What is the design life for a part-turn pneumatic actuator operating butterfly valves?. What is a endurance test for pneumatic actuators containg? What is the method?
I know a preliminairy standard prEN WI 0069096:2004. It mentions x cycles for x torque. Not enough.
Rack and pinion actuators last for millions of cycles.
I have documentation of a Worcester actuator going 15 million cycles in service (not a lab test). It was lightly loaded, and the cycle rate was obviously very frequent.
Usually what kills a rack and pinion actuator is contamination.  Water in the air causes rust in the air lines and rust flakes are abrasive.  THey chew up the cylinder walls, the o-rings, and the piston guide skirts.
If the actuator is spring-return, then the vent breathes IN when the pressurized part of the cylinder breathes OUT. If the air going into the vent is humid  or corrosive, it can cause internal actuator corrosion or rusting of the springs.  If the springs rust, they make rust flakes.  Also the rust causes pitting of the springs. The pits make a nice place for cracks to start. Rust springs break, and spring fragments are very damaging to the actuator internals.
A filter on the front-end air supply is a good thing-it keeps out the specks of rust from the air line. 
A rebreather gasket allows the vented air to pass over and to be inhaled by the vent port. The vented air expands to a much larger volume as it vents from 80 psi (typically) inside the actuator to atmospheric pressure.   The vented air is presumed to be cleaner and dryer than the ambient air, so corrosion from environmental cantaminats is avoided. 

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