Doc : Q066 Rev : 10
Issued : 01/11/13 Page : 4 of 25
4.1 Standard Components
4.1.1 Frame
The frame contains the scotch-yoke mechanism which converts the linear
movement of the pneumatic piston into a quarter-turn rotary movement. To
protect personnel from injury and the scotch-yoke mechanism from corrosion,
the frame is a fully enclosed carbon steel fabricated construction. To support the
transverse forces generated by scotch-yoke mechanisms, and to ensure correct
alignment of the piston rod, sliding blocks and yoke bushing, the body also
contains a chromium plated guide bar.
4.1.2 Pneumatic Cylinder
Contains the pneumatic piston, which generates the linear movement which is
converted into a quarter-turn rotary movement by the scotch yoke mechanism
housed in the actuator’s frame.
4.1.3 End Stop Screws
Allow precise adjustment of a scotch-yoke mechanism’s angular stroke by
means of two end stop screws. One end stop screw is always located on the
end flange of the pneumatic cylinder. For double-acting actuators, the remaining
end stop screw is either located on the body’s blanking flange, or in the event a
manual override is fitted, on the manual override. For spring-return actuators,
the remaining end stop screw is located on the outer spring-cylinder flange, or in
the event a manual override is fitted, on the manual override.
4.1.4 Local Position Indicator
Connected directly to the scotch-yoke mechanism, the local position indicator
protrudes through the top of the body to provide precise local visual indication of
the actuator’s angular position.
4.2 Optional Components
4.2.1 Spring Cylinder
Supplied with all spring-return PNS, PND and PNC Series valve actuators, the
spring cylinder comprises of a scragged pre-compressed spring contained within
a fully welded carbon steel container. WARNING! The spring cylinder is non-
serviceable; any attempt to disassemble it may result in serious injury or
death.
4.2.2 Manual Override
There are two types of manual override mechanisms which can be provided;
Jackscrew or Hydraulic Handpump. Jackscrew type manual overrides comprise
of a handwheel and jackscrew to create a linear motion, which is converted to a
rotary motion via the scotch-yoke mechanism. Hydraulic handpump manual
overrides comprise of a hydraulic cylinder and handpump to create a linear
motion, which is converted to a rotary motion via the scotch-yoke mechanism.
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