Some machine actions require rotary motion for only a portion of
a turn. Using a hydraulic motor to perform a partial-turn
function is expensive and it is difficult to accurately stop a
motor at a specified degree of rotation. A clevis-mounted
cylinder, attached to an arm and keyed to a shaft, produces
rotary action, but is limited to 90° or less. At 90° rotation, a
cylinder/lever arrangement has half torque or less when it
starts and nears the end of stroke. To obtain partial- or
multiple-turn rotary action and/or accurate stopping of rotary
output, use one of the rotary actuators shown in this chapter.
Figure 19-1 pictures the symbols for air- and hydraulic-operated
rotary actuators. Fig. 19-1. Rotary actuator symbols. Figure
19-2 provides simplified cutaway views of vane-type rotary
actuators. The figure depicts both single- and double-vane-
types. The vanes attach to an output shaft and have seals around
their periphery. When fluid pressure on a given vane area pushes
it through the body cavity, the output