A study of friction during low-velocity impact.

摘要

Low-velocity impact may occur in many industrial and mechanical systems due to clearances in joints, loading and unloading operations, and intermittent motion. In particular, flow induced repetitive impacts between heat exchanger tubes and their support plate hole surfaces is an operational problem in process equipment. To predict wear damage it is very important to know the contact forces which can be resolved into normal and tangential components. Most of the damage comes from the tangential component. Therefore, a more realistic prediction of the tangential force or the traction coefficient, which relates the tangential force to the normal force, during impact should lead to a more accurate prediction of wear and other associated effects of sliding and frictional forces.; An experimental study was conducted to generate detailed contact force waveforms during low-velocity oblique impact. A precise pendulum-based test apparatus and a new method for dynamic calibration of force sensors were developed. Results show that the tangential force can be smaller than the limiting Amontons-Coulomb friction value during some impact processes. The traction coefficient is a time dependent parameter which largely determines the impulse ratio. The impulse ratio is mainly determined by the approach angle.; Two new contact models were developed to simulate the experimental force waveforms. A damped normal contact model with unequal energy losses in compression and restitution phases which is based on the Hertz contact theory was developed for normal elastic impact. Based on the Cattaneo-Mindlin theory, a new solution method was developed for the tangential elastic contact problem. A more specific expression for the tangential force was deduced. There is very good agreement between the experimental and simulated waveforms of the normal force and the tangential force in the compression phase of forward slip. The agreement in the restitution phase for the tangential force is not as good as for the compression phase.; This study shows that elastic deformation cannot be neglected if accurate modelling of elastic impact is desired. Only the normal coefficient of restitution is needed in predicting elastic impact responses. Where reverse slip is observed, a change in the direction of velocity occurs before the tangential force direction reverses. Therefore, the tangential surface compliances offer some accommodation mechanism(s) that rigid-body theories do not capture. (Abstract shortened by UMI.)