What causes Stiction?

Published:  27 June, 2007

The answer to this month’s trouble shooting teaser is supplied by Parker Hannifin.

 

Q:

I know that stiction can cause problems in pneumatic valves and cylinders, but what causes the condition?

 

A:

The term stiction refers to a condition caused by friction occurring between the seal on a piston and the barrel of a cylinder, or between a valve spool seal and valve body.

In practice, the respective components tend to stick during the piston or spool stroke, normally at the beginning of the stroke, when the maximum breakaway force has to be applied to drive the piston or spool, but also sometimes during particularly slow piston movement.

Stiction is primarily affected by the design and construction of piston or spool seals, as there are the parts in direct contact with the cylinder or valve body. There is, however, a trade-off, as these seals must prevent air from leaking past the piston or spool yet ideally should be almost frictionless.

Traditionally, in conventional cylinders, reverse-facing flat rubber lip seals, separated by a wear strip, have been mounted around the piston to prevent air leakage. Unfortunately, these types of lip seal have a relatively large contact face, so there is greater potential for friction; consequently, higher air pressure will be required to apply a breakaway force, which may be higher than the pressure required for normal piston movement. The result can be a sudden and sharp initial motion before the piston resumes its normal stroke.

A condition know as stick-slip can also occur, where the levels of friction between the lip seals and cylinder barrel are so high that at low operating pressures or slow speeds the piston jerks sporadically as the seal binds and releases.

Similarly, valve spools are often manufactured with O-rings fitted onto each raised profile along the spool; if these O-rings are oversized or expand under high operating temperatures or pressures, they can bind against the valve body and cause stiction.

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