Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Failure criteria in thin film lubrication with EP additives

The scuffing behaviour of sliding couples made from 12 NiCr 13 steel, lubricated with SAE 90 and SAE 20 W30 oils with and without additives, was studied as a function of relative sliding speed and bulk oil temperature. It was found that the scuffing load decreases almost inversely with sliding speed. Calculations which take into account the decrease in hardness at increasing temperature show a fairly good constancy of total contact temperature (bulk + flash temperature) at scuffing (i.e. values ranging from 550 ° to 650 °C for all oils and test conditions). At low speeds (up to 1 m s^?1) a well-defined increase in scuffing load was found when EP additives were used; at higher speeds this effect was found to have vanished completely. Variations in nominal contact pressure in the range 1 to 3 had no appreciable influence on the scuffing load, indicating that, in the present case, scuffing was associated with a transition from the boundary lubrication regime to the severe wear regime.     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation of the temperature distribution in the elastohydrodynamic oil film

Results of investigations of the temperature distribution in elastohydrodynamic (EHD) contacts carried out on a two-disc machine for various contact parameters (rolling and sliding speeds and maximum Hertz pressures) are reported in this paper.The experiments were carried out with the aid of thin layer temperature transducers made of titanium that were evaporated onto the disc's surface in vacuum.Analysis of the errors affecting the temperature measurements in EHD contact was also carried out. It was shown that contact pressure and transducer insulation had a substantial influence on the precision of the temperature measurements made by the thin layer transducer. Error analysis has permitted the determination of the real value of the temperature rise in the contact zone by the application of suitable corrections.It was found that the relative sliding velocity and the contact loading force have substantial and similar influences on the maximum temperature rise in the EHD contact; however, the influence of the rolling velocity is relatively small.A simple empirical formula which permits the calculation of the maximum temperature rise in the contact zone as a function of load and velocity for the investigated oil is presented in this paper. The general formula as a function of non-dimensional parameters is also given. It was found that theoretical formulae which include the exact dependences of the viscosity variation on pressure and temperature (e.g. the Kannel equation) provide the best fit to the experimental results.Good agreement of the results obtained with other researchers' experimental results was also found.     

Effect of some material and operating variables on scuffing

This paper summarizes the results of scuffing tests performed on AMS 6260 steel disks, covering three oils (a MIL-L-7808G oil, a MIL-L-23699A oil, and a straight mineral oil), two oil supply temperatures, a variety of sliding and sum velocities, and two modes of operating the test disks such that the potential failure sites on the disk surfaces either do or do not synchronize precisely in repeated cycles of operation. It is shown that, under otherwise comparable situations, (a) different oil-steel combinations allow the operation to penetrate by different degrees into the boundary lubrication regime before scuffing occurs, (b) an increase in the sliding velocity, at constant sum velocity, decreases the scuff failure load and the critical temperature, (c) an increase in the sum velocity, at constant sliding velocity, increases the scuff failure load and the critical temperature, (d) the effect of changing the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, depends on the balance of the opposing effects of sliding ans sum velocities at the particualar velocity ratio of interest, and (e) the scuff failure load and the critical temperature are markedly increased when the potential failure sites on the disk surfaces do not precisely synchronize on repeated cycles of operation.It is further demonstrated that the variations of the oil film thickness at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to all surface and operating variables correlate satisfactorily with a dimensionless parameter ξ_f.     

Temperature—The Key to Lubricant Capacity

Failure of a nonreactive mineral oil can be predicted by Blok's formula for determining the maximum temperature between two bodies in rolling and sliding contact. Evaluation of many lubricants on a geared roller lest machine revealed that the lubricant failure for any particular lubricant-material combination occurs at a constant, critical contact temperature over wide ranges of load, sliding velocity, surface velocity, specimen temperature, film thickness, and viscosity grade. Coefficient of friction can be predicted by a parameter involving the unit load, inlet viscosity, sum velocity, and sliding velocity. The load capacity of a lubricant varies inversely with specimen temperature for a constant set of lest conditions. Electrical resistance measurements across the contact zone aided in identifying the lubricant failure point and in revealing the action of two deposit-forming additives.     

