Multi-valued friction is often observed in lubricated contact systems operating under unsteady sliding conditions. It the past, this behavior has been modeled using a measured time delay between changes in sliding velocity and resulting changes in friction. More recent work has revealed that fluctuations in normal separation are the principal cause of this deviation from steady friction behavior. This paper demonstrates that tangential contact dynamics can result in apparent deviations from steady friction which can resemble true deviations found from fluctuations in normal separation. Two different dynamic models are considered. One model accommodates the tangential rider dynamics of a friction test apparatus. The other model considers the prospect of tangential dynamics associated with asperity contacts.

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