Oscillations induced in EHD film thickness by a step in entrainment speed

Transient elastohydrodynamic (EHD) lubrication conditions occur in the contacts of many machine elements, such as gears, cams, and reciprocating devices, as a result of their working cycles. These conditions also occur in rolling‐element bearings at the onset or cessation of motion. The aspect of film thickness in elastohydrodynamically lubricated contacts subjected to a very rapid change in entrainment speed has not received much attention from researchers, probably because it is seen as less problematic than a sudden fall of the entrainment speed, which theoretically can lead to film failure. For a sudden stop, however, it has been shown previously that the lubricant forms an entrapment, which is able to protect the contact in many cases when the motion resumes. In this paper, EHD film behaviour under sudden acceleration is investigated; the study covers three cases ‐ starting from zero film, starting from an entrapped film, and starting from a continuous, steady film.     

零卷吸往复运动下的弹流润滑实验研究

使用常规球-盘光干涉试验机,研究零卷吸往复运动中油膜的变化情况。实验采用伺服电机驱动钢球与蓝宝石盘,以三角波的形式进行往复运动,两者速度相同但是方向相反。实验过程中采用光干涉技术测量球-盘之间的膜厚,实验后使用双光干涉法测量接触区中截面油膜厚度。实验发现,往复条件下的油膜凹陷小于对应的稳态油膜凹陷,而且接触区发生了速度滑移,导致所产生的油膜凹陷形状和位置区别于零卷吸凹陷;乏油的发生导致接触区中出现大面积的干接触和混合润滑接触。     

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.     

链条套筒与销轴铰链副冲击载荷-往复运动的弹流润滑数值模拟

数值模拟链传动中销轴与套筒之间的定载荷和变载荷弹流润滑接触问题,套筒相对于销轴做纯滑动往复运动。定载荷是假定往复运动过程中载荷恒定不变;变载荷是假定链节在啮入和啮出链轮过程中存在的冲击载荷按正弦函数规律变化。比较在定载荷和变载荷加载条件下线接触往复运动工况的弹性润滑油膜变化情况,分析在动载荷加载条件下不同行程长度对弹性流体动力润滑特性的影响。研究发现,动载荷对油膜的压力、膜厚影响较大:随着动载荷的增加,油膜中压力急剧增大,膜厚减小;但加载方式对摩擦因数的影响不大;在相同的加载方式下,随着行程长度的增加,油膜压力减小,中心膜厚和最小膜厚显著增加。     

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint.

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint has been studied. The poroelasticity equation of articular cartilage and the modified Reynolds equation for the synovial fluid lubricant have been successfully solved under squeeze-film motion and for the conditions experienced in a normal human hip joint. It has been shown that porosity of the articular cartilage depletes the lubricant film thickness, rather than increasing it, particularly when the lubricant film thickness becomes small. Furthermore, it has been shown that articular cartilage can be treated as a single-phase incompressible elastic material in the lubrication modelling under physiological walking conditions.     

Numerical analysis of TEHL line contact problem under reciprocating motion

This paper presents a full numerical analysis to simulate the thermal elastohydrodynamic lubrication (TEHL) of steel–steel line contact problem under reciprocating motion. The equation system is solved using multigrid techniques. General tribological behaviors of TEHL under reciprocating motion are explained. Comparison between thermal and isothermal results reveals the importance of thermal effect in prediction of the traction coefficient and film thickness. The influences of frequency, stroke length, and applied load on the variations of film thickness, pressure and traction coefficient during one working cycle are discussed. Furthermore, the influence of slide–roll ratio on tribo-characteristics of oil film under same entraining velocity is revealed.     

Elastohydrodynamic Lubrication of Rough Surfaces under Oscillatory Line Contact with Non-Newtonian Lubricant

This paper presents the results of a transient analysis of elastohydrodynamic lubrication (EHL) of two parallel cylinders in line contact with a non-Newtonian lubricant under oscillatory motion. Effects of the transverse harmonic surface roughness are also investigated in the numerical simulation. The time-dependent Reynolds equation uses a power law model for viscosity. The simultaneous system of modified Reynolds equation and elasticity equation with initial conditions was solved using the multigrid, multilevel method with full approximation technique. The film thickness and the pressure profiles were determined for smooth and rough surfaces in the oscillatory EHL conjunctions, and the film thickness predictions were verified experimentally.

