Computational Mechanics Squeak Project

Modeling and Prediction of Friction-Induced Vibrations

Sponsored by: Ford Motor Company
Project duration: 1994-1996

The results of COMCO's previous friction work have been applied in practical attempts to understand, model and eliminate friction-induced noises in industrial applications. In particular, Ford Motor Company sponsored a joint effort between COMCO and ORTECH International, decicated to precise experimental verification of the numerical models of friction-induced vibrations developed at COMCO. The primary objective was to verify and expand the understanding of friction-induced vibrations, and to confirm validity of the analytical predictive technology by a careful comparison with experiment.

A special test apparatus was built, which possesses desirable qualities of mechanical simplicity and controllable dynamic characteristics. The main part of the apparatus consists of a heavy rotating disk and a specially designed slider. The slider includes a metal block, supported on a cantilever arm. The pin is mounted on the block via a load cell, capable of measuring six load components: three forces and three moments. In order to vary the stiffness and the geometry of the system, an interchangeable bracket was introduced to support the pin. An acquisition system was provided to monitor forces and accelerations of various system components. Special care was taken to identify and define all the system parameters needed in the analytical model.
For this apparatus, an analytical and numerical model was devised. The model takes into account: (a) the complete dynamic characteristics of the system and (b) fully nonlinear constitutive properties of the contact interface, including normal compliance and sliding resistance. The constitutive characteristics of the contact interface were determined through asperity-based homogenization approach. The dynamic stability of the system was assessed via complex eigenvalue analysis followed by fully nonlinear transient analysis. By variation of selected mechanical details of the apparatus, both stable and unstable configurations of the system were devised, tested and correlated with numerical predictions. The primary mode of instability was identified as friction-induced dynamic coupling of selected modes of the system, leading to self-excited limit cycle oscillations. The validity of this approach is confirmed by excellent correlation of experimental and numerical results. In particular, the configurations of the system predicted to be unstable were indeed unstable, with very good agreement of predicted and measured frequencies and amplitudes of the slider.
Similarly, in the cases predicted to be stable, the self-excited oscillations did not initially occur. However, after some surface damage has occurred, the pin experienced seizure accompanied by bursts of chaotic unstable vibrations, different in their characteristics from the limit cycle oscillations. A simplified analytical modeling of this phenomenon was performed and produced results in qualitative agreement with the experimental observations.

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