Analysis of micro-elastohydrodynamic lubrication for engineering contacts

The paper describes methods which have been developed for theoretical analysis of elastohydrodynamic lubrication (EHL) in situations where the nominal oil film thickness is of the same order as the height of roughness asperities on the surfaces in contact. In such contacts the roughness significantly affects the distribution of pressure, and pressures much higher than predicted on the basis of smooth surface theory are obtained even under conditions where a full fluid film is present. This has consequences for surface fatigue processes such as pitting, and the presence of roughness is also considered to be a crucial factor in the occurrence of scuffing in which hydrodynamic film failure leads to adhesion and severe distress of the surfaces. The major aim of the work is to develop a better undertanding of the physics of scuffing based upon models of film failure in the presence of roughness.     

Investigation into the scuffing load capacity of environmentally-friendly lubricating oils

The present paper looks at the nature of scuffing, and the factors influencing it, for both hot scuffing and cold scuffing. The influence of surface layers and finishes in relation to this phenomenon are discussed, and some preliminary results are presented for cold scuffing tests, with various steels, for a mineral oil, a rapeseed oil, and a vegetable-based oil.     

Lubrication modes and the IRG transition diagram

The relationship between a Lubrication Mode Diagram (LMD) for concentrated contacts (LCC's) and the ‘IRG transition’ diagram has been studied. In addition, scuffing results, obtained by the IRG (International Research Group) have been analysed, as well as the results of scuffing tests performed by different workers using a two-disc apparatus. It has been found that in Region I of the IRG transition diagram, three sub-regions can be distinguished, representing elastohydrodynamic lubrication, mixed lubrication, and boundary lubrication, respectively. Thus, the IRG transition diagram is not comparable to a ‘lubrication mode diagram’. Furthermore, it has been found that failure of LCC's by scuffing most frequently occurs under conditions of mixed lubrication. Therefore, scuffing criteria based upon the assumption that the load at failure is carried entirely by the micro-contacts are doubtful, and probably wrong.     

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.     

Boundary lubrication of thermoplastic polymers in model fluids

Organic polymers, particularly thermoplastics, have traditionally been used in unlubricated contacts because of what is termed their ‘self lubricating’ character. Significant improvement in bearing performance may often be obtained by conventional liquid lubrication but a number of subtle interfacial phenomena are sometimes noted some of which are deleterious to good operation. A particular type of problem arises when the fluid migrates to a significant depth into the matrix of the polymer and causes a premature form of scuffing failure under intense levels of energy dissipation. This paper reviews the scuffing behaviour of several commercial polymeric matrices and speculates upon the fluid-polymer interactions which can occur at the onset of lubrication failure.     

Scuffing behaviour of salt-bath nitrided steel in loaded lubricated sliding/rolling contacts

The modified 3 in (76.2 mm) David Brown disc machine has been used to investigate the effect on scuffing performance of grinding away various amounts from the surface of nitrided steel discs. The results revealed a systematic deterioration in scuffing resistance as the surface nitrogen concentration decreased, but quantitative correlation between these parameters was hindered by the inadequacies of electron probe X-ray microanalysis as a tool for measuring low nitrogen concentrations. It was concluded that there is no safe depth to which Tenifer-treated (salt-bath nitrided) C15 steel can be ground without impairing its scuffing performance. Other disc tests were carried out to assess the effect on scuffing of phosphate treatment and oil formulation. Results showed that, while phosphate treatment is valuable when running against untreated steel, it is of no benefit against Tenifer-treated steel. On the other hand, the formulation of the lubricant was found to improve the scuffing resistance of Tenifer-treated steel     

The effect of base oil oxidation on scuffing

In this work, the effect of base oil oxidation on scuffing was investigated. It was found that under mixed lubrication conditions there is a direct relationship between the level of base oil oxidation and the onset of scuffing, i.e., the greater the oxidation the lower the likelihood of scuffing. An oxidised base oil forms, among other products, carboxylic acids, which reduce scuffing failure in comparison with the parent oil. This was confirmed by the addition of a carboxylic acid directly to the base oil operating under the same mixed lubrication conditions. The removal of the carboxylic acids from the oxidised oil via column chromatography resulted in a scuffing failure performance equivalent to the parent oil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of abrasive contaminants on scuffing

Four-ball machine tests have been performed to study the effect on scuffing of fine silica abrasive particles contained in the lubricant. The boundary lubrication zone that sets in after seizure recovery has been utilized in the study. The particles are found to increase abrasive wear but reduce adhesive wear and to cause scuffing at less severe conditions that with clean oil by contributing to frictional heating. When the heat generated cannot be dissipated from the contact zone the contact temperature is found to increase and lubricant desorption causes scuffing.     

Effect of temperature on the scuffing load capacity of EP gear lubricants

Scuffing of gears involves the welding together of locally unprotected metal‐to‐metal contacts when critical limits of pressure and temperature are exceeded. Protection can be maintained by a thick lubricant film, by physically adsorbed layers, or by chemical reaction layers. At higher temperatures, viscosity and film thickness decrease but, using an EP gear oil, due to higher chemical reactivity, the scuffing load capacity is not necessarily reduced accordingly. The reaction temperature of the additives is not always reached for large gears. In this paper the factors that influence the scuffing load capacity are investigated, and test possibilities and calculation methods are outlined.     

