| Elastohydrodynamic lubrication(EHL)is the main lubrication mechanism for mechanical components such as gears and bearings.At present most of the film thickness formulas of EHL are for the surface velocities of contact surfaces in the same direction.However in engineering practices,the velocities of the two contact surfaces of the components such as hypoid gears is not in the same direction,and the aforementioned film thickness formula can not be used.Therefore,studies of the lubricating oil films with non-collinear velocities of the contact surfaces is of great significance to reveal the lubrication mechanism of such components and to improve their comprehensive performance.On the other hand,various dimples in oil films have become a hot topic in the area of EHL.Researchers have proposed various explanations and ideas,where thermal viscosity wedge,wall slippage and speed effect are individually employed to explain the dimpled films.Therefore,the present work carries out comprehensive experimental and theoretical studies about the surface velocity,thermal viscosity wedge and interface characteristics of EHL,and systematically shows their influences on the generation of various dimples,and reveals the lubrication characteristics and mechanism of the transmission interfaces.Main works completed are as follows:(1)An optical interference test rig for film thickness measurement in a ball-on-glass contact,which is featured with a circular arc guiderail,has been manufactured.The test rig is capable of measuring EHL oil film under the conditions of non-collinearity surface speeds with a fixed angle,and common working conditions for EHL can be explored.(2)The oil film characteristics of EHL of PAO,PB(Polybutene)oils under conditions of pure ball/disk sliding,pure rolling,zero-entrianment,various slide-roll-ratios were studied.Experiments showed that under conditions of low speeds the EHL films,which were produced by pure sliding of the glass disk and the steel ball,were characterized by a wedge-shaped gap in the contact area and a crescent-shaped entrance dimple,and their formation was mainly controlled by interfacial slippage.When the dominant mechanism of the film dimple shifts from interfacial slippage to thermal viscosity wedge,a fusiform inlet film dimple emerges.Under the conditions of heavy loads and high speeds,the interfacial slippage effect is not dominant,and the circular dimple with increasing the oil film temperature comes from the thermal viscosity wedge.(3)The EHL oil film measurement of PAO,PB oils under the conditions of zero entrainment was completed.The traditional theory holds that the formation of oil film under the conditions of zero entrainment is always accompanied by the appearance of a film dimple.In contrast,the present work showed that the EHL film under zero entrainment velocity is not necessarily accompanied by the appearance of dimples.(4)For the PB and PAO oils,experimental study of the EHL films under non-collinear surface velocity of the same magnitude was carried out under fully flooded oil supply.The lubricating film shape showed some asymmetry,and this phenomenon was more obvious with the increase of speeds and loads.The asymmetry of the film shape and the position shift of the minimum film thickness were caused by thermal effect.The relative sliding speed is the dominant factor in the heat generation.Under the conditions of non-collinear ball/disk velocities,with the increase of the velocity difference,the EHL film showed obvious asymmetry along the direction of entrainment velocity.The change of the ball/disk velocity ratio has marked influence on the film thickness than that on the pressure and temperature.The symmetry of the oil film shape can be effectively improved by adjusting the speed ratio of the ball speed to the glass speed.(5)Wettability has great influence on the interfacial slippage.Under the AF-modified surface,the interfacial slippage was enhanced and the pure glass disk sliding presented more obvious inlet dimples for PB1300.Under the conditions of non-collinear speeds,the surface with wettability step by AS promoted the establishment of the inlet oil pool,which obviously improves the lubrication performance.The lateral surface tension at the AS boundary promotes the unidirectional flow of the lubricant,forming a anomalous dimpled oil film.The surface with a wettability step and thermal effect is more conducive to the formation of film dimples.(6)A mathematical model of thermal EHL lubrication with non-collinear surface velocities has been established.Coordinate transformation was used to solve the pressure,temperature and film thickness in two directions.Numerical solutions were obtained,which were in good agreement with the experimental results.The change of the ball-to-disk velocity ratio has greater influence on the film thickness.The surface temperature of the glass plate and the temperature of the middle layer of the oil film are higher than the surface temperature of the steel ball,resulting in the wedge-shaped gap of the oil film.Non-collinear velocity combined with thermal viscosity wedge and interfacial slippage contributes to the formation of film dimples.When the ball-to-disk velocity ratio increases to a certain extent,the contact solid with a smaller velocity has little effect on the oil film,and the film thickness can be corrected. |
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