椭圆接触弹流润滑油膜形状的实验研究

采用多光束干涉测量技术,在自制光弹流实验机上进行了椭圆接触弹流润滑油膜形状的实验测量,观察了椭圆接触区短轴与卷吸方向之间的夹角θ、速度、施加载荷等对油膜形状的影响。结果表明：夹角θ较小时,油膜厚度整体上更大,接触区较窄,入口区油膜更陡峭;低速时,夹角及载荷基本不影响膜厚;高速、轻载时,夹角θ对膜厚影响更显著;载荷及夹角越大,动压油膜越难建立。     

Experimental investigation of temperature rise in elliptical EHL contacts

A toroidal traction drive continuously variable transmission (CVT) transmits power by the shearing action of lubricant film under heavy loads at contact points on the CVT rollers. In other words, rolling and sliding motions produce traction force. Furthermore, due to the geometry of the toroidal CVT, spin motion is produced at the contact points. These contact points, which are elliptical in shape, are where shear stress of the lubricant generates frictional heat. Temperature rise at the contact point has never been measured under the conditions of high rolling speed (velocity) and minute amounts of sliding (slippage), nor has the influence of spin motion on temperature rise been examined thus far. The authors et al. measured temperature distribution at contact points under conditions of high rolling speed and minute amounts of sliding, such as what is found in a toroidal CVT, using an FZG twin-disk test machine and thin-film sensors. The influence of spin motion on temperature rise was also experimentally investigated.     

Experimental study of starved EHL contacts based on thickness of oil layer in the contact inlet

Starved lubrication conditions bring the risk of the damage of machine components. The experimental simulation of starved conditions is connected with the need to define the input conditions to joint the amount of the oil entering the contact with the resulting film thickness. This paper describes the experimental approach based on the thickness of the oil layer entering the starved contact. The use of multiple contacts optical test rig based on thin film colorimetric interferometry for film thickness measurement has enabled to simulate starved conditions and to compare the obtained results with theoretical prediction. The first elliptical contact between spherical roller and the glass disc is used to supply the defined oil layer to the second contact formed between the steel ball and the glass disc. Through the comparison with the theoretical prediction it was found that acceptable input conditions for the study of starved contacts can be achieved with such test rig configuration.     

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.     

A study of lubricant film thickness in compliant contacts of elastomeric seal materials using a laser induced fluorescence technique

A laser induced fluorescence technique was used to investigate the build-up of lubricant films in compliant contacts operating in the isoviscous elasto-hydrodynamic regime (I-EHL). The described technique utilises an optimised optical set-up with a relatively high signal-to-noise ratio and was shown to be able to produce film thickness maps of the complete contact area and measure a very wide span of thicknesses, from 50 nm to 100 μm. Maps of film thickness were obtained over a range of entrainment speeds and loads for three different contact configurations and two elastomer materials, polydimethylsiloxane (PDMS) and a fluorocarbon rubber (FKM) which is typically used in rotary seal applications. In a model contact of a nominally smooth PDMS ball sliding on a glass flat, a crescent shaped area of reduced film thickness was observed towards the contact exit. In contrast to typical elasto-hydrodynamic contacts, no side-lobes of reduced film thickness were recorded, while the central film region exhibited a converging wedge shape. The elliptical contact of an FKM O-ring rolling on a flat glass showed a central region of flat film while areas of minimum film thickness were located near the contact edges either side of the centre. The highly conformal contact of relatively rough FKM O-ring sliding against a concave glass lens, a geometry more representative of that found in elastomeric seals, showed discrete regions of reduced film, corresponding to surface roughness asperities. With rising entrainment speed, some lift-off was observed, with surface roughness asperities appearing to be increasingly compressed. Measured films thicknesses were compared to existing theoretical predictions for I-EHL contacts and the level of agreement was found to be highly dependent on contact geometry and applied conditions.     

Thermal elastohydrodynamic lubrication of starved elliptical contacts

For the starved thermal elastohydrodynamic lubrication in elliptical contacts, a numerical solver based on the multi-grid technique was developed and numerical solutions were achieved. The influences of the thickness of oil-supply layer, elliptical ratio, entrainment velocity, slide-roll ratio, and the maximum Hertzian pressure on the lubrication behaviour were investigated. The thermal solutions were compared with isothermal solutions. The numerical results show that, for an oil-starved contact, the central and minimum film thicknesses predicted by both thermal and isothermal solutions change at the same trend as the thickness of the oil-supply layer increases. That is, as the thickness of the oil-supply layer increases, in the beginning both the central and minimum film thicknesses increase rapidly, however, their increasing rates become smaller gradually, at last, when the thickness of the oil-supply layer reaches a certain value, both the central and minimum film thicknesses almost stop increasing. Such situation was called a quasi-fully flooded state in this study. It has been found that the amount of the supplied oil for the quasi-fully flooded state differs with the elliptical ratio, and the maximum amount of the supplied oil is required by a circular contact.     