缸套/活塞环摩擦学性能试验机的设计研究

根据缸套 /活塞环胶合失效试验研究的需求 ,设计了一台集试验、测量和控制于一体的往复式摩擦磨损试验机。该设备具有加热和温度控制功能 ,采用快速响应速度的微细热偶测量接触区温度 ,同时通过四臂电桥测电阻法测量缸套 -油膜 -活塞环之间的动态接触电阻 ,以反映接触区的润滑状态。试验表明该试验机性能稳定 ,可测物理量丰富 ,是胶合试验研究可靠的实验工具     

Model of scuffing based on the vulnerability of an elastohydrodynamic oil film to chemical degradation catalyzed by the contacting surfaces

A model of scuffing is developed based on the premise that metallic surfaces can catalyze degradation of the lubricant film in situ. A failure mechanism for elastohydrodynamic films based on rapid decomposition of mineral and synthetic oils involving chemical reaction between entrapped oil and the containing surfaces is proposed. It is suggested that this destruction of the elastohydrodynamic oil film allows adhesion between nascent metal of opposing surfaces in the contact which in turn causes scuffing. Suppression of scuffing by the application of coatings that do not catalyze the oil decomposition and by the action of some lubricant additives which may block the catalytic effect of metallic surfaces is discussed. Effect of solid lubricant films and contaminant layers on scuffing is also described.     

Method for scuffing propagation assessment

This paper describes a method for the determination of scuffing propagation using a four‐ball extreme‐pressure tester. The method has been developed at the authors' laboratory and is a completely new approach to the investigation of scuffing phenomena. A series of lubricants was prepared by blending antiwear (AW) and extreme‐pressure (EP) additives with a mineral base oil. Tribological experiments were then performed using the new method. The aim was to investigate the influence of such additives on scuffing propagation. It is shown that there is a significant influence of AW and EP additives on scuffing. Surface analyses (SEM, EDS) show the decisive role of the chemical reactions of AW and EP additives with the steel surface and their creation of a surface layer whose good antiseizure properties mitigate scuffing propagation and reduce wear intensity. It must be emphasised that the authors consider scuffing to be a process leading to the cessation of the relative movement of a tribosystem, known as seizure.     

镗杆和镗套烧结咬死后的快速修复方法

仅从镗杆镗套烧结咬死后修复的角度有针对性地提出了几种快速、适用的修复方法,这些方法在生产实际中已经过多次验证,采用这些方法可有效地解决镗杆和镗套烧结后快速恢复使用精度的问题,保证组合镗床在使用过程中不会因局部零件失效而长期影响生产.     

Influence of surface roughness on friction and scuffing behaviour of cast iron under sparse lubrication

Friction and scuffing behaviour of grey cast iron as influenced by the surface roughness under sparse lubrication conditions is studied. The studies are carried out on a three-shoe-on-disc machine under the conditions of parallel sliding. The experiments were conducted at three oil supply rates of 0.10, 0.22 and 0.36 μg/cm² per contact obtained through a mist oiling system and with specimens of cast iron shoes of different roughness values, Ra, between 0.04 and 2.0 μm against a 0.55% C steel disc. In a step load procedure, the friction torque at the end of each load step and the scuffing load are the major parameters measured. Results of friction and scuffing behaviour as a function of roughness and oil supply rates have been discussed.     

Global and local analysis of gear scuffing tests using a mixed film lubrication model

Gear tests were performed in a FZG test rig in order to evaluate the influence of the operating conditions (torque, speed and oil bath temperature), gear geometry and base oil viscosity on gear scuffing.A mixed film lubrication model was used to evaluate the normal pressures and shear stresses in several points along the gear meshing line, for each load stage and for all the gear scuffing tests performed.The gear scuffing results were analyzed using two different approaches: one considering global gear parameters defined at the meshing line scale and another based on local parameters at the roughness asperity scale, determined using the mixed film lubrication model.The analysis at the roughness asperity level was used to complete the scuffing study performed with global gear contact parameters, explaining the occurrence of scuffing during ‘running-in’, justifying the zones in teeth flanks where the first scuffing marks appear and supplying indicators for low scuffing resistance at high oil bath temperatures.     

The effects of lubricant additives on wear and scuffing resistance of copper alloys under boundary conditions

The effects of lubricant additives on alloy composition and structure, and on the wear and scuffing resistance of copper alloys, were investigated. The copper alloys used were Sn bronze, Sn-Zn-Pb bronze, Al-Fe bronze, and Si-Mn brass. The base oil used was straight mineral oil. The additives used were dimer acid, phosphonate ester, sulphide olefine cotton-seed oil, ZDDP and n-dibutyl phosphite. The test results revealed that Si-Mn brass shows the highest resistance to wear and scuffing, and Sn-Zn-Pb the lowest. It also revealed that oiliness additives are superior in wear and tear scuffing resistance to EP additives, especially sulphur-containing additives. The authors also noted that copper alloys show good resistance to wear and scuffing when coupled with a polished steel surface.     

