The tribology of rosin

Rosin is well known for its ability to excite stick-slip vibration on a vio lin string but the precise characteristics of the material which enable it to exhibit this behaviour have not been studied in any detail, A method is described in which the coefficient of friction of rosin is measured during individual cycles of a stick-slip vibration. Friction versus sliding veloci ty characteristics deduced in this way exhibit hysteresis, similar to that found in other investigations using different materials, No part of the hys teresis loops follow the friction/velocity curve found from steady-sliding experiments. Possible constitutive laws are examined to describe this frict ional behaviour. It is suggested by a variety of evidence that contact temp erature plays an important role, Friction laws are developed by considering that the friction arises primarily from the shear of a softened or molten layer of rosin, with a temperature-dependent viscosity or shear strength. T he temperature of the rosin layer is calculated by modelling the heat flow around the sliding contact, The temperature-based models are shown to repro duce some features of the measurements which are not captured in the tradit ional model, in which friction depends only on sliding speed. A model based on viscous behaviour of a thin melted layer of rosin gives predictions at variance with observations. However, a model based on plastic yielding at t he surface of the rosin gives good agreement with these observations, (C) 2 000 Elsevier Science Ltd, All rights reserved.