Experimental Study on the Physics of Thick EHL Film Formation with Grease at Low Speeds


An unexpected behavior of grease in rolling contact has become known among tribologists that it forms thick film at low speeds, but the nature of the film has still been in dispute. The dominance of EHL effects has been demonstrated in the previous works, in which the increase in the generalized viscosity of grease has been reasoned to cause the increase in the film thickness. This paper describes experiments to measure the film thickness with optical interferometry, to observe the flow in and around the contact with microscopy, and to determine the change in the concentration of the thickener by IR spectroscopy. On the basis of these findings, the physics of the thick film formation with grease at low speeds is discussed.

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Correspondence to Takashi Nogi.

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The Carreau–Yasuda viscosity equation

$$\eta^{*} = (\mu_{1} - \mu_{2} )(1 + (\lambda \dot{\gamma })^{a} )^{(n - 1)/a} + \mu_{2}$$

is an empirical equation being used for describing non-Newtonian rheology of polymer melts or polymer solutions. It gives a generalized viscosity η* as a function of the shear rate \(\dot{\gamma }\) such that a transition occurs from the first, high Newtonian viscosity μ1 at low shear rates to the second, low Newtonian viscosity μ2 at high shear rates. The three parameters are used to characterize the transition: it occurs at lower shear rate for larger λ, becomes smoother for smaller a and more gradual for smaller n. No means exist to give a priori the values of these parameters, and they were hypothesized through rheometry of the sample greases.

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Kochi, T., Sakai, M., Nogi, T. et al. Experimental Study on the Physics of Thick EHL Film Formation with Grease at Low Speeds. Tribol Lett 67, 55 (2019). https://doi.org/10.1007/s11249-019-1166-7

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  • Elastohydrodynamic lubrication
  • Grease
  • Film thickness
  • Low speed