Theoretical investigation on the dimple occurrence in the thermal EHL of simple sliding steel–glass circular contacts

The conical depression (surface dimple) phenomena observed by Kaneta et al. (Kaneta M, Nishikawa H, Kameishi K, Sakai T. Effects of elastic moduli of contact surfaces in elastohydrodynamic lubrication. ASME J. Tribol. 1992;114:75–80; Kaneta M, Nishikawa H, Kanada T, Matsuda K. Abnormal phenomena appearing in EHL contacts. ASME J. Tribol. 1996;118:886–892.) in optical interferometry experiments are simulated numerically by a complete solution to the simple sliding circular contact thermal elastohydrodynamic lubrication (TEHL) problem. Good agreement is displayed between the theoretical and experimental results. This agreement is explained by the “temperature–viscosity wedge” mechanism, which was first proposed by Cameron (Cameron A. Hydrodynamic lubrication of rotating discs in pure sliding, a new type of oil film formation. J. Inst. Petrol 1951;37:471.). Effects of the viscosity–pressure coefficient, the ambient viscosity, and the entrainment velocity on the behavior of the surface dimples are discussed.     

On the thermal elastohydrodynamic lubrication in opposite sliding circular contacts

Cases of elastohydrodynamic lubrication (EHL) point contacts running under opposite sliding conditions have been studied with consideration of the thermal effect for various loads and entrainment velocities. A thermal EHL solver has been developed and proven to be able to deal even with extreme cases under an infinite slide–roll ratio. Results show that film profiles featuring a dimple can be formed in the contact zone when the slide–roll ratio exceeds a certain level. Moreover, the present study provides theoretical evidence for the lubricating film build up in the case where the two bounding surfaces run with equal but opposite velocities. An effective lubricating film under zero entrainment speeds was experimentally proven by Dyson and Wilson [1] (Proc Instn Mech Engrs, 1968–1969 183(3P) 81) in the 1960's, which, however, cannot be explained by the isothermal EHL theory.     

变速点、线接触副弹流润滑油膜的观测

应用光干涉方法,在自制的光弹流试验机上分别对纯滚动条件下点接触和线接触形成的弹流油膜进行变卷吸速度实验,并进行油膜测量。结果表明,在卷吸速度为零时都有封油现象的出现。随着卷吸速度的增加,油膜厚度增加,点接触形成的弹流油膜具有典型的马蹄形特征,线接触形成的弹流油膜在接触区端部有类似点接触的马蹄形收缩。要达到同样的最大赫兹接触压力,施加在线接触实验上的载荷要比施加在点接触实验上的载荷大40倍左右。     

A numerical study of fatigue life in non-Newtonian thermal EHL rolling–sliding contacts with spinning

This paper presents a study on fatigue life in non-Newtonian thermal elastohydrodynamic lubrication (TEHL) point contacts with spinning. A numerical procedure is developed and extended to rolling contact fatigue (RCF) life. The results show that the effect of entraining velocity on the RCF life is closely related to ellipticity. The RCF life first decreases steeply and then gradually with increase in slide–roll ratio. However, the RCF life may increase slightly at a large slide–roll ratio. Spinning is beneficial for reduction of longitudinal friction coefficient; however, even for smooth surface contact, the RCF life can be slightly reduced by spinning.     

界面滑移条件下点接触Stribeck曲线的实验研究

采用高黏度聚异丁烯润滑油,在光学弹流实验机上考察球-盘接触纯滑条件下的摩擦因数随卷吸速度和载荷的变化。结果表明,随着卷吸速度的增加,球-盘接触副进入弹流润滑并向流体动压润滑转变的过程中,摩擦因数并不像传统的Stribeck曲线一样,随着卷吸速度的增加而单调增加,而是呈现先上升、后下降、再上升的趋势,其中摩擦因数下降时的起始速度大致为凹陷出现的速度,摩擦因数再上升时的速度大致是润滑进入动压润滑的速度。初步论证界面滑移为产生上述波动的主要原因。     

基于推力球轴承的点接触弹流润滑分析

研究了推力球轴承的钢球与滚道间形成的椭圆接触等温弹流润滑问题。据此问题建立的数学模型既有沿接触椭圆短轴方向的卷吸速度分量，又有沿其长轴方向的卷吸速度分量，且速度值处处各异。运用多重网格法求得了几组参数下的完全数值解，并分析了转动速度、中心距、椭圆比等参数对结果的影响。     

Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

Improving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range dependent. Furthermore, for the test conditions used in this study, grease-lubricated contacts appear to shift from the initial fully flooded condition to starved condition over a prolonged running of 2 h. The results from this study concur with those reported in the literature that fully flooded oil elastohydrodynamic lubrication (EHL) theory or film thickness cannot be directly applied or taken as a guideline in grease-lubricated contacts.     

The influence of surface roughness in elliptical contacts

The influence of surface roughness on contact behaviour is of great importance in many tribological situations. In the last decade several methods to calculate the pressure distribution and the real contact area in contacts between rough surfaces have been described. A problem arising for slender elliptical contacts, such as between gear teeth, roller and raceway, cam and follower, etc., is that the size of the contact is much greater than the size of the asperities. Accordingly the number of contact nodes necessary for an accurate solution to the problem becomes excessively large. This paper describes a method to calculate the influence of three-dimensional surface roughness in contacts that are very long in one direction. The method is based on restricting the calculations to a subpart of the real contact area, while the rest of the contact is taken into account by mirroring techniques. The results show that the real contact area is very sensitive to the amplitude of the roughness, while the waviness is less important. An equation is suggested from which the real area can be calculated if the smooth case contact parameters and two roughness parameters are known.     