For an increase in the applied load on the cylinders or a decrease in the lubricant viscosity, there is a reduction in the minimum film thickness, as expected. The predicted film thickness for smooth surfaces is slightly higher than the film thickness obtained experimentally, owing primarily to cavitation that occurred in the experiments. The lubricant film under oscillatory motion becomes very thin near the ends of the contact when the velocity goes to zero as the motion direction changes, but a squeeze film effect keeps the fluid film thickness from decreasing to zero. This is especially true for surfaces of low elastic modulus. Harmonic surface roughness and the viscosity and power law index of the non-Newtonian lubricant all have significant effects on the film thickness and pressure profile between the cylinders under oscillatory motion.     

A Simplified Model to Study EHL Film Collapse During Rapid Halting Motion

A theoretical model for describing the EHL film thickness during rapid deceleration is presented. The theory is based on the pioneer work of Ertel (1939) and Grubin (1949), who gave the first analytical solution for the elastohydrodynamic lubrication of a line contact under stationary operating conditions. An extension is made here for rapid halting motion. The proposed model is well adapted when the halting period is small in comparison to the transit time (i.e. 2b/u, ratio between the contact width and the rolling speed). This work completes the model of Glovnea and Spikes (2001b), appropriate for slow halting motion but which suffers from experimental fitting, and the model of Chang (2000) that is more suitable for speed or load oscillations at a wavelength close to the transit time.

This behavior implies that stop-start, reciprocating or rapidly halting machine components may be able to maintain a separating film for longer than would be expected based on steady-state EHL theory. An application to a ball bearing arrangement in a space mechanism is finally made in order to assess the model capabilities.     

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     

Synergetic representation of stick-slip mode of boundary friction

The melting of an ultrathin lubricating film clamped between two atomically smooth solid surfaces that are in relative motion is studied based on the Lorentz model for the approximation of a viscoelastic medium. An equation of motion for the stresses has been derived in the form of a three-order differential equation and analyzed at various friction surface temperatures. In all cases, the phase portraits and the time dependences of the stresses have been plotted. It has been found that, depending on the temperature and the lubricant parameters, either the damped oscillation mode or the stochastic oscillation mode may occur. The stochastic oscillation mode is presented in the phase plane as a strange attractor. It has been shown that initial conditions have a critical effect on the system behavior. Based on the model, the behavior of two types of tribosystems, i.e., with the unidirectional shear of the surfaces and under an alternating external effect, has been described.     

往复运动条件下润滑油膜特性的实验观察

采用多光束干涉技术观察往复运动条件下润滑油膜的滑移及黏弹特性,研究振幅和频率对往复动态润滑弹流油膜的影响。结果表明:往复运动过程中,在特定时刻气穴的出现使油膜厚度逐渐减小,削弱了滑移程度;因润滑油的黏弹性而引起的运动滞后导致了油膜的非对称性;频率增大时,正行程末端时膜厚明显增大,油膜输送速度也随着增大;而负行程末端油膜受气穴的影响膜厚增大较慢;振幅(输入位移)增大时,正行程末端时油膜整体平移,而负行程末端入口凹陷呈现先变明显而后消失的现象。     

Theoretical Analysis of Piston-Ring Lubrication Part I—Fully Flooded Lubrication

A one-dimensional analysis for lubrication between the piston ring and cylinder wall has been developed. A fully flooded inlet condition and axisymmetric geometry are considered. The piston ring is treated as a reciprocating, dynamically-loaded bearing with combined sliding and squeeze motion. A system of two nonlinear differential equations is used to model the lubrication including the Reynolds cavitation boundary condition. A numerical procedure is then developed to obtain the cyclic variations of film thickness, frictional force, power loss, and oil flow across the ring.

