The Steady-State Dynamics of Jewel Bearing with a Free Ball

A type of jewel bearing employing two sapphire cup surfaces (one fixed and the other movable) which were in contact with a free steel ball were analyzed. This arrangement reduced wear substantially as compared to conventional jewel bearings and had more degrees of freedom. The analysis of the ordinary jewel bearing was extended to the present case in order to determine the bearing steady-state motion reached asymptotically from an arbitrary starting state. Two different steady-state motions were investigated, i.e., when the ball was motionless (stationary), and when the ball attained pure rotation with reference to the frame. In the first case, there was no relative motion at the contact area with the fixed cup, and the relative motion at the contact area with the rotating cup consisted of rolling, sliding and spinning. In the second case, the relative motion at the contact area with the fixed cup consisted of rolling, sliding and spinning and the relative motion at the contact area with the rotating cup was pure rolling. The analysis was supported and validated by numerical simulation.     

Simulating angular ball bearing lubricated elliptical contacts Film thickness and traction measurements

A high performance barrel and plate apparatus was built to study film formation and traction by simulating the real situation of a lubricated elliptical contact in an angular ball bearing under general kinematic conditions. Simultaneous measurements of load, speed of each surface, traction, and film thickness by optical interferometry can be performed. The sapphire disc plate and the steel barrel are driven independently at constant controlled speeds. Small relative sliding, lateral sliding and spinning near pure rolling conditions can be imposed by controlling barrel shaft angle contact location. Tests were performed at ambient temperature for a small barrel whose principal radii are 1.34 mm and 9.7 mm, for applied loads which generate Hertzian pressures up to 2 × 10⁹ N/m², and for a low viscosity mineral oil. Typical experimental results show that under elasto-hydrodynamic conditions, the centre film thickness is slightly below the values calculated from classical elastohydrodynamic theories and that oil starvation occurs at high speeds. Traction curves versus slide/roll ratio are presented for different loads and under spinning and lateral sliding conditions.     

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.     

往复冲击对脂润滑成膜性能影响的试验研究

冲击现象广泛存在于工业链、滚动轴承等机械零部件中,严重的情况下会引起冲击磨损。为探究冲击载荷对脂润滑条件下成膜性能的影响,在点接触光干涉弹流试验台上对锂基脂润滑条件下的膜厚演化进行冲击试验研究。试验时钢球和玻璃盘的初始间隙设置为0,冲击载荷按三角波往复变化。结果发现：在第一个冲击周期内,接触区存在大块的增稠剂纤维团,该纤维团造成接触区内的脂膜凹陷;随着冲击周期的增加,接触区内的大块增稠剂纤维团消失,脂膜厚度逐渐降低,润滑状态进入到薄膜润滑状态,最后发生了表面损伤;在任何一个冲击周期内,中心膜厚和最小膜厚大部分的时间都呈现固定值;中心膜厚随着冲击周期数的增加而减小,最小膜厚在最初的100个周期内变化很小,此后逐渐降低,最后为0。     

Film-forming Characteristics of Grease in Point Contact under Swaying Motions

The technique of relative optical interference intensity (ROII) and simple numerical calculations were used to investigate the lubricating behavior of grease lubricant films in the rolling direction under swaying motions (acceleration/deceleration). Experimental results indicate that at a same entrainment velocity of the inlet, the central film thickness under deceleration is larger than that under acceleration. The minimum central film thickness in one swaying cycle does not occur at the moment of zero entrainment, but at the initial period of acceleration. At the moment of zero entrainment, the central film is thicker than its peripheries, and the value of the central film thickness increases with increases in the changing rate of the entrainment velocity. It is thought that the transient behaviors of the grease lubricant film deviate from those in steady state conditions. The profiles of the transient film thickness and the approximate thicknesses of elastohydrodynamic contact in the rolling direction calculated by using a simple numerical method are supported by the experimental results. The numerical method can also be used to explain the behavior of the grease lubricating film under non-steady state conditions. An erratum to this article can be found at     

Power law fluid model incorporated into elastohydrodynamic lubrication theory of line contact

An algorithm is developed for the study of the infinitely long slider bearing in general form, considering the lubricant to be an incompressible power law fluid in isothermal conditions. The earlier works on this topic were considered by taking cavitation boundary conditions when a cylinder moves over a plane lubricated with a power law fluid and in EHL solution in a particular case, viz. pure rolling of a cylinder over an identical cylinder. We have considered a general solution including elastohydrodynamic lubrication (EHL) for different values of power law exponent. Deviation of values of central film thickness for different values of power law exponent from those for Newtonian lubricants are presented. The effects of the power law exponent on the central film thickness, minimum film thickness and load capacity are analysed. The effects of rolling and sliding velocities of contact surfaces are also analysed in terms of an equivalent radius of a cylinder moving over a moving plane. Film shapes and pressure distributions are also calculated numerically and presented graphically for various values of central film thickness considered in this paper. A number of observations obtained here with pseudoplastic nature of lubricants are in good agreement with the experimental results. The theoretical observations suggest the behaviour of common lubricants as pseudoplastic fluids in the cases of slowly moving surfaces and motion under heavy load.     

