1994-03-01

Static Friction - What It Is, What Affects It 941034

The purpose of this paper is to improve the understanding of static friction and the factors which influence it and its measurement. This paper also evaluates the relationship between a variety of measurements to quantify the static friction level.
The static friction level is very important in equipment such as an automatic transmission where the clutch pack must have sufficient holding capacity to transmit power from the motor to the wheels.
A test procedure was developed to evaluate three commonly reported measurements of static capacity. They were: the endpoint of a dynamic engagement (lock-up) where the relative velocity approaches zero, the breakaway torque required to initiate relative motion in an engaged clutch, and the steady-state torque generated during a low speed slip.
A Taguchi array was used to evaluate the effects of the following variables: dwell time between the dynamic engagement and the subsequent static pull, static pull speed, apply pressure, energy of the dynamic engagement (speed, inertia), groove pattern, plate flatness, fluid type, and friction material.
The apparatus used was an SAE #2 Machine. A thermocouple was imbedded in the steel reaction plate for clutch pack temperature measurements.
It was found that temperature plays a significant role in affecting the static friction level of an engaged clutch. Also, the relationship between three typical static friction measurements (endpoint, breakaway and steady-state) vary significantly with friction materials, fluids and test conditions.

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