Many studies and organizations have defined stiction, or static friction, in different ways. A few examples of the various definitions are as follows:

  • According to Entech [1], “stiction is a tendency to stick-slip due to high static friction. The phenomenon causes a limited resolution of the resulting control valve motion. ISA terminology has not settled on a suitable term yet. Stick-slip is the tendency of a control valve to stick while at rest and to suddenly slip after force has been applied.”
  • The Instrument Society of America (ISA)(ANSI/ISA-S51.1-1979) [2], described “stiction as the resistance to the start of motion, usually measured as the difference between the driving values required to overcome static friction upscale and downscale.” The definition was first proposed in 1963 in American National Standard C85.1-1963.
  • According to Ruel: “stiction is a combination of the words stick and friction, created to emphasize the difference between static and dynamic friction. Stiction exists when the static (starting) friction exceeds the dynamic (moving) friction inside the valve. Stiction describes the valve’s stem (or shaft) sticking when small changes are attempted.”
The above definitions all seem to agree that stiction is the act of being “stuck” by static friction, which prevents one surface from moving against another. Furthermore, if the external force becomes greater than the static friction, the stiction between two surfaces will be overcome and the object will begin to move again.

Stiction between two surfaces can be the result of many factors that may include but are not limited to:

  • Corrosion
  • Cold welding, i.e., metal on metal adhesion
  • Breakdown of the lubrication boundary layer
  • Change in the viscosity of the lubrication
  • Build-up of deposits
  • Contamination or foreign material on the contact surfaces
  • Chemical reactions between the metal or lubrication
  • Deterioration and breakdown of the valve’s sealing components
For general purpose, stiction can be defined as the resistance to the start of motion usually measured as the difference between the external force being applied in order to overcome the static friction and the force to maintain movement between the two contacting or working surfaces.

Stay tuned for more entries on stiction. I will also be presenting a white paper at GCPS/AIChE conference on 5/1/13 in San Antonio, TX.

[1]. EnTech (1998), EnTech Control Valve Dynamic Specification (version 3.0).

[2]. ANSI/ISA (1979), ANSI/ISA-S51.1-1979 Process Instrument Terminology. The Instrument Society of America, Research Triangle Park, NC.

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