Spring Return Fail Clockwise Actuator Sizing

Spring Return Fail Clockwise Actuators are the most common Spring Return Actuator. In the event of a loss of air supply, the actuator spring will turn the valve clockwise. This is a failsafe operation is to guarantee the valve fails in a predetermined position on air supply loss. Spring Return Fail Clockwise Actuators are often refereed to as Spring Return Fail Closed.  This can be misleading depending on the operating direction of the valve. “Fail Clockwise” is a more precise way to refer to this operation.

Follow these steps to find the appropriate actuator size.

1. Determine the MINIMUM operating pressure. (This is the lowest air supply available)

2. Determine the torque required to operate the valve. For safety, always increase the torque to provide a safety factor. (torque x 1.25)

3. Select which actuator you will be using. (Rack and Pinion, Scotch Yoke, Vane, Manual, etc.)

4. Obtain the torque charts for the actuator being used. (For this explanation we will use the Rack and Pinion torque charts from Triac Actuators to the right.)

5. Find the SPRING END torque value that is greater than you valve torque with safety factor. (Refer to step 2) We look to the SPRING END because we need the valve to fail clockwise and the spring needs to have enough end torque to operate the valve.

6. Find the AIR SUPPLY BREAK torque valve that is greater than the valve torque with safety factor. (Refer to step 2)

7. The proper actuator will exceed the valve torque and a safety factor in both the SPRING END and AIR SUPPLY BREAK torque columns.

For example:

My valve has a break torque of 40in lbs and I have a minimum air supply of 80psi. I will take 40 x 1.25 = 50 in lbs with safety factor. Then I will look tot he SPRING END Torque and find the torque that exceeds 50 in lbs. I will then look to the AIR SUPPLY BREAK to find the actuator that exceeds 50 in lbs. In this case all the sizes exceed in the AIR SUPPLY BREAK torque, but only one size exceeds in the SPRING END torque. I would select the TR20SR with 6 springs.