Influence of pressure and temperature dependence of thermal properties of a lubricant on the behaviour of circular TEHD contacts

The aim of this paper is to study the effects of pressure and temperature dependence of a conventional lubricant's thermal properties on the behaviour of heavily loaded thermal elastohydrodynamic lubrication (TEHL) contacts. For this purpose, a typical mineral oil (Shell T9) is selected and the dependence of its transport properties on pressure and temperature is investigated. Appropriate models are then developed for these dependencies. The latter are included in a TEHL solver in order to investigate their effect on the behaviour of circular EHD contacts. The results reveal the necessity of a thermal analysis including the pressure and temperature dependence of thermal properties for a good estimation of film thicknesses and mostly traction coefficients in circular EHD contacts operating under severe conditions. Numerical results are compared with experiments, showing a very good agreement over the considered ranges. This thorough validation of a thermal EHL framework for the calculation of film thickness and friction offers a previously unavailable opportunity to investigate the effects of variations in material properties.     

Numerical analysis of grease thermal elastohydrodynamic lubrication problems using the Herschel-Bulkley model

Grease thermal elastohydrodynamic lubrication (TEHL) problems of line contacts are analysed numerically. The effects of temperature and rheological parameters on grease TEHL are investigated using the Herschel-Bulkley model as a rheological model of greases. The pressure distribution, the shape of grease film, mean film temperature and surface temperature of solid wall in line contacts are obtained. It is found that thermal effects on the minimum film thickness become remarkable at high rolling speeds. The effect of yield stress of the Herschel-Bulkley model on minimum film thickness is negligible, while the flow index and viscosity parameter have significant effects on minimum film thickness.     

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。     

基于拟动力学的航空发动机主轴滚子轴承热弹流润滑分析

以D1842926航空发动机主轴滚子轴承典型工况为算例,基于拟动力学分析结果,获得滚动体与套圈之间的接触微区运动和受力状态,分别用Hamrock-Dowson(H-D)拟合公式,Wilson-Sheu(W-S)热修正公式和热弹流数值法得到最小膜厚,并与试验测试数据进行了对比,结果表明,H-D最小膜厚误差非常大,W-S热修正最小膜厚比H-D误差要小,但误差仍在40%以上,文中数值结果与试验数据较为一致,误差10%以内;对比不同速度下三种算法的最小膜厚分布,低速时三者较为一致,随着速度的提高,H-D最小膜厚误差越来越大,而W-S最小膜厚在速度增大到一定程度后反而降低;随着径向载荷的增大,润滑膜压力增大,膜厚减小,两端的压力略高于中间;随着转速增大,润滑膜膜厚增大,压力基本没有变化。     

Elastohydrodynamic problem for journal sliding bearing under reciprocating motion

A plane elastohydrodynamic problem for a radial sliding bearing with a thin liner under reciprocating motion under a constant load is considered. It has been shown that at the moment of time when the velocity becomes equal to zero the thickness of the lubricating film exceeds zero and that it continues to decrease during a period of time after changing the direction of rotation of the shaft. Film thickness and pressure distributions at various moments in time, as well as time dependences of the eccentricity and the minimum film thickness along the spatial coordinate, are presented for various values of the dimensionless parameter, which depends on the period of reciprocating motion. It has been shown that the smallest value of film thickness over the entire period of reciprocating motion increases with shortening period of reciprocating motion.     

Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.     

Non-newtonian thermal analysis of an EHD contact lubricated with MIL-L-23699 oil

A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φ_T), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation.     

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.     

Thermoelastohydrodynamic Analysis of Spur Gears with Consideration of Surface Roughness

Thermoelastohydrodynamic lubrication (TEHL) analysis for spur gears with consideration of surface roughness is presented. The model is based on Johnson’s load sharing concept where a portion of load is carried by fluid film and the rest by asperities. The solution algorithm consists of two parts. In the first part, the scaling factors and film thickness with consideration of thermal effect are determined. Then, simplified energy equation is solved to predict the surfaces and film temperature. Once the film temperature is known, the viscosity of the lubricant and therefore friction coefficient are calculated. The predicted results for the friction coefficient based on this algorithm are in agreement with published experimental data as well as those of EHL simulations for rough line contact. First point of contact is the point where the asperities carry a large portion of load and the lubricant has the highest temperature and the lowest thickness. Also, according to experimental investigations, the largest amount of wear in spur gears happens in the first point of contact. Effect of speed on film temperature and friction coefficient has been studied. As speed increases, more heat is generated and therefore film temperature will rise. Film temperature rise will result in reduction of lubricant viscosity and consequently decrease in friction coefficient. Surface roughness effect on friction coefficient is also studied. An increase in surface roughness will increase the asperities interaction and therefore friction coefficient will rise.     

