In applications, such as gears and rolling bearings, where concentrated contact occurs, extreme loading can result in damaging asperity contact. To study this, an adapted elastohydrodynamic rig was used to measure the film using the reflection of an ultrasonic pulse. Ultrasonic reflection depends on the stiffness of the contact interface. The stiffness of an oil film depends on its thickness. A single contact was operated dry, full film lubricated, and in the mixed regime, by changing the sliding speed. The reflection then depends on the stiffness of both the liquid contact (oil film) and the solid (asperity) contact, acting as two springs in series. When dry, the solid stiffness dominates. As an oil film starts to form, a high stiffness thin liquid film forms. As the speed is increased the stiffness of this layer reduces, as its thickness increases, until it becomes thicker than the roughness and the stiffness then originates from liquid contact alone. By comparing dry, wet and sliding conditions the contributions of solid and liquid stiffness can be separated.

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