基于动态子结构的主轴承热弹性流体润滑研究

研究基于动态子结构缩聚的轴承热弹性流体动力学(TEHD)基本理论和求解方法;建立某V型8缸内燃机主轴承的TEHD仿真模型,分别计算得到各主轴承在最大载荷工况下的油膜压力、油膜厚度、摩擦功耗、轴心轨迹和油膜温度等润滑特性;针对润滑状况较差的第3主轴承,进行TEHD、EHD(弹性流体动力学)和HD(流体动力学)不同仿真求解方法的对比研究。研究结果表明,该内燃机的第3主轴承最小油膜厚度和最大油膜压力等润滑性能最差,需要进行相应的改进设计;TEHD求解中计及了润滑油和轴瓦热效应的影响,能获得更高的轴承润滑特性计算精度。     

An experimental study of elastohydrodynamic central and minimum film thicknesses for various material parameters

A newly developed experimental technique, based on colorimetric interferometry, has allowed accurate measurement of central and minimum elastohydrodynamic (EHD) film thickness for a wide range of operating conditions. This study was aimed at determining the influence of various material parameters on central and minimum EHD film thicknesses. Obtained experimental data were compared with Hamrock and Dowson film thickness equations and a numerical solution presented by Venner. The results obtained confirmed that the ratio between the central and minimum film thicknesses changes significantly with both load and material parameters.     

滚珠型弧面凸轮机构的摩擦学设计

在滚珠型弧面凸轮机构的设计中引入摩擦学设计方法,推导出钢球和凸轮滚道接触的最小油膜厚度计算公式。为了保证机构的润滑状态,采用膜厚比进行润滑状态的判断,并以此作为设计准则。同时为了防止油膜温升导致润滑失效,计算出润滑油膜的最大温升,利用润滑油的黏温特性,计算出油膜厚度降低系数,对常温下油膜厚度进行修正。在设计中兼顾强度设计和压力角校核,满足机构的使用性能,减少摩擦和磨损。     

考虑混合润滑的飞机燃油泵径向滑动轴承静特性分析

针对飞机燃油泵径向滑动轴承润滑油黏度极低、润滑油膜形成难、表面易磨损等问题,通过综合考虑紊流、热效应、质量守恒、温黏效应及混合润滑边界等因素,建立轴承的热流体动力学润滑分析模型,采用有限元方法联立求解雷诺方程、能量方程、接触方程得到轴承的静态特性,研究轴承间隙比、宽径比、转速、载荷、进油温度等对轴承静态润滑性能如油膜厚度和油膜压力的影响规律。结果表明：由于航空煤油动力黏度低,造成轴承的浮起转速高（大于5 500 r/min）,极限承载力低（小于37 N）、油膜厚度过低;降低进油温度、适当减小间隙,增加轴瓦宽度有利于增加油膜厚度,提高轴承可靠性。研究结果对飞机燃油泵径向滑动轴承设计与运行维护具有一定的工程借鉴价值。     

超环面行星蜗杆传动弹流润滑状态研究

在超环面行星蜗杆传动啮合原理和载荷分布研究基础上,给出了该种传动弹流油膜厚度计算公式,分析了该种传动的弹流润滑状态,揭示了油膜厚度的分布规律,并分析了油膜厚度的影响因素和影响规律。研究结果对于该种传动的设计与制造具有指导意义。     

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.     

点接触弹流润滑性能及应用分析

牵引式无级变速器的传动零件间处于点接触状态，在某一传动比时，相对自转速度为零。本文研究了该状况下各种工况参数如滑动率、滚动速度和载荷等对点接触的弹流润滑性能的影响。研究表明：随着滑动率的增大，摩擦因数增大，油膜最大温升增大；在相同滑动速度下，随着滚动速度的增大，油膜厚度增大，但摩擦因数减小；随着载荷的增大,油膜厚度减小,摩擦因数增大，油膜最大温升增大。     

Scuffing failure analysis and experimental simulation of piston ring–cylinder liner

The scuffing failure phenomenon of piston ring–cylinder liner is studied theoretically and experimentally. The load and bulk temperature when scuffing failure occurs are measured under different engine speed, lubricant, and environmental temperature in a bench test. Based on the experimental results, the asperity capacity when scuffing occurs is evaluated. Surface contact temperature is determined with the measured bulk temperature and the surface flash temperature calculated by Blok theory. The scuffing failure threshold of piston ring–cylinder liner is established by using specific oil film thickness. This revised version was published online in June 2006 with corrections to the Cover Date.     

Elastohydrodynamic Film Thickness Measurements of Artificially Produced Surface Dents and Grooves

Elastohydrodynamic (EHD) film thickness measurements using optical interferometry have been made of artificially produced dents and grooves under rolling and sliding conditions. These measurements are compared to stylus traces of the dent and groove profiles to determine the local deformation associated with micro-EHD pressure generation. The surface geometry associated with the dents and grooves is seen to become intimately involved in the lubrication process itself, creating local pressure variations that substantially deform the local surface geometry, particularly under sliding conditions. The rolling results have implications concerning surface initiated fatigue and the sliding results show clearly the EHD surface interactions that must occur prior to scuffing failure.     