Effects of Slide/Roll Ratio on the Behaviours of Grease Reservoir and Film Thickness of Point Contact

The film formation mechanism of lithium complex grease under starved condition was proposed based on the analysis of the relationship between grease reservoir and the finger-shaped lubricant along the rolling track using a laboratory built ball-on-disc test rig. Film thicknesses with rolling time at different slide/roll ratios were measured and discussed in detail. Experimental results showed that starvation occurred soon after the operation under pure rolling condition. In contrast, the contact remained fully flooded under slide–roll condition. The measurement of grease fingers proved that slide/roll ratio contributed to replenishing the contact by transferring more grease to the vicinity of the contact to form a lager lubricant reservoir. The volume of grease fingers, the inlet lubricant supply and the film thickness at different slide/roll ratios were found to be in good agreement.     

Influence of temperature distributions in EHL film on its thickness under high slip ratio conditions

This paper describes the influence of temperature rise of oil film in Hertzian contact area on the film thickness or profile under high slip ratio conditions. Temperatures of both surfaces of a ball and a disk as well as average temperature across the oil film were measured by means of an improved infrared. Two kinds of optical band-pass filters were used to separate the radiations from the ball surface and the oil film through an infrared transparent disk made of sapphire glass. In case of temperature measurement of the disk surface, another sapphire glass disk was coated with 300 nm chromium layer on the contact surface to radiate the infrared from the disk surface and also to intercept the radiation from the ball surface and the oil film. Temperature profiles across the oil film were estimated by assuming a parabolic profile with the measured three kinds of temperatures.For case within 200% in slip ratio, both minimum and central film thickness decreased under constant entrainment velocity as slip ratio increased. Measured film shapes were not flat at central Hertzian contact region under high-slip condition and differed from the results by the conventional EHD theory assuming constant viscosity in the direction of film thickness. The profile of Couette flow varied due to the distribution of oil film temperature in thickness direction. The viscosity wedge action, that is the variation of the profile of Couette flow causes reduction of film thickness or deformation of film profile. For case over 200% in slip ratio, the relation between central film thickness and slip ratio under constant relative slide speed had a great difference from the results calculated from the formula presented by Chittenden et al.     

Measurements of Two-Dimensional Distribution of Refrigerant Concentration in EHL Film Using Micro FT-IR and Effect of Variation of Concentration on Oil Film Thickness

The dissolution of refrigerant into a lubricant causes a decrease in viscosity of the oil and it gives a large effect on the lubrication of sliding parts in a refrigerant compressor. This paper describes an application of micro FT-IR to measure the two-dimensional concentration distribution of refrigerant held in solution in the EHL film surrounded by the refrigerant gas and discusses the refrigerant concentration variation in the vicinity of the Hertzian contact area. In order to measure the concentration distribution, an apparatus which can observe the EHL film in a point contact in the refrigerant atmosphere was developed. The refrigerant concentration was measured using micro FT-IR through a CaF₂ window from outside of the apparatus with polyol ester as a base oil in an atmosphere pressurized with HFC-134a refrigerant gas. The results indicate that the concentration of HFC-134a refrigerant reduces in the inlet boost region of EHL contact and the Hertzian contact area but in the side region of Hertzian contact area it is greater than that in the bulk fluid. In addition, the effect of the variation on the oil film thickness is discussed.     

Starvation and Reflow of Point Contact Lubricated with Greases of Different Chemical Formulation

Starved film thickness measurements are presented in grease-lubricated point contact under pure rolling condition. The starvation process of different types of greases is recorded and analysed in detail. Results show that the film decay rates for lithium greases and SiO₂ greases are quite similar regardless of the rolling speed. Urea grease shows strong adsorbability to the metal surface, which can be clearly observed during the test. Oil separation tests and scanning electron microscopy observations proved the differences in structure and stability of the tested grease. A close relationship between starved film thickness and oil bleeding rate is found in the present research.     

