Sunday, February 26, 2017

Avoid Fretting and Backlash with Rotary Vane Actuators

fretting and backlash shorten actuator life
Fretting and backlash
shorten actuator life and
negatively affects controllability
over time.
The American Society for Metals Handbook on Fatigue and Fracture defines fretting as: "A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force."

In pneumatic valve actuators, fretting wear is caused by the repeated cyclical rubbing between gears in scotch yoke or rack and pinion actuators. Over a period of time, fretting will remove material from one or both surfaces. This extra material, or debris, is usually harder than its source material due to work hardening and oxidation. The resulting debris becomes an even more effective abrasive, increasing the rate of mechanical wear and continued fretting.

Backlash happens when gears change direction. It is caused by the gap between the trailing face of the driving tooth and the leading face of the tooth behind it. The gap must be closed before force can be transferred in the new direction, hence the phenomena of backlash. This is also sometimes referred to as "slop".

Both fretting and backlash significantly effect the service life of an pneumatic valve or damper actuator. Both are also detrimental to controllability when actuators are used to accurately manage the percent open status of a valve or damper.

rotary vane actuators
Internal view of a rotary vane actuators.
Notice the vane is constructed from
a single piece of stock and
contains no gears.
An alternative actuator design is the rotary vane design that uses a single piece of machined steel for both the vane body and drive shafts. With this design, the shaft and vane are not affected by backlash, friction or wear. Vane actuator's incorporate a design with only a single moving part that provides very repeatable, smooth movement that will not decay over time.

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