A statistical study of the coefficient of friction under different loading regimes

Shrink-fits are common engineering fastening systems that comprise a cylindrical hub and shaft locked together by a radial pressure due to interference in size at their interface diameter. Frictional forces at the interface allow the transmission of a torque or resistance to axial movement in the assembly. The measurement of the coefficient of friction under simulated shrink-fit conditions is difficult and time consuming. A flexible experimental approach is presented that can test over a range of pressures, sizes, loading directions, i.e. torsion, compression and tension, and which has the ability to detect changes in the normal load applied due to loading direction. A series of statistical validation studies are conducted on 15 mm diameter low carbon steel specimens to verify theoretical formulations that suggest the coefficient of friction is different for the different loading regimes, and also identifies the surface roughness measured axially or radially as being contributory to the final coefficient of friction value. The experimental approach is applied to a novel laminate sleeve for an electrical machine in order that high confidence is achieved in measuring the coefficient of friction under realistic service conditions.