The Failure of Fluid Film at Nano-Scale

The determination of hydrodynamic film failure has become one of the key aspects in the study of thin film lubrication (TFL) since the hydrodynamic effect of fluid film at nano-scale can be observed with recently developed experimental techniques. In the present paper, the relative optical interference intensity (ROII) technique with a resolution of 0.5 run in the vertical direction has been used to measure the film thickness. Experimental results show that the hydrodynamic effect can be clearly observed even at very low speed if the contact pressure is sufficiently low or if the viscosity of lubricant is comparatively high. When the pressure increases to a certain degree, the film will suddenly drop to the dimension of several layers of molecules and this is where the failure of the fluid film has taken place. For different viscosity of lubricants, the fluid film failure occurs at different rolling speeds and pressures. In addition, when the normal load becomes higher, a higher speed or larger viscosity is required to form the fluid film in the contact region. Finally, the effects of pressure, viscosity, and velocity on the occurrence of fluid film failure have been examined and a relationship involving the three parameters is proposed.     

Starvation and Reflow in a Grease-Lubricated Elastohydrodynamic Contact

The lubrication mechanisms of a grease in a rolling-element bearing has been studied through the measurement of film thickness in a rolling point contact. To simulate bearing conditions the contact runs under fully starved conditions; there is no attempt to maintain bulk flow of the grease into the inlet using an external supply. In consequence the film thickness drops off rapidly as the contact progressively starves. After a few minutes rolling (at constant speed) an equilibrium film thickness is attained which has two components: a residual film (h_R) comprised of degraded grease thickener and a hydrodynamic component (h_EHD) due to the liquid phase from the grease. The hydrodynamic contribution represents a balance between lubricant lost from the contact and replenishment from the grease close to the track. The ability of the grease to replenish the rolling track has been inferred from measurements of lubricant reflow around the static contact. These results are discussed in light of current starvation and grease lubrication models.     

Boundary Film Formation by Lubricant Base Fluids

The measurement of lubricant film thicknesses in a rolling steel ball on glass flat contact, down to one nanometer is now possible by using ultrathin film interferometry. This technique has been used to study the film-forming properties of a range of synthetic base fluids in the mixed elastohydrodynamic and boundary lubrication regimes.

For a very highly purified sample of hexadecane, it was found that classical EHD theory was obeyed down to less than one nanometer, indicating that any boundary film formed by this fluid was less than I nm thick. Most other synthetic fluids tested gave thicker films than predicted from EHD theory under very thin film conditions. This deviation from theory occurred at between 1 and 10 nm for different fluids. The effect can be interpreted as resulting from the presence of boundary layers, one or two molecular layers thick of the fluid on each solid surface, which are more viscous than the bulk lubricant.     

蜗杆传动的弹性流体动力润滑

从弹性流体动力润滑理论和蜗杆传动的特点出发，分析了有关文献对蜗杆传动弹性流体动力润滑条件分析中的不足，指出了滚动速度是建立润滑油膜和油压的重要条件，论证了滑动速度对润滑油膜和油压的形成也有较大影响。采用Dowson公式计算油膜厚度时，为了考虑滑动速度的影响可引入滑动速度系数Ks予以修正。     

Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts

The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within non-conformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11 mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc.Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions.From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts.     

EHL Performance of the Lubricant With Shear Strength: Part I — Boundary Slippage and Film Failure

This paper analytically investigates the isothermal line contact elastohydrodynamic lubrication of three lubricants with much different shear strengths under the nondimensional operating parameters of w = 2.15e-4 and U = 2.53e-10 applying the lubricant ideal viscoplastic rheological model. The boundary slippage of the low-shear-strength lubricant occurring in the EHL inlet zone was found and results in a much thinner film compared to the classical EHL theory prediction. The film boundary slippage and its growth with the slide/roll ratio variation of tile low-shear\- strength lubricant exhibit special phenomena, which are much different from those of the high-shear-strength lubricant. The easy occurrence of film failure in concentrated contact in the case of high sliding speed, heavy load, large slide/roll ratio, and low-shear-strength lubricant was concluded due to the severe friction heating on the surface conjunction and the lubricant thermal desorption on tile lubricant/surface boundary. The EHL film failure mechanism was further recognized.     

