High frequency noise generation from components in sliding contact: Flutter instabilities including the role of surface roughness and friction

One type of troublesome friction-induced noise, common in brakes, clutches and mechanical seals, is high frequency chirp or squeal. Chirp frequencies typically range from around 1 kHz to more than 10 kHz. We have found that the essential physical ingredients needed to model this problem are two finite distributed mass elastic systems coupled by friction and an interfacial contact stiffness, transverse to the direction of sliding. The contact stiffness is associated with the roughness of the sliding surfaces and, sometimes, with the presence of wear particles within the contact. Lumped parameter models are not adequate to capture the dynamics. Our approach is to perform an eigenvalue analysis, using finite elements, of pairs of coupled sliding elastic rings. Due to the presence of friction, the stiffness matrix is asymmetric and mode coupling or mode splitting can occur. Typically, around ten per cent of the first forty or so vibratory modes appear prone to instability. Generally one or two of these show up as instabilities in the actual physical system. No stick-slip action needs to be invoked and these instabilities can occur with a single constant coefficient of friction.