It has been reported that up to 50 variables might influence the magnitude and rate of the fretting process.

Predictions of the fatigue strength under fretting conditions were reported to be successful when based on “stress models” (e.g., the fracture mechanics approach). These predictions use only a few variables of the fretting process such as external load, contact pressure, coefficient of friction, and the amplitude of relative slip.

A concept of “primary” and “secondary” sets of fretting variables has been proposed. The “primary” set of variables includes the coefficient of friction, magnitude of slip and contact pressure acting at the fretting interface. The “secondary” set of variables affects the fretting process through changes these variables cause in the “primary” set of variables.

The coefficient of friction has been identified as the main variable of the fretting process. Analysis of the literature shows that the effect which many other variables have on the fretting process can be explained by the changes these variables cause in the value of the coefficient of friction.