Friction Characteristics and Mechanisms of Two Lithium Greases in Elastohydrodynamic Lubrication
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TECHNICAL ARTICLE—PEER-REVIEWED
Friction Characteristics and Mechanisms of Two Lithium Greases in Elastohydrodynamic Lubrication Yanshuang Wang . Baojie Wu
Submitted: 24 June 2020 Ó ASM International 2020
Abstract Two types of lithium greases were fabricated by using PAO and polyol ester as two base oils. The friction tests for the lithium soap-based greases were performed under different operating conditions by minitraction machine, and the data were compared and analysed. The results show that when the lithium-based soap thickens different types of base oils with the same viscosity, the greater the elastohydrodynamic friction coefficient of the base oil, the greater the elastohydrodynamic friction coefficient of the grease. In a low-speed bearing or contact pair, both base oil and thickener go through the contact zone to contribute to the friction of the grease. When the same lithium-based thickener thickens the different base oils, the greater the interactions between the base oil molecules, the greater the interactions between the base oil molecules and the thickener molecules, the wider the soap fibres, and the greater the friction coefficient of the grease. The friction coefficient of oil PAO–lithiumbased soap grease is higher than that of its base oil PAO. The friction coefficient of oil polyol ester–lithium-based soap grease is less than that of its base oil polyol ester when the viscosities of the two base oils are 68 mm2/s. Keywords Friction characteristics Lithium grease Elastohydrodynamic lubrication Friction mechanisms
Y. Wang (&) School of Mechanical and Automotive Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, Shandong, China e-mail: [email protected] B. Wu China Sinopec Lubricant Co., Ltd. Tianjin Branch, Tianjin 300480, China
Introduction 90% of rolling bearings are lubricated by grease in practical engineering applications [1–3]. When the bearing is installed correctly without any material or structural problems, the service life of the bearing is mainly determined by the service life of the grease and its friction characteristics. Fan Xiaoqiang [4] measured the tribological properties of four types of self-made greases on an SRV friction and wear test machine, and pointed out that the thickener in the grease formed a physically deposited film and a chemical reaction film with the contact surface under the condition of boundary lubrication. In the literature [5], an SRV friction and wear testing machine was used to test the friction characteristics of a calcium–lithium mixed grease, and it was also considered that the components of the grease formed a layer of friction chemical reaction film with the friction surface. In many reports, an SRV friction and wear testing machine and four-ball friction and wear testing machine were used to study the influence and improvement of various grease additives on friction and wear properties under boundary lubrication states [6–11]; however, the motion of the rolling elements relative to the racewa
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