When two parallel surfaces slide parallel to each other in the presence of a liquid, classical lubrication theory shows that no load carrying ability of the lubricant should result. In experiment after experiment it has been clearly demonstrated that a large and useful load carrying capability often does develop in such situations. Since the successful operation of a significant fraction of sliding bearings and face seals may depend upon this phenomenon, a better understanding would be very useful for bearing and seal design. In this paper much of the known data on parallel sliding experiments are reviewed and compared. Seals as well as bearings are included in this data base. A wide range of conditions and viscosities are included. Some recent work on parallel sliding in water is examined. A comparison of parallel sliding to tilted sliding is also made. It is shown that a strong load support mechanism is present in all of the experimental results. The experiments clearly show that as speed is increased the bearing surfaces are lifted up such that asperity contact and friction are reduced. While explanations are often given for individual sets of experiments, the pervasiveness of this behavior suggests that perhaps there is some mechanism common to all parallel sliding which is not well understood. A mixed friction model is developed and used to explain some of the results. Conclusions are reached concerning several characteristics of parallel sliding. The paper concludes that given the importance of this phenomenon, careful evaluation of the various possible load support mechanisms should be made so that an effective direction for further research can be established.

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