A Broader Look at Pedestrian Friction

Abstract

Abstract The classical concept of friction having just two constant coefficients, static and dynamic, with static friction always greater than dynamic, is totally inadequate for polymers, especially for rubbers and plasticized PVC. The view that static friction is always greater than dynamic cannot be supported by experimental evidence. Theories of pedestrian stability based solely on this view are incorrect, and experimental programs based on them are inadequate and misleading. Under conditions of true lubrication the classical view remains more or less true, but the presence of a liquid between two test surfaces is no guarantee of lubrication, and some footwear materials in the presence of water have a higher friction than when dry. Stability when walking depends on several factors. Firstly, the level of friction at a velocity close to zero must be at a certain minimum level which exceeds the maximum H/V values (friction values) indicated in force plate experiments. Secondly, as velocity increases and as the area of contact and pressure of the foot change, this friction value must be maintained (a minimum requirement) or preferably should increase. For safety the coefficient of friction should increase as velocity increases, so that sudden and uncontrollable slipping cannot occur. A dangerous situation arises when friction is barely adequate at low velocities (nominally the static friction of the literature) and falls below the minimum acceptable H/V value as velocity increases. Experiments that measure single point friction values and take no account of the change of friction with velocity are misleading. Only in those cases where friction is largely independent of velocity will the results of such tests be comparable either with service conditions or with other single point tests. Arbitrary comparisons of different test methods in which single point results are obtained under very different test conditions are largely meaningless. Few of the test machines at present used for pedestrian friction are adequate for their purpose. Two types of tester are needed (a) a laboratory test for quality control of either footwear materials or flooring materials as appropriate, (b) a portable tester for checking installed floors and the effect of floor treatments. In either case only instruments capable of measuring over a range of test conditions, in particular a range of velocities, should be considered. In comparative tests some form of practical service test, such as the SATRA ramp, should be included. Whether any test has been accepted as a Standard Method by any country is irrelevant and acceptance of a test by a Standards Committee should not be taken as a measure of its correctness. Methods of specimen preparation, test conditions, standard test surfaces and contaminants need to be closely specified. It is suggested that for quality control purposes some universally available standard material, such as plate glass be chosen, the appropriate friction characteristics for safety on such a surface being established initially by practical experiment. As friction measured on clean, new, test pieces bears little relation to values which prevail in service, it is also necessary to define some universally available contaminant which bears some resemblance to the dirt or dust picked up by shoe soles in practice. It is hoped that this broader look at pedestrian friction will draw closer together those engaged in research work and engineers in the field concerned with establishing practical standards. As Westover and Vroom have said, the work of those doing research into the mechanism of friction seems too abstract in nature to be of use. However, it has been shown here that much valuable information, long available to engineers, has been ignored and the subject of pedestrian friction has for too long been isolated from that of polymer friction in general.