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Title: Considerations on the Lift-up effect in Sliding Friction
Authors: Janssens, Thierry
Al-Bender, Farid #
Issue Date: 6-Sep-2009
Publisher: Japanese Society of Tribologists
Host Document: Proceedings of the 4th World tribology Congress pages:222
Conference: World Tribology Congress edition:4 location:Kyoto, Japan date:6-11 September 2009
Article number: P-132
Abstract: One characteristic of sliding friction is that the apparent load carrying capacity of the contact increases with the sliding speed. This is true for both dry and lubricated friction. In dry conditions this is so because, owing to decreasing adhesion influence with speed, a single asperity will be more active, so more in contact, per unit displacement, or more "available" for load carrying. In lubricated conditions a pressure build up can be observed with rising sliding speed due to the hydrodynamic effect of the fluid. In this configuration at high speed or low load the contact can be seen as a hydrodynamic bearing. The lubrication regimes are commonly divided into: Boundary Lubrication (BL), Mixed Lubrication (ML) and Full Lubrication or Hydrodynamic Lubrication (FL, HL). In the BL regime the lubricant’s hydrodynamic action is negligible and the load is carried directly by the surface asperities, similar to dry friction contact. In de ML regime the load is carried by the lubricant’s hydrodynamic action and/or directly by surface asperities. When the load is carried totally by the lubricant film the contact enters FL regime. Consequently, when the normal load is constant, for a given slider, the number of contacting asperities will decrease with increasing sliding speed, i.e., the slider will lift up. This behaviour has already been observed by several researchers. Several explanations have been offered in the past, notably (i) Amontons’ classical, but controversial hypothesis that the asperities on the mating surfaces have to surmount each other (the mean asperity slope representing then the coefficient of friction), and (ii) Tolstoi’s questionable hypothesis that the effect be owing to induced normal vibrations coupled with the non-linear asperity stiffness behaviour. Another explanation of the phenomenon could be given, namely that it is a direct consequence of adhesion, deformation, creep and pressure build up, which in fact resolves the contradiction of the first view without excluding the second. Here, we present experimental evidence of the increasing bearing capacity which results in a lift-up effect for dry as for lubricated friction. We performed dry and lubricated, sliding rubbing experiments that show a relative normal displacement associated with the tangential motion. In particular, with periodic tangential input, the normal motion describes regular, hysteresis, butterfly like curves (similar in nature to those found in piezo-electric and magnetic materials). This paper explores the basic behaviour of the lift-up effect and those butterfly curves experimentally.
Publication status: published
KU Leuven publication type: IC
Appears in Collections:Production Engineering, Machine Design and Automation (PMA) Section
# (joint) last author

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