振子驱动型牵引传动研究

在常规牵引传动和往复牵引传动的基础提出了一种依赖反复微小振动进行动力传动的振子型牵引传动。先通过振子振动形成油膜实验得出在接触区施以微小振动更易形成油膜的结论,又通过圆周运动型牵引传动实验得出了其相对于摩擦传动磨损小的优点。利用微小振动形成润滑油膜的方法引起的磨损小,可广泛应用于光学聚焦等精密运动领域。     

一种离子液体的热弹流润滑数值模拟

离子液体因微观结构与普通润滑油不同,使其具有较低的黏压系数。采用光干涉油膜测量技术标定一种离子液体的黏压系数,并通过等温数值计算验证其可靠性。使用标定的黏压系数,对该离子液体进行膜厚、温升、摩擦因数和接触区中心黏度等热弹流润滑数值计算,并与具有相同黏度的普通润滑油的算例进行比较。计算结果显示,离子液体与同黏度润滑油相比具有突出的摩擦学性能,体现在离子液体在较宽速度和滑滚比范围内有非常低的温升和摩擦因数,而膜厚仍保持在同黏度润滑油的40%以上。离子液体的这种热弹流特性主要归因于其较低的黏压系数。     

Comparisons of TEHL Simulations of Newtonian Fluids in Line Contacts between the 3D FEM and the Reynolds Equation–Based Approaches*

Thermoelastohydrodynamic lubrication (TEHL) analysis of line or point contacts is usually done by simultaneously and numerically solving the Reynolds equation, the Boussinesq equation of an elastic semi-infinite body, the energy conservation equation, and the load balance equation. Although a number of publications are available in this field, there is still a lack of general-purpose and widely used TEHL software for engineering applications. On the other hand, commercial software for both the solid structure and fluid flow analyses have become easy design tools. To expand the application of the commercial software to TEHL simulation, coupling of structure and fluid analyses is required. This study gives some demonstrations of the 3D finite element method (FEM) simulations of line contact TEHL problems using ANSYS version 13.0. The equilibrium equations of momentum and continuity and the energy conservation equation of lubricating fluids are solved with CFX. The elastic deformation of solids is calculated with the ANSYS Structure module. Through the fluid–solid coupling interfaces, the fluid pressure, solid deformation, and thermal flow are transferred between the fluid and solid domains. The computational fluid domain is enlarged, enclosing the contact zone, in the 3D model. Further, the 3D model can treat the realistic constraint conditions of solid deformation, whereas conventional TEHL analysis uses the assumption of semi-infinite body. The simulation results for pressure, lubricant film thickness, and temperature distributions are compared with the traditional Reynolds approach, and reasonable agreement for pressure and film thickness distributions has been obtained.     

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.     

Comparative analysis based on adiabatic shear instability for scuffing failure between unidirectional and reciprocating sliding motion

The principal goal of the experiments described here is to study the sliding motion effects on the scuffing life on the basis of adiabatic shear plastic instability. Experimentally we observed that the load capacity of the surface decreased and micro-scuffing initiated frequently under the reciprocating sliding motion as compared to under the unidirectional sliding motion. According to the adiabatic shear instability model, the scuffing initiation occurs when the rate of thermal softening exceeds that of work hardening due to plastic deformation. In order to ascertain the thermal softening in sliding surfaces, the contact temperatures were calculated. We found that the higher friction coefficient under the reciprocating sliding motion caused the higher contact temperature than that under the unidirectional motion. Therefore, the rate of thermal softening could exceed that of work hardening easily under the reciprocating sliding motion owing to frictional heating. We speculated that the scuffing initiation could roughen the sliding surfaces rapidly under the reciprocating sliding motion and confirmed that our assumption demonstrated above, was consistent with the experimental observation. In conclusion, there is a synergy effect in relation to scuffing failure because the frictional heating, surface roughening, and scuffing initiation function together to enhance each other, and consequently, the load capacity of surfaces could decrease under reciprocating sliding.     

