Tuesday, January 7, 2014

Considerations for Sizing Spring Return Pneumatic Actuators

Pneumatic Actuator
Springs, Piston & Seal 
Sizing direct-acting pneumatic actuators is pretty straightforward. The actuator only has the fixed air supply to energize the actuator. If you have enough actuator torque at a given supply pressure to open and close your valve (plus a safety factor), you're good to go. Spring-return actuators, however, present an additional challenge when sizing.

A spring-return actuator uses the supply air to energize the actuator in one direction (open/close) and the internal springs in the opposite direction (close/open). Because we have these two energy sources, there are four torques to consider and compare to the required valve torque:

  1. Spring Start - naturally provides higher value because it starts compressed and then expands.
  2. Spring End - naturally has lower value because it loses "strength" as it decompresses.
  3. Air Start - naturally has higher value because it is working opposite a decompressed spring.
  4. Air End - naturally has lower value because its working opposite a compressed spring.

Most actuators also come with multiple "spring packages" - these are different spring combinations to produce more/less spring torque. However, don't forget the opposing relationship between air and spring.

A spring torque value may be high enough at the start, but as the spring "relaxes" the torque will decrease and may not overcome the valve torque threshold. Conversely, the torque provided by the air supply at its start may be high enough to overcome the valve and the relaxed spring, but may fail as the spring compresses.

Because of this spring/air torque relationship, you may find yourself having to jump several actuator model sizes to provide the required torque. This of course is a simplified explanation of how spring return actuators work. There are other factors that must be considered for proper sizing. We always advise getting professional advice from an expert before applying an actuator.