193
Self-Contained Steering Gear
The self-contained power steering gear is a conven-
tional recirculating-ball steering gear that contains a power
piston and a control valve.
Power Pistons
A piston with Teflon® seals is attached to the ball nut,
dividing the interior of the steering gear into upper and
lower chambers, Figure 9-56. Pressure directed to either of
the power chambers causes it to move the ball nut. Moving
the ball nut moves the sector gear.
When the driver makes a left turn, pressure is directed
to the upper chamber, creating a pressure differential
between the upper and lower chambers. This pressure
differential assists the driver effort in moving the ball nut
down. See Figure 9-57A. The fluid in the lower chamber
exhausts through the passages in the rotary valve.
In Figure 9-57B the vehicle is making a right turn.
Pressure is directed to the lower chamber, causing the ball
nut to be assisted in moving up. Fluid in the lower cham-
ber exits through the rotary valve to the pump reservoir.
Rotary Control Valve
Most self-contained power steering gears use a rotary
valve similar to that used on rack-and-pinion power steer-
ing. The valve assembly consists of a spool valve inside a
valve body. The spool valve can turn a small amount inside
of the valve body. Passages in the valve and valve body
direct fluid to the power chambers. The outer valve spool
turns the stationary steering gear housing, and Teflon®
sealing rings seal the pressure passages
The series of illustrations in Figure 9-58 shows the
action of the rotary valve. Note that when the steering
wheel is in the straight ahead position, fluid circulates
through the valve with no effect on either power chamber.
When the steering wheel is turned to the left, fluid passages
in the valve align with passages in the valve body to direct
fluid to the upper chamber. This presses on the top of the
power piston. Fluid in the lower chamber exhausts through
the valve passages to the pump reservoir.
When a right turn is begun, the valve moves slightly
to the right. Fluid passages in the valve and valve body
align to direct fluid to the lower chamber. Pressurized fluid
against the lower end of the power piston assists the driver
with the turn. Fluid in the upper chamber returns to the
pump reservoir through exhaust passages in the rotary
valve.
When the driver returns the steering wheel to the
straight ahead position, the valve twists to the straight
ahead position inside of the valve body and fluid pressure
is exhausted from the power chambers.
Linear Control Valve
Another power steering gear uses a linear control
valve. The linear control valve assembly is a small spool
valve that can move in a linear direction (back and forth)
in a valve bore. A lever connected to the steering shaft
moves the linear valve. A typical linear valve steering gear
is shown in Figure 9-59. The operation of the power
chambers and pistons is identical to the operation of the
power chambers and pistons used in the rotary valve
power steering gear. This steering gear was used on some
older cars and on pickup trucks until the early 1990s.
When the steering wheel is turned, the resistance of
the sector gear against the ball nut causes the shaft to move
either up or down according to whether the wheel is being
turned right or left. This causes the lever to pivot and move
the valve in its bore. The valve then directs fluid to the
proper power chamber as necessary. Turning the steering
wheel in the opposite direction causes the lever to pivot
and return the valve to the centered position.
Linkage-Mounted Power Steering
Linkage-mounted power steering is a conventional
steering system with a separate power piston and control
valve mounted on the linkage. The steering gear is a
manual design, although it may have a different steering
ratio than one used with manual steering. Linkage power
steering was the first kind of power steering, but it has not
been used on cars since the late 1980s. Pressure from the
power steering pump is delivered to the valve assembly
through high-pressure hoses.
Power Cylinder
The power cylinder is a machined cylinder containing
an internal piston connected to an output rod. The power
cylinder is attached to the steering linkage, usually at the
center of the relay rod. The cylinder output rod is attached
to the vehicle frame, usually through a double bushing.
Hydraulic fluid enters the power piston through high
pressure hoses from the valve. See Figure 9-60.
O-ring
Valve
spool
Teflon rings (4)
O-rings (4) - installed
under teflon rings
Valve body ball nut
Stub shaft
Figure 9-56. An exploded view of one particular power piston
assembly. Styles will vary. (General Motors)