A quarter turn pneumatic actuator, whether direct-acting or spring-return, has a given output torque based upon the air supply pressure provided. This output torque can be fairly linear, or very non-linear depending on the style of the actuator (rack and pinion vs. scotch yoke) or if the application calls for springs to be used for opening or closing.
It is critically important to properly select and match the design and torque requirements of the pneumatic actuator and the valve in any given process. Improper selection will result in failure, much higher installed cost, not to mention lost production.
If the actuator is too small (underpowered at a given line pressure to provide ample seating and unseating torque), it will fail and need to be replaced. If an actuator is severely oversized, the cost of the actuation package is higher, and space is wasted accommodating the larger physical size.
Quarter turn valves and actuators are often used in demanding application requiring high cycle rate, exposure to harsh environments and control-critical processes such as hazardous chemicals. For these reasons, a great deal of attention and care must be given to proper sizing.