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Hydraulic
and Pneumatic Knowledge | Hydraulic Actuators
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Pneumatic actuators are normally used to control processes
requiring quick and accurate response, as they do not require a large amount of
motive force. However, when a large amount of force is required to operate a
valve (for example, the main steam system valves), hydraulic actuators are
normally used. Although hydraulic actuators come in many designs, piston types
are most common.
A typical piston-type hydraulic actuator is shown in the
illustration below. It consists of a cylinder, piston, spring, hydraulic supply
and return line, and stem. The piston slides vertically inside the cylinder and
separates the c y l i n d e r i n t o two chambers. The upper chamber
contains the spring and the lower chamber contains hydraulic oil.
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