A comparison of boundary wear film formation on steel and a thermal sprayed Co/Cr/Mo coating under sliding conditions

In this paper, the action of the zinc dialkyl dithiophosphate (ZDDP) anti-wear additive has been examined on two different materials (Steel AISI 52100 and a Co/Cr/Mo thermal spray coating) sliding against cast iron in reciprocating mode. Tests have been conducted under lubricated wear conditions at relatively low (20, 50 °C) and elevated (up to 100 °C) bulk oil temperatures. A comparison is made between the friction, wear and chemical nature of the wear film formed under varying temperatures, on two materials, in two lubricants (one free from and one containing ZDDP) and after different test durations. The wear film has been examined by energy dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS).In this work, it has been shown in this work that the friction coefficient is dependent on the temperature, the lubricant and the nature of the contacting surfaces. In the presence of ZDDP, a wear film, comprising Zn, S and P, forms even at the lowest bulk oil temperature of 20 °C. The nature of the film is dependent on the substrate material and the steel and Co/Cr/Mo coating showed contrasting film characteristics. In this paper, the wear and friction results for each couple in oil containing and free from additives is discussed with reference to the nature of the wear film. A correlation has been made between the wear, friction and chemical analysis measurements.     

The properties of a thermal semielastohydrodynamic oil film with fully flooded and with starved lubrication

A study of thermal semielastohydrodynamic oil films including changes in the viscosity and the density with oil pressure and temperature is reported. The phenomena associated with sliding are also discussed. Approximate formulae for the minimum oil film thickness, the heat generated, the maximum temperature in the oil film and the friction factor are given. The effect of starved lubrication on the properties of the oil film was also investigated.     

Failure modes of sliding lubricated concentrated contacts

The mechanisms of film failure of lubricated concentrated contacts are discussed in relation to critical failure data of operating variables, calculations of scoring surface temperatures and scanning electron photomicrographs of worn contact zone topographies. Depending on the critical values of operating variables failure modes in the form of local break through of EHD oil film at the outlet end of the contact zone as well as gross thermal desorption effects were found.     

Measurement of lubricating film thickness,temperature and pressure in gear contacts with surface topography as a parameter

Thin film transducers, sputtered onto a tooth flank, are used to measure temperature, pressure and oil film thickness profiles in gear contact. The former two are resistance sensors, while the film thickness sensor functions as a capacitive transducer. Experiments were carried out with ground and hobbed gears. The measurements are repeated after the tooth flank has been polished. The differences of the measured values for the ‘rough’ and ‘smooth’ surfaces are used for comparisons between differently machined surfaces. An EHD simulation programme has been developed which calculates film thickness and contact pressure, also taking surface roughness into account.     

Computational fluid dynamics and full elasticity model for sliding line thermal elastohydrodynamic contacts

Classically, the EHD problem is solved using the Reynolds assumptions to model the fluid behaviour, and the Boussinesq elastic deformation equation to model the solid response, both being coupled with the load balance equation. The development of an alternative approach is presented here in order to solve at once the Navier-Stokes equations (mass conservation and momentum equilibrium), the full elasticity and energy equations for the line EHD problem in a fluid-structure interaction approach.The Finite Element Method is used to solve the mathematical formulation in a fully coupled way, inspired from Habchi et al. (2008) [1]. After linearisation with the Newton procedure, all the physical quantities (pressure, velocity field, deformations and temperature) are solved together in a unique system. An important benefit of this approach is the possibility to implement in a simple manner the non-Newtonian and thermal effects; in fact all the quantities can vary through the film thickness. The extension to non-Newtonian rheology and the pressure and temperature dependencies for the viscosity and density are taken into account in a direct way to allow an acceptable prediction of the friction coefficient. Gradients across the film thickness and temperature fields in both the fluid and the two solids are naturally computed and analysed. As a case study, we focus first on the pure sliding cylinder-on-plane contact. It is shown that thermal effects due to friction in the central zone of the contact play a role in heating the lubricant at the inlet zone, via heat conduction in the solids. By increasing the Slide-to-Roll Ratio (SRR), the occurrence of dimples and the subsequent effects in different parts of the contact under zero entrainment velocity conditions are then studied.