Page 321 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Rack and pinion actuator
Another type of pneumatic actuator is rack and pinion actuator which is used mostly for on-off application (i.e. to
open and close the valve) of quarter turn valves like Butterfly, Ball, Plug valves & dampers. . “Rack and pinion” is
a generic term for a pair of gears that convert linear motion into rotary motion. A linear gear bar called the “rack”
engages the teeth on a circular gear called the “pinion”. The linear force applied on the rack will cause rotary
motion of the pinion. A figure showing a simple rack and pinion mechanism is given below-

Rack and pinion mechanism

“The rack and pinion mechanism uses two piston-type racks moving in opposite directions to ensure balanced
forces on the pinion. normally, pneumatic air pressure is used to power the actuator. By applying pressure on the
piston rack, the pinion can be changed to the desired position. The pinion bottom connects to the valve stem to
open and close the valve as the pinion turns. These actuators are available in two constructions-
spring return – Mechanical spring return is for fail-safe applications and can be assembled for “fail close” or “fail
open” safety function, and
Double acting – Double acting actuators can be used for “fail to lock position” safety function.
Parts of rack and pinion actuator

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CITS : E & H - Electronics Mechanic - Lesson 188 - 198