Grease film evolution in rolling elastohydrodynamic lubrication contacts
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Grease film evolution in rolling elastohydrodynamic lubrication contacts Xinming LI1,*, Feng GUO1, Gerhard POLL2, Yang FEI1, Ping YANG1 1
School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao 266250, China
2
Institute of Machine Design and Tribology, Leibniz University Hannover, Hannover 30167, Germany
Received: 18 December 2019 / Revised: 13 February 2020 / Accepted: 07 March 2020
© The author(s) 2020. Abstract: Although most rolling element bearings are grease lubricated, the underlying mechanisms of grease lubrication has not been fully explored. This study investigates grease film evolution with glass disc revolutions in rolling elastohydrodynamic lubrication (EHL) contacts. The evolution patterns of the grease films were highly related to the speed ranges and grease structures. The transference of thickener lumps, film thickness decay induced by starvation, and residual layer were recognized. The formation of an equilibrium film determined by the balance of lubricant loss and replenishment was analyzed. The primary mechanisms that dominate grease film formation in different lubricated contacts were clarified. Keywords: grease lubrication; rolling contacts; starvation; replenishment; elastohydrodynamic lubrication (EHL)
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Introduction
As a primary method for rolling element bearing lubrication, grease lubrication has been extensively studied both on model test apparatus and on a full bearing testing bench [1, 2]. However, the underlying mechanisms of grease lubrication are too complicated to be fully explored. Unlike conventional oil lubrication where film thickness can be calculated through a simple formula [3], grease film thickness can be predicted with less confidence. Apart from the dimensionless parameters of speed, load, material, and geometry [3], additional parameters due to the nature of the grease, such as structure, thickener type, concentration, and so on [4–11], and especially the inlet grease amount [12–15], will have an equal or even more significant influence on film thickness determination. Greases are usually classified as non-Newtonian substances with yield shear stress (Bingham plastic) [16], providing different rheological responses to both shear rate (viscoelasticity) [17] and shear duration (thixotropy) [18]. Below its yield shear stress, the bulk grease is * Corresponding author: Xinming LI, E-mail: [email protected]
retained, and acts as a reservoir and seal [19], and it does not readily reflow to induce starvation [12–15] and form a corrugated cavitation pattern at both sides of the rolling track [12, 13, 20, 21]. Under fully flooded conditions, the variations of effective/apparent viscosity with shear rate (shear thinning effect) cause the film thickness to decrease initially and then increase with speed, forming a “V”-shaped curve [8, 21–24]. Under constant speed with single charge, the film thickness initially exceeds that of the corresponding base oil and then rapidly decreases to below
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