Chapter 25 Steering Systems 511
Power Piston
The power piston on both rack-and-pinion
and parallelogram steering systems is a simple
hydraulic device. If pressure on one side of the
piston is greater than pressure on the other side,
the piston moves toward the side with less pres-
sure. Seals on the power pistons prevent pres-
sure leakage between sides.
Parallelogram Linkage Power Steering Gear
A self-contained parallelogram linkage
steering gear contains a control valve mecha-
nism, a power piston, and gears. Pressure devel-
oped by the unit is applied to the pitman shaft.
All modern conventional power steering gears
are inline units. Figure 25-24 shows a basic
inline unit. Notice that all the basic elements of
the conventional manual steering gear are used.
A pitman shaft sector gear engages a recirculat-
ing ball nut that rides on a typical worm gear
or shaft. The basic differences between an inline
power steering gear and manual steering gear
include the use of the ball nut as a power piston
and the addition of a rotary spool valve.
Rack-and-Pinion Power Steering
The power rack-and-pinion power steering
system uses a rotary control valve that directs
the hydraulic fl uid from the pump to either side
of the rack power piston. Steering wheel motion
is transferred to the pinion. From there, it is sent
through the pinion teeth, which are in mesh
with the rack teeth. See Figure 25-25.
The integral rack piston, which is connected
to the rack, changes hydraulic pressure to a lin-
ear force (back and forth movement in a straight
line). When the steering wheel is in the straight
ahead position, the rotary control valve delivers
equal pressure to both sides of the power piston.
As the steering wheel is turned, the rotary con-
trol valve delivers greater pressure to one side of
the power piston. This, in turn, moves the rack
in a right or left direction. The force is transmit-
ted by the inner and outer tie rods to the steer-
ing knuckles, which, in turn, move the wheels.
See Figure 25-26.
Power Steering Pumps
The power steering pump provides the
hydraulic pressure needed to operate the power
steering system. The power piston of a steering
system must be supplied with approximately
125 psi (861.87 kPa) for normal driving, 400 psi
B
A
Control
valve
Piston shaft
sector gear
Ball nut
Worm
shaft
Figure 25-24.
Inline power steering gear. A—Action of an inline power
steering gear during a left turn. B—Action of an inline
power steering gear during a right turn.
Goodheart-Willcox Publisher
Rotary
control
valve
Valve
unit
Reservoir
Vane-
type
pump
Pressure
regulator
Steering
gearbox
Pinion
Power cylinder
Rack
Integral
power
piston
Figure 25-25.
Cutaway view of a power rack-and-pinion steering system.
Note the rotary control valve, which regulates fl uid fl ow
to the power cylinder. This system is able to function as a
manual rack-and-pinion unit if fl uid operating pressure is
lost due to a faulty pump or fl uid loss.
Honda