Measurement of interface pressure in interference fits

When components such as bearings or gears are pressed onto a shaft, the resulting interference induces a pressure at the interface. The size of this pressure is important as many components fail because fatigue initiates from press-fit stress concentrations. The aim of the present work was to develop ultrasound as a tool for non-destructive determination of press-fit contact pressures. An interference fit interface behaves like a spring. If the pressure is high, there are few air gaps, so it is very stiff and allows transmission of an ultrasonic wave. If the pressure is low, then interface stiffness is lower and most ultrasound is reflected. A spring model was used to determine maps of contact stiffness from interference-fit ultrasonic reflection data. A calibration procedure was then used to determine the pressure. The interface contact pressure has been determined for a number of different press- and shrink-fit cases. The results show a central region of approximately uniform pressure with edge stress at the contact sides. The magnitude of the pressure in the central region agrees well with the elastic Lamé analysis. In the more severe press-fit cases, the surfaces scuffed which led to anomalies in the reflected ultrasound. These anomalies were associated with regions of surface damage at the interface. The average contact pressure in a shrink-fit and press-fit joint were similar. However, in the shrink-fit joint more uneven contact pressure was observed with regions of poor conformity. This could be because the action of pressing on a sleeve plastically smooths out long wavelength roughness, leading to a more conforming surface.