flow of fluid to and from the two chambers of
the actuating cylinder.
Since the two control valves operate
independently of each other as far as hydraulic
pressure is concerned, failure of either hydraulic
system does not render the actuator inoperative.
Failure of one system does reduce the output force
by one-half; however, this force is sufficient to
permit operation of the actuator.
The rack-and-pinion-type actuators, also
referred to as limited rotation cylinders, of the
single or multiple, bidirectional piston are used
for turning, positioning, steering, opening and
closing, swinging, or any other mechanical
function involving restricted rotation. Figure
10-10 shows a typical rack-and-pinion double-
The actuator consists of a body and two
reciprocating pistons with an integral rack for
rotating the shaft mounted in roller or journal
bearings. The shaft and bearings are located in
a central position and are enclosed with a bearing
cap. The pistons, one on each side of the rack,
are enclosed in cylinders machined or sleeved into
the body. The body is enclosed with end caps and
static seals to prevent external leakage of
Only a few of the many applications of
actuating cylinders were discussed in the preceding
paragraphs. Figure 10-11 shows additional types
of force and motion applications.
In addition to its versatility, the cylinder-type
actuator is probably the most trouble-free
component of fluid power systems. However, it
is very important that the cylinder, mechanical
linkage, and actuating unit are correctly aligned.
Any misalignment will cause excessive wear of the
piston, piston rod, and seals. Also, proper
adjustment between the piston rod and the
actuating unit must be maintained.
Figure 10-10.Rack-and-pinion double-piston rotary actuator.