Results are presented for a typical automotive engine. The effects of ring profile, ring tension, and engine speed are examined. It is shown that this analysis can be used to study the influence of ring design parameters in order to improve the design of the ring pack in reciprocating engines.     

Film Formation in EHL Point Contacts under Zero Entraining Velocity Conditions

The contacts of adjacent balls in a retainerless bearing are subjected to the zero entrainment velocity (ZEV). The existence of an effective elastohydrodynamic lubrication (EHL) film between contacts running under ZEV conditions has long been proven experimentally. However, the classical EHL theory predicts a zero film thickness under ZEV conditions. Mechanisms, such as the thermal viscosity wedge effect and immobile film theory, have been proposed to tentatively explain the phenomenon. However, detailed numerical results are needed to provide theoretical evidence for such film formations. This paper aims to simulate, based on the viscosity wedge mechanism, the film formation of EHL point contacts under ZEV conditions. Complete numerical solutions have been successfully obtained. The results show that the thermal viscosity wedge induces a concave film profile, instead of a parallel film (Hertzian) as postulated by some previous researchers. By the simulation solver developed, the variation of film thickness with loads, oil supply conditions and ellipticity parameters have been investigated. Some unique lubrication behaviors under ZEV conditions are demonstrated. Furthermore, preliminary quantitative comparisons with the latest optical EHL experiments are finished. Both results are in good correlation.     

Structure and kinematics of a planetary converter of the rotational motion into the reciprocating rotary motion

The problem of the structural synthesis of a mechanism for converting rotational motion into reciprocating rotary motion has been considered. In accordance with the synthesis conditions and the found solutions, a planetary mechanism with elliptical gears has been proposed and its kinematic analysis has been performed. It has been shown that the rocking angle and the asymmetry factor of the working stroke of the mechanism depend on the selection of the eccentricities of elliptical gears.     

Optimum film thickness of thin metallic coatings on silicon substrates for low load sliding applications

The frictional behaviour of thin metallic films on silicon substrates sliding against 52100 steel balls is presented. The motivation of this work is to identify an optimum film thickness that will result in low friction under relatively low loads for various metallic films. Dry sliding friction experiments on silicon substrates with soft metallic coatings (silver, copper, tin and zinc) of various thickness (1–2000 nm) were conducted using a reciprocating pin-on-flat type apparatus under a controlled environment. A thermal vapour deposition technique was used to produce pure and smooth coatings. The morphology of the films was examined using an atomic force microscope, a non-contact optical profilometer and a scanning electron microscope. Following the sliding tests, the sliding tracks were examined by various surface characterization techniques and tools. The results indicate that the frictional characteristics of silicon are improved by coating the surface with a thin metallic film, and furthermore, an optimum film thickness can be identified for silver, copper and zinc coatings. In most cases ploughing marks could be found on the film which suggests that plastic deformation of the film is the dominant mode by which frictional energy dissipation occurred. Based on this observation, the frictional behaviour of thin metallic coatings under low loads is discussed and friction coefficients are correlated with an energy based friction model.     

仿生织构形态对缸套-活塞环摩擦学性能的影响

为了提高缸套-活塞环的摩擦学性能,设计了一种仿生排布的菱形凹坑织构,并通过激光刻蚀技术在缸套表面进行加工;在同一转速和不同载荷下,在MWF-10往复式摩擦磨损试验机上进行试验,以探究仿生排布的菱形织构对缸套-活塞环摩擦副摩擦磨损性能的影响,并与使用阵列排布的纹理的缸套以及未经处理的原始缸套进行比较。结果表明:织构的排布形式对油膜厚度的影响较大,尤其在重载荷工况下,合理地优化排布形式能够实现较好的动压润滑效果;仿生排布的菱形织构实现了往复运动方向上纹理特征的全覆盖,能够极大程度上限制磨屑的移动并对磨屑进行收集,有效降低磨损后的表面粗糙度,从而减少磨粒磨损;仿生排布的菱形织构在各试验工况下能够有效提高油膜厚度,提升表面承载能力,实现最佳的润滑效果。     