Influence of temperature on cam-tappet lubrication in an internal combustion engine

The transient thermo-elastohydrodynamic (TEHL) lubrication simulation and isothermal elastohydrodynamic (EHL) simulation were performed on the exhausting camtappet friction pair of an internal combustion engine. Although by employing the two models the center pressure, the thickness of the lubricant film and friction coefficient obtained were similar in the changing trend during a rotating cycle, the parameters make a great difference, especially for the thickness of the lubricant film; the TEHL was four times thicker than the EHL. These results show that the temperature should not be neglected in the study of the lubrication of cam-tappet pairs. __________ Translated from Tribology, 2006, 26(4): 362–366 [译自: 摩擦学学报]     

Thermal effect in hydrodynamic lubrication of line contacts-Piezoviscous effect neglected

Thermal effect in high-speed rolling element bearings has been investigated numerically following a computationally efficient method developed by Elrod and Brewe [11. Viscous shear heating effects on both film thickness and rolling friction are investigated for a line-contact geometry assuming fully flooded lubrication. Thermal load-carrying capacity and rolling friction of the line contact have been numerically calculated for varying rolling speeds from 5 to 40 m/s and dimensionless film thickness between 10⁻⁴ and 10⁻³. Results indicate marked influence of viscous shear heating on the load-carrying capacity, film thickness and rolling traction at high rolling speeds. Neglecting thermal effect at high rolling speeds would lead to gross overestimation of load capacity, film thickness and traction. Results are presented for pressure and temperature distribution within the contact for various rolling speeds and film thicknesses.     

Experimental Investigation of Lubrication Failure of Polyalphaolefin Oil Film at High Slide/Roll Ratios

Lubrication failure of polyalphaolefin (PAO) oil film at high slide/roll ratios of more than 1.9 has been experimentally investigated based on a ball-on-disc configuration. The central film thickness and the oil pool shape in the contact were determined by optical interferometry and fluorescence microscopy, respectively, and the traction coefficient and surface profiles of the tribopairs were measured using a tension-compression sensor and light surface profilometer, respectively. The results demonstrate that the central film in the contact was in the elastohydrodynamic lubrication regime at the initial stage under high slide/roll ratios. Variation in the traction coefficient and the presence of wear scars on the disc surface shows that the sliding speed played an important role in lubrication failure. We propose that a thermal effect explains the obtained results at high slide/roll ratios. The fluorescence technique combined with optical interferometry was also used to gain more understanding of lubrication properties under mixed slide/roll conditions.     

The Elastohydrodynamic Traction of Synthetic Base Oil Blends

The aim of this study was to investigate the elastohydrodynamic (EHD) properties of lubricant blends. Three base fluids of very similar viscosities, a polyalphaolefin, a diester and an alky lated aromatic, have been obtained and their EHD film thickness and traction behavior measured at a range of pressures. Blends of these fluids have been prepared and the influence of blending on film thickness and traction has been investigated. Traction measurements were conducted at film thicknesses between 100–200 nm and thermal analysis was incorporated to correct for in-contact shear heating. The blends showed a broadly linear relationship between the inlet pressure-viscosity coefficient and blend composition. Isothermal traction comparisons revealed that traction is not an additive property of lubricant blends.     

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.     

A chemical study of wear particles and other features associated with the wear of ceramics

A sapphire convex surface was loaded against a reciprocating flat SiC counterface material. In this particular study the chemical nature of the wear surfaces and associated features such as the wear debris and local areas of material transfer have been studied using analytical techniques such as EDX, XPS and AES. Prior to wear tests the SiC substrate is covered with a thin (1–2 nm) layer of SiO₂. During wear the thickness of this layer is substantially reduced, and wear debris of a cylindrical morphology is produced. Examination of the outer 1–2 μm of the wear debris, as well as the first few atomic layers, by EDX and AES, respectively, showed very similar results in areas rich in oxygen accompanied by varying quantities of Al and Si but litte carbon. It is proposed that the wear debris is initially produced by the fragmentation of asperities on the two wear surfaces followed by the transfer of a wear film of SiO₂. Such equiaxed debris is then agglomerated into a characteristic cylindrical particle that lies normal to the reciprocating motion.