Ultrasonic Measurement of Contact Pressures in Automotive Component Contacts

The measurement of pressure at a contact in engine parts is important, because it is frequently contact stresses that lead to failure by seizure, wear, or fatigue. Whilst an interface might appear smooth on a macro-scale, it consists of regions of asperity contact and air gaps on a micro-scale. The reflection of an ultrasonic pulse at such a rough contact can be used to give information about the contact conditions. The more conformal the contact, the lower the proportion of an incident wave amplitude that will be reflected. In this paper, this phenomenon has been used to produce maps of contact pressure at three automotive component contacts: a cam/follower interface, a valve tip contact and a tripode drive joint.

An ultrasonic pulse is generated and reflected at the interface, to be received by the same piezo-electric transducer. The transducer is scanned across the interface and a map of reflected ultrasound (a c-scan) is recorded. The proportion of the wave reflected can be used to determine the stiffness of the interface using a spring model. Stiffness correlates qualitatively with contact pressure, but unfortunately there is no unique relationship. In this work, contact pressure is obtained from results of an independent calibration experiment used to relate the two. The information obtained can be used to redesign geometries and alter surface finish etc. to reduce high contact stresses that can lead to wear or fatigue occurring.