胆甾醇润滑添加剂的摩擦噪声与胶合分析

在N15机械油中添加胆甾醇,用标准的立式万能摩擦磨损试验台实验分析了N15机械油、N15油机械 胆甾醇的摩擦磨损性能。针对磨合阶段、稳定磨损阶段、胶合失效与摩擦噪声进行分析。结果表明,在磨合阶段摩擦因数有较大变化,随后的稳定磨损阶段中摩擦因数变化减小;在摩擦副出现胶合失效前,摩擦副的摩擦因数急剧上升;N15机械油 1.7%胆甾醇的抗胶合性能较好,但N15机械油 0.106%胆甾醇的摩擦噪声特性较好;胆甾醇添加剂可以显著改善N15号机械油的抗胶合能力。     

A method for testing lubricants under conditions of scuffing. Part II. The anti-seizure action of lubricating oils

This paper describes an investigation of lubricating oils under extreme-pressure (EP) conditions in a specially modified four-ball tester. A new test method developed at the Tribology Department of ITeE described in Part I of this paper was used. In this, during a test run, the applied load is increased continuously and the friction torque is measured. A sudden increase in the friction torque indicates the collapse of the lubricating film — where scuffing is initiated. The load at this moment is called the scuffing load. If the load is increased further, it is possible to observe scuffing propagation until seizure occurs, i.e., a defined, maximum friction torque is reached. Thus, scuffing is considered as a process leading to seizure. Using the method, tribological experiments were performed employing various lubricating oils consisting of viscosity-index improvers and antiwear (AW) and extreme-pressure (EP) additives added to a base oil. Mineral and synthetic base oils of different kinematic viscosities were used. The aim was to investigate the influence of such lubricants on scuffing initiation and propagation with the present methodology. In Part I it was shown that scuffing initiation depends strongly on the kinematic viscosity of the lubricant; the higher the viscosity, the greater the scuffing load. The presence of AW and EP additives in the lubricant increases the scuffing load significantly. It was also shown that the kinematic viscosity of the lubricant oils has no effect on scuffing propagation. However, scuffing propagation is significantly mitigated by AW and, to a greater extent, by EP additives. The results of surface analyses show the decisive nature of the chemical reactions of AW and EP additives with the steel ball surface under scuffing conditions, as well as the possible diffusion of sulphur and phosphorus. Chemical reactions and diffusion lead to the creation of an inorganic surface layer (probably iron sulphide), the good anti-seizure properties of which limit scuffing propagation.     

Friction,Wear, and Scuffing Characteristics of Marine Engine Lubricants with Nanodiamond Particles

Many kinds of additives are generally added to engine lubricants to improve performance. These chemical additives are harmful to both humans and the environment. For this reason, the research trend in the lubricant industry is to reduce the use of chemical additives in engine oils. Carbon materials like nanodiamonds are candidates among many physical additives. Nanodiamond particles are round, very hard, chemically stable, and highly heat conductible. In this research, nanodiamond particles were uniformly dispersed in marine engine lubricants. A matrix synthesis method was used for dispersion with various concentrations. Friction and wear tests were performed to measure the friction and wear amounts, and scuffing tests were performed. The friction coefficients were decreased with the addition of nanodiamond particles. Due to their octagonal and almost spherical shape, the particles could act as rolling contact elements between two lubricated sliding surfaces. In addition, it was found that there was a proper concentration of nanodiamond to minimize the wear amounts, which was 0.3 wt%. From the scanning electron microscopy (SEM) analysis many agglomerated particles were found on the sliding surfaces with a high concentration of particles over 0.3%. The excessive amount of nanodiamonds acted as abrasive debris and ploughed the contact surfaces. Finally, as the concentration of nanodiamonds increased, the scuffing life increased due to a reduction in friction, and the rate of temperature increase was reduced due to the high heat conductivity of nanodiamonds.     

低噪声润滑脂的摩擦磨损特性分析

用标准的立式万能摩擦磨损试验机的销盘实验,分析了低噪声润滑脂EMQ2与RS3润滑脂的摩擦磨损特性,针对磨合阶段、稳定磨损阶段、摩擦噪声与胶合失效进行分析.结果表明,在磨合阶段摩擦系数有较大变化,随后的稳定磨损阶段中摩擦系数变化减小;脂润滑的摩擦副的摩擦系数比油润滑显著增大、变化频率增加,在试验过程中出现嗡嗡声时,而在摩擦系数下降阶段不出现摩擦噪声;在摩擦副出现胶合失效前,摩擦副的摩擦系数急剧上升;在摩擦系数变化幅度达到10%左右时,摩擦副就出现胶合失效.并用扫描电镜（SEM）分析了试盘、试销摩擦副运转2 h后的磨斑表面形貌.