Starved Grease Lubrication of Rolling Contacts

Many grease lubricated roller bearings operate in the starved elastohydrodynamic (EHL) regime where there is a limited supply of lubricant to the contact (1). Under these conditions the film thickness drops to a fraction of the fully flooded value (2) and, thus, it is difficult to predict lubrication performance, or bearing life, from conventional EHL models. In this regime film thickness depends on the ability of the grease to replenish the track rather than the usual EHL considerations. The conventional view of grease lubrication is that base oil bleeds from the bulk reservoir close to the track, replenishing the inlet and forming a fluid EHL film (3). Resupply, under starved conditions, will thus depend on both operating conditions and grease parameters. The aim of this paper is to evaluate the influence of these parameters on starved lubrication in a rolling contact. Starved film thickness has been measured for a series of greases and the results have been compared to the fully flooded values. These show that the degree of starvation increases with increasing rolling speed, base oil viscosity and thickener content but decreases at higher temperatures. In many cases an increase in absolute film thickness is obtained when moving from high viscosity base oil to a low one, this result is the reverse of normally accepted EHL rules. Taking the fully flooded film thickness as a guide to lubrication performance is therefore not valid as grease film thickness in the starved regime is determined by local replenishment rather than bulk rheological properties.     

Velocity fields in grease lubricated contacts

This paper examines the behaviour of a grease lubricant in open contacts. Analytical expressions of velocity profiles for a Bingham fluid are presented for three cases: flow where all the film thickness is sheared; where a non-sheared core zone is attached to one contact surface; and where a core zone floats in the middle of the film. An experimental study is described which used visualization through illumination across contact surfaces. Results are presented for two modes of operation: contact of cylinder and fixed plane surface, and pure rolling of two cylinders. In the former case, nine zones of operation are identified.     

急停问题的椭圆接触弹流润滑分析

将机械急停时滚动轴承、齿轮等的弹流润滑油膜的渐变过程简化为从稳恒状态突然转化为恒载荷纯挤压状态,然后随时间的推移挤压效应逐渐消失的过程,建立了椭圆接触瞬态弹流润滑模型。假设润滑油为牛顿流体,在等温条件下用多重网格技术进行了动态数值求解,讨论了急停前卷吸速度分别沿椭圆接触区的长轴和短轴方向时,不同参数条件下的残留弹流润滑膜的压力和膜厚随时间的变化规律。数值计算结果表明,急停后润滑油会逐渐被挤出接触区,因此残留油膜只能保持很短的时间;较高的Hertz接触压力会通过增加润滑油粘度而延长残余油膜的维持时间,但不能根本上改变上述变化趋势,卷吸速度的方向也不能改变上述变化趋势。     

新型光干涉法纳米级润滑膜厚度测量仪

润滑膜厚度的测量是开展纳米量级薄膜润滑状态研究的关键问题。利用光干涉法相对光强原理研制出一种纳米级润滑膜厚度测量仪,在低速低载荷条件下对点接触纯滑动润滑接触中心区润滑膜厚度进行测量,并讨论接触中心区和润滑膜厚度与速度和载荷之间的关系。结果表明:已测量的膜厚值已达到纳米量级,在设定工况下润滑膜厚度随速度增大而增大,随载荷增大而减小;比较Hamrock-Dowson公式计算结果和实验结论证明,这种仪器能有效实现对纳米级润滑薄膜厚度的测量。     

Occurrence of Wall Slip in Elastohydrodynamic Lubrication Contacts

Preliminary experimental work has been carried out to identify some of the boundary slip phenomena of highly pressurised polybutenes in an elastohydrodynamic lubrication (EHL) conjunction. The movement of the oil is signified using an entrapment that can be readily formed by the impact of a steel ball against a layer of oil on a glass block in an optical EHL test apparatus. The post-impact lateral movement of the entrapment was investigated under the conditions: (i) pure rolling, (ii) pure glass block sliding (steel ball stationary) and (iii) pure ball sliding (glass block stationary). It was observed that under pure rolling the entrapped oil travels within the contact region at the entrainment speed, which is correlated with EHL theory. Under pure glass block sliding conditions, the speed of the entrapped oil core is less than the entrainment speed, and in the extreme cases, this core can be nearly stationary. Under pure ball sliding conditions, the oil core moves at a speed greater than the entrainment speed. The observation indicates that the oil/steel ball interface can sustain higher shear stress than the oil/glass (chromium coated) interface and there is a boundary slip in terms of relative sliding at the latter interface under the experimental conditions. Furthermore, the amount of slip increases with an increase in the pressure. These experiments provide evidence of the existence of wall slippage, which leads to the abnormal EHL film profile characterised with an inlet dimple as reported earlier.     

