What is Rotary Pneumatic Actuator? How is the Working Principle?
Pneumatic Rotary Actuators are devices used to convert compressed air or process gas energy into rotary motion in a cylinder. These devices provide advantages such as high power, simplicity and longevity according to their durability dimensions. Rotary Pneumatic Actuators are collected in 3 main groups as their working principle. These are as follows:
- Scoth Yoke (Scottish Yoke)
- Vane (Winged)
- Rack and Pinion
Rotary Pneumatic Actuators are generally designed to make only ¼ turn to remotely open/close Butterfly and Ball valves. For this reason, the movement in rotary actuators is limited between 0-90° with a 5% plus-minus tolerance. Rack and Pinion pneumatic actuators are known as quarter turn pneumatic actuators. Butterfly and ball valves are the most known and used models in terms of cost and ease of accessibility compared to others. Rotary pneumatic actuators can be equipped with dry contact or proximity limit switch boxes for on/off information, I/P positioner for modulating, proportional control, and 5/2 or 3/2 namur directional valves for opening and closing.
For the valve connection of the actuators, the valve connection shafts are produced in accordance with the diagonal, square, star shapes, and the connection size diameters, which are called square mile diameter connection, increase depending on the valve shaft whose diameter increases as the actuator piston diameter increases.
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What is the Working Principle of Quarter Turn (Rack Rack) Pneumatic Actuators?
In pneumatic actuators, rack gear is designed to convert linear motion into rotary motion.
As can be seen in the image, this very common design consists of 3 basic components: Piston, rack and pinion gear. When the chamber on one side of the piston is pressurized with compressed air, it moves the rack gear inward and the movement output rotary shaft converts this movement into rotary motion through the gear connection with the pawn gear. Double pistons are used to double the output power. In addition, this design provides a bearing and double-sided balance in the piston. Then it is provided by pressurizing the other chamber to move in the opposite direction or by placing springs in the chambers.
The design formed by pressurizing both chambers in the pneumatic cylinder is defined as Double-acting, the design formed by placing springs in the chambers is defined as single-acting.
What are Double Acting Quarter Turn Pneumatic Actuators?
Double-acting pneumatic actuator can be operated with 5/2 Directional valve. By changing the connection of the supply air, the working direction can be changed clockwise or counterclockwise. In all pneumatic actuators, it is mandatory to have a discharge so that the actuator can work in two directions and the air in the room does not block the movement in the opposite direction. Higher torque values can be obtained by increasing the air pressure and piston diameter. If requested, stroke limitation can be made in both working directions.
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What is a Single Acting Quarter Turn Pneumatic Actuator?
ccw (counterclockwise) cw (clockwise)
In single-acting quarter-turn pneumatic actuators, air is taken into only one chamber and is responsible for only one-way movement. Movement in the opposite direction is provided by a spring group placed in the chamber on both sides. The disadvantage of single-acting pneumatic actuators is the reduced output force throughout the entire stroke due to counter-spring forces.
Quarter Turn Pneumatic Actuator Torque Characteristics
Although a constant torque value can be obtained throughout the stroke (at 0 and 90°) in double-acting pneumatic actuators, maximum torque can be obtained at the beginning of air opening (0°) in single-acting pneumatic actuators, while at the end of the stroke (90°) it decreases due to the counter-spring force. The torque value increases from (0°) when the spring is fully tensioned (90°). For this reason, in practice, single-acting actuators are chosen with larger diameters than double-acting actuators. An important issue to be considered in practice is the selection should be made by using a value as large as 30% safety factor of the current torque value while selecting the actuator due to friction losses and tolerances of the components.
What are the Standards?
Pneumatic actuators have been standardized to be compatible and interchangeable with products from various manufacturers. These standards specify the dimensions of various components such as the actuator, air ports, position indicator, accessory mounts, shaft shafts.
What is ISO 5211:2017?
It specifies the requirements for connecting actuators to industrial valves. Specifies the dimensions and torque values of flange and drive components.
NAMUR VDI/VDE 3845: 2010-09 What is it?
This standard includes specifications between the actuator and a process valve supplying compressed air. Namur valves offer a mounting application according to VDI/VDE 3845, which includes two vents on the flat side of the valve and additional holes to secure the valve to the actuator stem.
What is VDI/VDE 3845 (Namur)?
It covers the connection standards of the directional valves on the body.
What is ISO 5211/DIN3337?
It covers the connection standards of the pneumatic actuator to the process valve.