Reference Manual

2−5
Figure 2-3. The Fisher 2052 spring-and-
diaphragm actuator has many features to provide
precise control. The splined actuator connection
features a clamped lever and single-joint linkage
to help eliminate lost motion.
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Figure 2-4. Double-acting piston actuators such
as the Fisher 1061 rotary actuator are a good
choice when thrust requirements exceed the
capability of spring-and-diaphragm actuators.
Piston actuators require a higher supply pressure,
but have benefits such as high stiffness and small
size. The 1061 actuator is typically used for
throttling service.
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services where high ambient temperatures are a
concern.
The main disadvantages of piston actuators are
the high supply pressures required for positioners
when used in throttling service and the lack of
fail-safe systems.
Figure 2-5. Spring fail-safe is present in this
piston design. The Fisher 585C is an example of
a spring-bias piston actuator. Process pressure
can aid fail-safe action, or the actuator can be
configured for full spring-fail closure.
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Figure 2-6. Since the requirements for accuracy
and minimal lost motion are unnecessary for
on-off service, cost savings can be achieved by
simplifying the actuator design. The Fisher
1066SR incorporates spring-return capability.
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There are two types of spring-return piston
actuators available. The variations are subtle, but
significant. It is possible to add a spring to a piston