A mechanism of high friction in dry sliding bearings

The conventional definition of friction coefficient as the ratio of the frictional force to the initial normal load is misleading when complete mechanical interlocking between two surfaces occurs. In that case, even after the initial normal load is released, the frictional resistance of surfaces to relative motion remains high and the magnitude of friction coefficient, by the conventional definition, is infinity. It is shown in this paper that the entrapment of wear particles at the sliding interface of a geometrically constrained bearing operating under dry conditions leads to increased normal load. The higher normal load in turn results in a rapid increase in the frictional torque of the bearing and can cause seizure. A model is developed to predict the normal load increase. The model is based on the assumption that the entrapped wear particles are compacted at the initial contact point of the bearing. The model predicts that the large increase in the frictional torque of bearings experiencing seizure is mainly due to the increased localized normal load and not necessarily due to higher friction coefficient.