Simple Method for Determining the Dynamic Coefficient of Friction of Vulcanized Rubber

Abstract

Abstract A method for measuring the coefficient of friction at low speeds by means of a normal dynamometer for rubber testing is described. To this end a couple of molded rubber wheels are pulled over a piece of roadlike surface. At the same time the wheels are forced to rotate with a speed nonconcordant with the linear speed on the surface, so that a certain amount of friction occurs. The force necessary to turn the wheels over the surface is recorded on the dynamometer; the average is proportional to the average coefficient of friction. Preliminary experiments were carried out to prove the usefulness of the apparatus. A number of compounds of natural rubber, GR-S, and “cold” rubber were tested on four surfaces: asphalt, asphalt with rubber, concrete, and ground glass. Although the apparatus allows variation of load and of speed, the experiments were carried out at a speed of 100 cm. per minute and only with a load of 4 kilograms. On dry surfaces, the highest coefficient of friction was found with a natural-rubber compound on an asphalt material in which rubber powder had been dispersed. With most rubbers this surface showed a somewhat higher coefficient of friction than did the asphalt without rubber-powder, which in turn was better than the other two surfaces. On the average there was little difference between the various rubber compounds, though natural rubber seemed to yield the highest values. Natural-rubber pure gum compound did not show higher values than the carbon black mixes, but a vulcanizate with cyclorubber instead of carbon black was definitely better.