Inverse approach for calculating temperature in EHL of line contacts

This paper proposes a thermal elastohydrodynamic lubrication (TEHL) inverse approach to estimate the pressure, temperature rise, and apparent viscosity distributions in an EHL line contact. Once the film shape is measured, the pressure and estimated film thickness distributions can be calculated from force balance and elastic deformation theories. By using these smoothing pressure and film thickness distributions, the Gauss-Seidel iteration is employed to calculate the temperature rise distribution from energy, surface temperature, and rheology equations. This approach overcomes the problems of pressure and temperature rise fluctuations, and generates accurate results of pressure and temperature rise distribution from a small number of measured points of film thickness, which also saves computing time. Results show that the direct inverse method requires a lot of measured points to establish the amplitude and location of the pressure and temperature rise spikes, whereas the inverse approach can obtain the accuracy results with only 31 measured points. With the error from the resolution in the film thickness measurements, this approach also presents a smooth curve of the pressure and temperature rise distributions with a small error. Furthermore, this approach still provides a good solution in apparent viscosity, whereas the direct method provides a much larger error in apparent viscosity.     

Calculation of Stribeck curves for line contacts

A mixed lubrication model is presented by which Stribeck curves can be calculated. By means of the Stribeck curve the transitions from boundary lubrication to mixed lubrication and the transitions from mixed lubrication to elasto-hydrodynamic lubrication can be predicted and subsequently the lubrication regime in which a particular contact operates can be predicted. Calculations are restricted to line contacts, because of the wide range of applications of line contacts or of very wide elliptical contacts.     

Flash temperatures due to friction and Joule heat at asperity contacts

Approximate solutions, reliable within a few per cent, have been obtained for the theoretical flash temperatures at circular and elliptical contact spots between two homogeneous materials when heat is evolved at the (assumed to be mathematically flat and uncontaminated) interface at a uniform rate, either independent of velocity as in the case of Joule heat or in proportion with the relative speed, i.e. as is the case for friction heat. The parameters appearing in the corresponding equations have been expressed in terms of known materials properties plus the number of contact spots and their axis ratio. The accuracy with which the flash temperatures can be known in practice is limited by uncertainty regarding those parameters and by the idealizations embodied in the model. Those theoretical uncertainties are probably no larger than the actual momentary fluctuations in the conditions at the contact spots.     

Effects of Surface Roughness on Point Contact EHL

The effects of orientation of surface roughness, entrainment (rolling) velocity, and slide/roll ratio on micro-elastohydrodynamic lubrication (micro-EHL) are investigated under pointcontact conditions using the optical interferometry technique. Long bumps with constant height and wavelength produced artificially on the surface of a highly polished steel ball are used as a model roughness. It is shown that the asperities are elastically deformed and the magnitude depends on the film factor A, defined by the ratio of the central film thickness based on smooth surfaces to the composite surface roughness, as well as the surface kinematic conditions and the orientation of the asperities. It is also found that a thin or thick oil film formed at the inlet of the contact by a moving rough surface travels through the contact region at a speed very close to the average speed of the contacting surfaces. The possible mechanism is discussed.     

Inverse approach for calculating pressure and viscosity in pure squeeze EHL motion of circular contacts

The inverse approach is proposed to the pure squeeze EHL motion of circular contact problems, where the pressure is approximated by polynomial form in terms of the radial coordinate and time to provide a smoothing algorithm in calculating pressure from the elasticity equation. The least-squares method is used to calculate the optimum value of the pressure–viscosity index from the transient Reynolds equation. Results show that the estimated pressure can be obtained accurately with a few measuring film thickness points, but it is hard to obtain the correct viscosity for the direct inverse method. By using the inverse approach, even though the measurement error in the film thickness is present, the fluctuations in the pressure can be overcome, and it predicts the pressure–viscosity index with very good accuracy. The post-impact high-pressure condition for the constant load case is suitable to evaluate the pressure–viscosity index in a wide range along the radial axis with very good accuracy. Subsequently the inverse approach is successfully used to estimate the pressure distribution resulting from the film thickness map from the interferometry measurements. The agreement between the actual value of the test fluid and the estimated value is quite good.     

Experimental study on lubrication regime variation in point contacts

The change between elastohydrodynamic lubrication (EHL) and hydrodynamic lubrication (HL) under a wide range of entrainment speeds and applied loads was studied using an optical EHL apparatus. A log-log scale linear relationship was demonstrated in the two lubrication regions between the film thickness and the entrainment speed (or load). A transition region can be clearly discerned between these two regions in which the film thickness no longer bears a linear relationship with the entrainment speed (or load). It is shown that a piezoviscous effect can be distinguished in the HL region by the speed exponent or the load exponent, and that relative sliding has a significant influence on the transition region.