Behavior of thin viscous boundary films in lubricated contacts between micro-textured surfaces

Thin film colorimetric interferometry was used to map changes in film thickness in the vicinity of micro-dents of various depths produced on rubbing surfaces. It has been shown in recent studies that shallow micro-features within concentrated contact can increase mean film thickness by supplying more lubricant to the contact; however, this beneficial effect can also be accompanied by a local film thickness reduction. Nevertheless, these observations were done with mineral base oils that exhibited no boundary films formation. In this study the behavior of micro-textured surfaces are observed using formulated lubricant containing polyalcylmethacrylate (PAMA), viscosity index improver with boundary film forming properties. Obtained results show that an enlarged film thickness due to the presence of viscous boundary films is formed within the whole contact and these boundary films minimize the local film thickness reduction caused by micro-dents and further increase the efficiency of surface texturing within non-conformal contacts. It can be suggested from the obtained results that joint action of both boundary film formation and surface texturing combines both contributions that can help to increase tribological performances in different stages of machine parts operation by increasing lubrication film thickness.     

Effect of surface texturing on elastohydrodynamically lubricated contact under transient speed conditions

Effect of surface topography modifications on lubrication film thickness within non-conformal lubricated contact operated under transient speed conditions is observed. Optical test rig is used to observe the lubricant film behaviour between the flat surface of a chromium coated glass disc and a steel ball under simplified operational conditions modelling the cam and tappet contact. Numerical simulation was used to be able to choose the operating conditions suitable for experiments. An array of micro-dents was produced on the ball surface to be able to demonstrate the effect of surface topography on lubrication film formation. Experiments were carried out under elastohydrodynamic lubrication conditions. Obtained results have shown that surface texturing could represent the way how to increase lubrication efficiency of rolling/sliding non-conformal contacts under transient operational conditions through the lubricant emitted from micro-dents. It was found that the lubricant emitted from the micro-dents helps to separate rubbing surfaces especially under thin film lubrication conditions where the rubbing surfaces moves in the opposite direction.     

Behavior of real roughness features within mixed lubricated non-conformal contacts

A combination of thin film colorimetric interferometry and phase shifting interferometry was used to study the effect of slide-to-roll ratio on the micro-elastohydrodynamic action and asperity-contact mechanism on the real asperity scale. The behavior of the roughness features of different scales in very thin film, real rough surface elastohydrodynamic contacts was observed from chromatic interferograms evaluated by thin film colorimetric interferometry. Obtained film thickness distribution was compared with undeformed ball surface topography measured by phase shifting interferometry. It was confirmed that the presence of deep grooves within lubricated contact can result in lubrication film breakdown under positive slide-to-roll ratio conditions when the rough surface is moving slower than the smooth surface. Negative slide-to-roll ratio conditions are much less critical from this point of view. Moreover, shallow pits formed naturally on rubbing surface as a result of surface finishing process were observed to significantly influence the film thickness formation. They act as lubricant micro-reservoirs and emit the lubricant into the contact under rolling/sliding conditions that enlarges film thickness. Such a behavior also suggests the possible beneficial tribological effect of surface texturing based on shallow micro-cavities under mixed lubrication of non-conformal surfaces.     

Hydrodynamic lubrication analysis for tube spinning process

A theoretical analysis based on the two dimensional isothermal Reynolds equation was developed for the hydrodynamic lubrication of the tube spinning process. The linear velocity of the forming tool and rotational velocity of the mandrel both influence the establishment of a hydrodynamic lubricant film thickness at the inlet zone. Formation of a hydrodynamic lubricant film thickness at the inside of the tube is ruled by the eccentricity of the mandrel and tube. The theoretical and experimental estimates of film thickness were compared and are in agreement.     

A Two-Wavelength Model of Surface Flattening in Cold Metal Rolling with Mixed Lubrication

A new model of surface flattening is developed for cold metal rolling in the mixed regime. Longitudinal surface roughness is modeled by two separate wavelengths. The new model follows the asperity crushing analysis of Sutcliffe (1999) for unlubricated rolling but additionally includes a hydrodynamic model to account for the effect of the lubricant. The effect of various parameters including speed, reduction in strip thickness, roughness wavelength and lubricant properties is examined. The results show similar behavior to previous models of mixed lubrication, with a speed parameter A_s having the most influence, and confirm the results for unlubricated rolling that the short wavelength components of the surface roughness persist more than the long wavelength components. The predicted changes in roughness are in good agreement with experiments.     

超薄膜润滑径向轴承模拟计算

粘度是润滑剂的主要性能参数，随着油膜厚度的改变而在改变，在超薄膜润滑中特别突出，采用粘度修正雷诺方程来求解轴承的性能，对无限宽轴承来说，可得出其解析解，从而对其它轴承的计算提供依据。     

柴油机滑动轴承热流体动力润滑仿真研究

根据径向滑动轴承热流体动力润滑理论,基于JFO理论提出的质量守恒边界条件,建立同时包含油膜完整区和空 穴压力变化的单缸柴油机滑动轴承热流体动力润滑模型,采用有限差分法求解模型方程,仿真分析滑动轴承的油膜厚度、油膜压力、润滑油流量和温度等参数对润滑性能的影响,分析内燃机滑动轴承润滑特性,为轴承润滑可靠性设计提供一定的理论依据.     

