A substantial body of qualitative information concerning highly viscoelastic materials suggests they may be much superior to Newtonian fluids in some lubrication applications. Contrarily, most quantitative studies have predicted negligible or even adverse effects of the non-Newtonian behavior. This paper is accordingly addressed to a broad consideration of rheological properties and a critical analysis of the validity of common simplifying assumptions, in order to educe major effects which have not been well understood. Largely omitted are those rheological factors which have already been subjected to intensive scientific study. It is seen that the behavior of viscoelastic fluids in elongational deformations and in deformations of short duration are of primary interest; positive effects of two orders of magnitude have been shown in closely related problems. It is shown that the exploitation of these non-Newtonian rheological properties requires favorable geometric and kinematic conditions in a bearing which may differ appreciably from those employed with Newtonian fluids. The conclusions reached are speculative rather than definitive because of the primitive state of the development of this branch of rheology at the present time. They may be helpful, however, and do serve to direct attention to several kinds of lubricant-rheological studies requiring special emphasis.

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