The influence of dust particles on the contact of solids

Abstract

An investigation has been made of the manner in which two solids touch when one of the contacting surfaces is contaminated with dust. The behaviour is discussed in terms of the prob­ability that intimate contact will be established between the solids in any particular closing operation. It is shown that this probability is related in a simple manner to the number and size of the contaminating particles, to the load between the solids, and to the nature of the contacting surfaces. The manner in which a dust particle can become trapped between the approaching bodies and thus prevent direct contact occurring is considered and a simple physical model of the processes is proposed. For this model, the dependence of the nature of the contact on the number and size of the contaminating particles is derived. The experimental behaviour is in good agreement with that predicted. The theoretical discussion indicates that the effectiveness of the contamination in preventing direct contact should fall off rapidly if the surface roughness is increased until the height of the irregularities is comparable with that of the particles. The effect has been verified experimentally. It has also been demonstrated that there is a sharp change in the probability of the occurrence of intimate contact if the area of contact between the solids is made comparable with the cross-sectional area of the particles. The practical significance of these observations is discussed. The various possible methods of increasing the probability that direct contact will occur are considered, and general relations are given by means of which the behaviour of two contacting solids under any given concentration of dust may be predicted.