轧钢机动压轴承油膜补偿与临界轧速分析

采用有限元方法求解雷诺方程,对形成动压润滑的临界轧制速度以及影响板厚轧制精度的油膜厚度进行计算机数值模拟以评价不同轧制工况下油膜轴承的承载能力性特性参数。计算得出的不同轧速下厚预报值以及临界轧速一实际数据吻合。     

Effect of surface texturing on mixed lubricated non-conformal contacts

The behaviour of surface texturing based on shallow micro-dents was observed within mixed lubricated non-conformal contacts and compared with results obtained under thin film elastohydrodynamic conditions. Thin film colorimetric interferometry was used to observe the changes in lubrication film thickness. It was found that lubricant emitted by micro-dents could effectively lift off the real roughness features that provided an increase in average but also the local minimum film thicknesses. On the contrary to smooth contact conditions no film thickness reduction is obvious either downstream or upstream the micro-dent. The possible beneficial effect of surface texturing on mixed lubricated contact was checked through the qualitative wear test. It confirmed that an array of shallow micro-dents reduced asperity interactions of rubbing surfaces. Moreover, the effect of micro-dents on rolling contact fatigue was also considered in this study. It has been shown that individual dents would have to be much deeper compared to those used in surface texturing experiments to cause reduction in contact fatigue life. It can be suggested from the obtained results that properly designed surface texturing could help to increase the separation of rubbing surfaces under mixed lubrication conditions.     

A Lubrication Deviation From the Classical EHL Theory by the Lubricant Viscoplasticity: Part II — Boundary of Lubrication Regimes

This paper distinguishes elastahydrodynamic lubrication (EHL) regimes in isothermal pure rolling case as three kinds according to lubricant rheology. In these regimes, the lubricants are respectively viscoelastic, viscoplastic, and non-continuum. Mathematical expressions are introduced ta describe the boundaries among these three regimes. H_c - r_it and U - W charts respectively plot the operational scopes of these lubrication regimes. The present study holds significance to understanding EHL film formation and more clearly embodies elastohydrodynamic film failure stage transitions.     

Theoretical Analysis of Piston-Ring Lubrication Part II—Starved Lubrication and Its Application to a Complete Ring Pack

Most studies of piston-ring lubrication assume fully flooded lubrication in the cylinder-bore and piston-ring conjunction. However, the lubricant supply is not always adequate for fully flooded lubrication. This study incorporates the effects of lubricant starvation into the one-dimensional piston-ring analysis developed earlier and applies it to a complete ring pack. A system of three nonlinear equations is derived to solve the starved lubrication problem. A postulate for lubricant transport in a complete ring pack is also proposed. A computer code is developed to apply the starved lubrication model to a complete ring pack.

The findings reveal that lubricant starvation has an important effect on the minimum film thickness of a ring pack, especially at mid-stroke for compression rings. The role of oil-control rings and the evaluation of ring performance as a complete ring pack are also discussed.     

滑动条件下弹流润滑的屈服膜厚与屈服边界

本文讨论了弹流润滑的屈服问题，从润滑剂的极限剪应力可以得到屈服膜厚的表达式。屈服膜厚是弹流润滑膜的下降 ，当利用拟合公式得到的膜厚小于屈服膜厚时，由于润滑已经处在非牛顿区，所拟全公式不再适用。本文还给出了用膜厚形式表示的屈服准则，并详细讨论了最大压力，平均速度和滑滚比对屈服膜厚的影响。     

A Transient Mixed Lubrication Model of a Rotary Lip Seal with a Rough Shaft

A rotary lip seal, widely used in machines containing rotating shafts, is usually protected from mechanical and thermal damage by a thin film of lubricant under the lip, separating the lip and the shaft surfaces. However, under some transient conditions such as those during startup and shutdown, the fluid film is not fully established or it breaks down, and the seal operates in the mixed lubrication regime. To simulate such cases, a transient mixed lubrication analysis has been developed. It generates predictions of such seal operating characteristics as load support sharing between hydrodynamic and contact pressure, contact and cavitation area ratio, the reverse pumping rate, and the average film thickness. In most previous numerical simulations of the rotary lip seal, the shaft surface is modeled as perfectly smooth. In the present study, a more realistic shaft surface with asperities is used, and the effect of the shaft surface roughness on the behavior of the seal is investigated.