Experimental and numerical investigation on the behavior of transverse limited micro-grooves in EHL point contacts

This study presents experimental and numerical investigations on the effects of transverse limited micro-grooves on the behavior of film thickness and friction in EHL point contacts. The tribological performance has been compared for smooth and textured surfaces in sliding and reciprocating motion and under starvation. The measurements were conducted by using a ball-on-disk tribometer equipped with a high speed camera and torque sensor. The results show that the transverse shallow micro-grooves with a length less than the diameter of the Hertzian contact are efficiently able to enhance the film thickness under different operating conditions. The beneficial effect under starved lubrication requires a mechanism for filling the depleted micro-grooves entering the contact with fresh lubricant. This mechanism can be attributed to the capillary effect in the inlet zone under starvation. The numerical simulation of the transient behavior of transverse limited micro-grooves shows agreement with experimental results. On the other hand, introducing micro-grooves as closed texture cells on one of rubbing surfaces results in a friction reduction in the reciprocating motion. The reduction of friction is substantially attributed to the film thickness enhancement.     

Acoustic emission of rolling bearings lubricated with contaminated grease

In elastohydrodynamic lubrication (EHL), an extreme smoothness of the contacting surfaces is essential for the formation of a thin lubricant film, which separates the moving surfaces from each other. Sharp asperities, in particular, are very detrimental for the sliding surfaces. Because of the limited thickness of the lubricating film, the contact zone is very vulnerable if the lubricant contains contaminants, especially if hard particles are present in the lubricant.The present paper focuses on the acoustic emission (AE) measurement method for the monitoring of the lubrication situation in a grease lubricated rolling bearing. The aim of the investigation was to clarify how the contaminants in the grease influence the acoustic emission of the rolling bearing. In the paper, the results of tests with clean greases and with deliberately contaminated greases, and the influence of the cleaning and re-greasing of the bearing are discussed. The results showed that the AE measurement indicated very clearly the lowest contaminant concentration included in the study that was as low as 0.02 weight-%. Small size contaminant particles generated a higher AE pulse count level than large size particles. The AE time signal analysis method proved to be a suitable method to indicate the hardness of the contaminant particles. Cleaning the bearing of contaminants and re-greasing with a clean grease reduced the AE level of the bearing.     

Film pressure distributions in point contacts predicted by thermal EHL analyses

In general, lubricated rolling/sliding contact fatigue problems have been investigated by assuming Hertzian contact pressure distributions. In this paper, thermal EHL analyses in consideration of the variations in oil properties in all directions within the film have been carried out under conditions of circular and elliptical contacts. It has been found that the actual film pressure distributions differ markedly from the Hertzian pressure distribution depending on the thermal conductivities of both contacting surfaces, slide–roll ratios and viscosity–pressure coefficients.     

Residual stress in some elasto-plastic problems of rolling contact with friction

The problem or rolling contact is of a great interest from the practical and cognitive points of view. It concerns for example a wheel passage on a rail, as well as many other processes where rolling or rolling with sliding is involved. Particularly, residual stresses distributions in the subsurface area due to plastic deformations are of significant importance: these stresses strongly influence fatigue limit and cracking tendencies of the material. A motion of the wheel on the rail (or of a roller on a strip) often has a very complex character: it consists of rolling, sliding in the longitudinal direction due to brakings and accelerations, sliding in the transversal direction, etc. Investigations conducted on this field by many authors have not yet delivered a unique solution. The aim of this paper is to determine the influence of some of these components of the wheel motion (separately and in common action) on residual stresses distributions. The problem has been investigated numerically by the FEM method, using the program SEGLA developed by the authors. The program can be applied to solving linear and nonlinear 2D, axisymmetric and 3D problems with elasto-plastic effects in the nonlinear contact and fracture mechanics. It uses three, four, six, eight and nine-node elements and also the special “spring” elements to model contact. The applied algorithm allows automatic analysis of these fragments of boundary, which enter and leave the contact; in the area where the “new” contact is created the program introduces additional elements.     

Movement of Entrapped Oil Under Pure Rolling Conditions

This article presents a study on the movement of an oil entrapment (or impact dimple) in a pure rolling elastohydrodynamic lubricated (EHL) contact. The oil entrapment was formed by impacting a steel ball against a lubricated glass disc. The contact was then activated under pure rolling conditions, and the movement of the entrapped oil was visualized by optical interferometry. It was found that during the movement of the dimple within the EHL contact, there exists a critical value for the displacement of dimple core. For the displacement of the dimple core less than the critical value, the dimple moves at the entrainment velocity and the film thickness of dimple core remains almost constant. For displacement beyond the critical value, the dimple slows down and its depth decreases rapidly. The effects of influential factors such as speed, initial dimple depth, load, and initial gap size were theoretically and experimentally investigated.