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。     

Reduction of Friction by Functionalised Viscosity Index Improvers

A range of functionalised polymethacrylate copolymers have been synthesised with different functionalities, polymer architecture and molecular weight. It is shown that appropriately functionalised block copolymers give enhanced film thickness and greatly reduced friction under low entrainment speed conditions, even with polymer concentration as low as 1% wt. This behaviour almost certainly results from the formation of an adsorbed brush-like film of thickness ca 20 nm on each polar surface. These films provide a highly viscous inlet that promotes fluid entrainment and thus maintains a separating film down to very low entrainment speed. The adsorbed polymer films are also able to maintain separation in stationary contact conditions. Randomly distributed copolymers do not show this type of behaviour. The friction reduction observed is more effective in unidirectional, mixed sliding–rolling than in reciprocating, sliding conditions. However, it is found that functionalised polymers and conventional organic and molybdenum-based friction modifiers can be combined to provide effective friction reduction over the whole range of rubbing conditions.     

Fluid-solid interaction model for hydraulic reciprocating O-ring seals

Elastohydrodynamic lubrication characteristics of hydraulic reciprocating seals have significant effects on sealing and tribology performances of hydraulic actuators,especially in high parameter hydraulic systems.Only elastic deformations of hydraulic reciprocating seals were discussed,and hydrodynamic effects were neglected in many studies.The physical process of the fluid-solid interaction effect did not be clearly presented in the existing fluid-solid interaction models for hydraulic reciprocating O-ring seals,and few of these models had been simultaneously validated through experiments.By exploring the physical process of the fluid-solid interaction effect of the hydraulic reciprocating O-ring seal,a numerical fluid-solid interaction model consisting of fluid lubrication,contact mechanics,asperity contact and elastic deformation analyses is constructed with an iterative procedure.With the SRV friction and wear tester,the experiments are performed to investigate the elastohydrodynamic lubrication characteristics of the O-ring seal.The regularity of the friction coefficient varying with the speed of reciprocating motion is obtained in the mixed lubrication condition.The experimental result is used to validate the fluid-solid interaction model.Based on the model,The elastohydrodynamic lubrication characteristics of the hydraulic reciprocating O-ring seal are presented respectively in the dry friction,mixed lubrication and full film lubrication conditions,including of the contact pressure,film thickness,friction coefficient,liquid film pressure and viscous shear stress in the sealing zone.The proposed numerical fluid-solid interaction model can be effectively used to analyze the operation characteristics of the hydraulic reciprocating O-ring seal,and can also be widely used to study other hydraulic reciprocating seals.     

The effect of surface texturing on thin EHD lubrication films

Surface texturing has been successfully used for conformal contacts in many tribological applications in an effort to diminish friction and wear. However, the use of such a surface modifications are still in nascent as far as highly loaded contacts between non-conformal surfaces are concerned. It is mainly caused by the fact that the presence of such micro-features within these contacts can significantly influence the pressure distribution within the contact. Nevertheless, it has been shown in recent studies that the surface texturing can also have beneficial tribological effects if the depth of micro-features is properly designed. This paper is devoted to the experimental study of the effect of the micro-dents of various depths on thin lubrication films to find an experimental evidence of the micro-feature depth threshold for surface texturing applications in highly loaded non-conformal surfaces. The behaviour of an array of micro-dents within thin EHD contacts has been studied by thin film colorimetric interferometry. The influence of surface texturing on lubricant film formation has been observed under sliding/rolling conditions. The significant effect of micro-dents depth on lubricant film thickness is observed for positive slide-to-roll ratio when the disc is moving faster than the micro-textured ball. The presence of deep micro-dents within lubricated contact results in film thickness reduction downstream. As the depth of micro-dents is reduced, this effect diminishes and beneficial effect of micro-dents on film thickness formation has been observed. No significant influence of micro-dents depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when micro-dents do not cause film thickness reduction regardless of their depths.