Spring Return Fail Counter-Clockwise Actuator Sizing

Spring Return Fail Clockwise Actuators is not the standard configuration for Spring Return Actuator, but it is not unusual. In the event of a loss of air supply, the actuator spring will turn the valve counter-clockwise. This is a failsafe operation is to guarantee the valve fails in a predetermined position on air supply loss. Spring Return Fail Counter-Clockwise Actuators are often refereed to as Spring Return Fail Open.  This can be misleading depending on the operating direction of the valve. “Fail Counter-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 BREAK torque value that is greater than you valve torque with safety factor. (Refer to step 2) We look to the SPRING BREAK because we need the valve to fail counter-clockwise and the spring needs to have enough start torque to operate the valve.

6. Find the AIR SUPPLY END 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 START and AIR SUPPLY END 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 BREAK Torque and find the torque that exceeds 50 in lbs. I will then look to the AIR SUPPLY END to find the actuator that exceeds 50 in lbs. In this case there are there options in each category that exceed the required 50 in lbs of torque. For our purposes it would be best to select the TR20SR with 5 springs, because this is the standard configuration.