The frictional behaviour of wood

Abstract

The work described was primarily concerned with determining the more important variables affecting friction between wood and steel, but friction between wood and non-ferrous materials including wood itself was also investigated briefly. Deviations from the classical laws of friction were of interest, the most significant being variation of friction coefficient with sliding speed. With an increase in sliding speed up to 4 m/sec the curves for highly polished steel showed undulations, but with unpolished surfaces there was a monotonic reduction, somewhat greater in wet wood than dry. Other important factors, interacting with sliding speed, were steel roughness and wood moisture content. Effects of load, nominal contact area and fibre direction were minor. The results appear to be adequately explained in terms of adhesion and lubrication. A world wide selection of species was tested, and it appears that the nature and amount of extractives in most woods is such that they have similar friction coefficients except on very smooth steel, and only a few “greasy” species, have significantly lower coefficients. For most air-dry wood in contact with unpolished steel, the coefficient decreases from a static value about 0.65 to a value about 0.4 at 4 m/sec. For wet wood, the corresponding values are about 0.7 and 0.15. Coefficients of friction between wood and wood were similar to those between wood and rough steel, and those between wood and other materials varied in a manner which may be related to strength of adhesion.