Avoid Fretting and Backlash with Rotary Vane Actuators

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.

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.

For more information, visit http://www.KinetrolUSA.com or call 972-447-9443.

Elegantly Engineered Pneumatic Valve Actuators. A Simple, Single Moving Part

One moving part. Simple. Elegant.

According to Wikipedia, elegance is "a synonym for beauty that has come to acquire the additional connotations of unusual effectiveness and simplicity." In engineering terms, "a solution may be considered elegant if it uses a non-obvious method to produce a solution which is highly effective and simple. "

When you compare the mechanics of various other pneumatic valve actuators, such as rack and pinion or scotch yoke with their internal gears, bushings and bearings, you immediately understand that simplicity is sacrificed and the design is certainly not elegant.

Actuators with gears wear.

When gears mesh, there is friction. Friction causes heat and wear, which effects the mechanical life of the actuator. Friction converts kinetic energy into thermal energy and can have dramatic consequences if left unchecked. Another important consequence of friction is wear, which may lead to performance degradation and/or damage to the internal components of a rack and pinion or scotch yoke actuator.

"Fretting wear" is caused by the repeated cyclical rubbing between two surfaces (gears in the case of scotch yoke or rack and pinion actuators) and over a period of time, will remove material from one or both surfaces.

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".

A well designed vane actuator 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.

For more information, contact:

Kinetrol USA
1085 Ohio Drive
Plano, Texas 75093
(972) 447-9443 phone
(972) 447-9720 fax
sales@kinetrolusa.com