Figure 8-27.The three major power steering systems. (A) Integral piston (linkage type), (B) External cylinder (linkage type),
and (C) Rack and pinion type.
External Cylinder (Linkage Type)
The external cylinder power steering system has
the power cylinder mounted to the frame and the center
link. In this system the control valve may be located in
the gearbox or on the steering linkage. Operation of
this system is similar to the one previously described.
Power Rack and Pinion
Power rack-and-pinion steering uses hydraulic
pump pressure to assist the operator in moving the rack
and front wheels. A basic power rack-and-pinion
assembly consists of the following:
POWER CYLINDER (hydraulic cylinder
formed around the rack)The power cylinder is
precisely machined to accept the power piston.
Provisions are made for the hydraulic lines. The
power cylinder bolts to the vehicle frame, just
like the rack of a manual unit.
POWER PISTON (a double-acting hydraulic
piston formed on the rack)The power piston is
formed by attaching a hydraulic piston to the
center of the rack. A rubber seal fits around the
piston to prevent fluid from leaking from one
side of the power cylinder to the other.
HYDRAULIC LINES (steel tubing that
connects the control valve and power cylinder).
CONTROL VALVE (a hydraulic valve which
regulates hydraulic pressure entering each end
of the power piston)There are two types of
control valvesrotary and spool. Using a
torsion bar connected to the pinion gear
operates the rotary valve, whereas the spool
valve is operated by the thrust action of the
Other components of the power rack and pinion
are similar to those that are found on manual
rack-and-pinion steering system.
Power rack-and-pinion operation is fairly simple.
When the steering wheel is turned, the weight of the
vehicle causes the front tire to resist turning. This
resistance twists a torsion bar (rotary valve) or thrusts