Catalin I. Serpe

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

Abstract 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.

Thermomechanical finite element analysis to describe the engagement and wear of electromagnetic clutches

Abstract The engagement of electromagnetic clutches is modeled as transient sliding of smooth and concentric mild steel rings. The coupled thermomechanical problem is solved by finite element analysis. Calculations of transient stresses and temperatures are presented. Wear predictions, based on a local Archard model, are compared with measurements. The principal finding of the work is that smooth surface coupled thermomechanical finite element analysis fails to accurately capture thermal distortions or lack thereof as observed in tests. The asperity contacts reach high temperatures, hundreds of degrees Celsius, while the surrounding material remains much cooler and exhibits low temperature gradients. Thermal distortions are suppressed and the system operates successfully under far more severe conditions than expected. Isothermal analysis better captures the macroscopic pressure distributions in these devices than does the thermomechanical model. We also suggest a modification to local Archard model, which relates local normal pressure to wear depth. Inclusion of the macroscopic (von Mises) stress distributions, which are different on two the sliding surfaces, better captures the shapes of the observed wear tracks. Accurate modeling of such contacts will ultimately require the simultaneous analysis of microasperity contacts and the full component geometries.

Contact Stiffness of New and Worn Surfaces

The presence of surface texture/roughness on engineering surfaces results in contacts between surfaces being considerably more compliant than if the interfaces were smooth and flat. The inclusion of a local contact stiffness can be critical to the accurate analytical or computational modeling of mechanical contacts. We present measurements of contact stiffness for five pairs of freshly prepared and worn of steel surfaces. The wom surfaces variously contain surface glazes, oxide layers, subsurface cracks, inclusions and wear particles. Contact resonance frequencies between sample interfaces are measured at various applied pressures. With known modal masses, the contact stiffness is easily calculated and presented on a per unit area basis. For a given contact pair, the contact stiffness is nonlinear, increasing with nominal pressure and decreasing with increasing surface roughness. We compare these results with the Greenwood-Williamson (G-W) Theory of rough surface contact. The expected pressure and summit height dependencies are observed in the measured data. When there are wear particles within the contact, the stiffness is reduced when the underlying surfaces are very smooth. If the underlying surfaces are rough, the presence of wear particles have little effect on the contact stiffness.Copyright

Understanding the Effects of Friction and Surface Roughness on Noise Generation From Elastic Components in Sliding Contact

One type of troublesome friction-induced noise, common in brakes, clutches and mechanical seals, is high frequency chirp or squeal. The frequencies at which these noises and underlying vibrations occur 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 elastic systems coupled by friction and a distributed 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. 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 ten per cent of the first forty or so vibratory modes are potentially unstable. Generally one or two of these appear as instabilities in the actual physical system being modeled. No stick-slip action needs to be invoked and these instabilities can occur with a single constant coefficient of friction.Copyright