Reference Manual

2−4
DIAPHRAGM
DIAPHRAGM CASING
DIAPHRAGM
PLATE
ACTUATOR SPRING
ACTUATOR STEM
SPRING SEAT
SPRING ADJUSTOR
STEM CONNECTOR
YOKE
TRAVEL INDICATOR DISK
INDICATOR SCALE
W0363-1
LOWER DIAPHRAGM CASING
W0364-1
Figure 2-1. Spring-and-diaphragm actuators offer an excellent first choice for most control valves.
They are inexpensive, simple and have built-in, fail-safe action. Pictured above are cutaways of the popular
Fisher 667 (left) and Fisher 657 (right) actuators.
Figure 2-2. Spring-and-diaphragm actuators
can be supplied with a top-mounted handwheel.
The handwheel allows manual operation and also
acts as a travel stop or means of emergency
operation.
W0368-2
Actuators are available for either fail-open or
fail-closed action. The only drawback to the
spring-and-diaphragm actuator is a relatively
limited output capability. Much of the thrust
created by the diaphragm is taken up by the spring
and thus does not result in output to the valve.
Therefore, the spring-and-diaphragm actuator is
used infrequently for high force requirements. It is
not economical to build and use very large
spring-and-diaphragm actuators because the size,
weight and cost grow exponentially with each
increase in output force capability.
Piston Actuators
Piston actuators are generally more compact and
provide higher torque or force outputs than
spring-and-diaphragm actuators. Fisher piston
styles normally work with supply pressures
between 50 and 150 psi and can be equipped with
spring returns (however, this construction has
limited application).
Piston actuators used for throttling service must be
furnished with double-acting positioners that
simultaneously load and unload opposite sides of
the piston. The pressure differential created across
the piston causes travel toward the lower pressure
side. The positioner senses the motion, and when
the required position is reached, the positioner
equalizes the pressure on both sides of the piston.
The pneumatic piston actuator is an excellent
choice when a compact unit is required to produce
high torque or force. It is also easily adapted to