Rolling contact fatigue in lubricated contacts

This work presents and discusses the results of rolling contact fatigue tests (IP-300) with six different lubricants using a four-ball E.P. lubricant tester. These results tended to confirm the mechanism for rolling fatigue proposed by Jin and Kang. The lubricants tested were two mineral oils (SN 350 and SN 600) and four synthetic oils (PAG-9, PAG-12, PAO 6, and PAO X), the test machine used was a Four-Ball E.P., and the IP 300/87 standard was applied. The results indicated that: 1) lubrication has an important influence on the rolling fatigue life of mechanical components; 2) normally, in oils of the same family, the higher the viscosity is, the higher the rolling fatigue life is; 3) besides viscosity, other lubricant properties such as the pressure–viscosity coefficient (α), compressibility (B), and the EHL friction coefficient (γ), should all be taken into account for lubricated contact design.     

Tribological Study of a Tetrahedral Diamond-Like Carbon Coating under Vegetable Oil–Based Lubricated Condition

Maintaining a clean environment is the major concern of industries that produce fuel and lubricants for automotive applications. Thus, vegetable-based oils are being explored for the preparation of biobased lubricants because of their biodegradability and nontoxicity. Despite their low thermal stability, vegetable oils show better tribological characteristics than mineral oils. Nonetheless, the thermal stability of vegetable oils could be improved by transesterification. In this study, three vegetable-based oils (sunflower, palm, and coconut) were used to investigate the tribological properties of ta-C diamond-like carbon (DLC) coating under DLC–steel contact condition. A BICERI ball-on-a plate tribotesting machine was used to conduct experiments. During the experiment, test contacts of tribopairs lubricated with sunflower oil exhibited better tribological characteristics than those using coconut oil as a lubricant.     

Friction coefficient in FZG gears lubricated with industrial gear oils: Biodegradable ester vs. mineral oil

Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, were characterized in terms of their physical properties, wear properties and chemical contents and compared in terms of their power dissipation in gear applications [Höhn BR, Michaelis K, Döbereiner R. Load carrying capacity properties of fast biodegradable gear lubricants. J STLE Lubr Eng 1999; Höhn BR, Michaelis K, Doleschel A. Frictional behavior of synthetic gear lubricants. Tribology research: from model experiment to industrial problem. Elsevier 2001; Martins R, Seabra J, Seyfert Ch, Luther R, Igartua A, Brito A. Power Loss in FZG gears lubricated with industrial gear oils: biodegradable ester vs. mineral oil. Proceedings of the 31th Leeds-Lyon symposium on tribology. Elsevier; to be published; Weck M, Hurasky-Schonwerth O, Bugiel Ch. Service behaviour of PVD-coated gearing lubricated with biodegradable synthetic ester oils. VDI-Berichte Nr.1665 2002.]. The viscosity–temperature behaviors are compared to describe the feasible operating temperature range.Standard tests with the Four-Ball machine and the FZG test rig [Winter H, Michaelis K. FZG gear test rig—desciption and possibilities. In: Coordinate European Council second international symposium on the performance evaluation of automotive fuels and lubricants; 1985.] characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants.Power loss gear tests are performed on the FZG test rig using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils. Lubricant samples are collected during and at the end of the gear tests [Hunt TM. Handbook of wear debris analysis and particle detection in liquids. UK: Elsevier Science; 1993.] and are analyzed by Direct Reading Ferrography (DR3) in order to evaluate and compare the wear particles concentration indexes of both lubricants.An energetic model of the FZG test gearbox is developed, integrating the mechanisms of power dissipation and heat evacuation, in order to determine its operating equilibrium temperature. An optimization routine allows the evaluation of the friction coefficient between the gear teeth for each lubricant tested, correlating experimental and model results.For each lubricant and for the operating conditions considered, a correction expression is presented in order to adjust the friction coefficient proposed by Höhn et al. [Höhn BR, Michaelis K, Vollmer T. Thermal rating of gear drives: balance between power loss and heat dissipation. AGMA Technical Paper; October 1996. pp 12. ISBN: 1-55589-675-8.] to the friction coefficient exhibited by these lubricants. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized operating temperature.     

Effects of End Groups on the Tribochemical Reactions of Lubricants at Head-Disk Interface

Mass spectrometry was used to monitor in-situ gaseous species that were generated at the head-disk interface (HDI) in a high vacuum. It was found that the end groups of the lubricants significantly affected the wear durability at the HDI; piperonyl (–CH₂-phe = (O)₂ = CH₂) terminated Fomblin AM3001 lubricant exhibited longer life than hydroxyl (–OH) terminated Fomblin ZDOL lubricant. The continuous removal of the lubricants resulted in a continuously increasing friction coefficient. Further, the characteristics of tribochemical reactions of the lubricants (Fomblin Z series) with different end groups was investigated in details using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) just after the sliding tests. It was found that the decomposition of the end groups was more significant than that of the backbone. The lubricants terminated with the following groups showed the following order of increasing decomposition: –CH₂O-CH₂-phe = (O)₂ = CH₂ (AM3001), –CH₂OH (ZDOL) < –CH₂OCH₂CH(OH)CH₂OH (Z Tetraol) < –CH₂(OCH₂CH₂)_nOH (ZDOL-TX), –COOH (Z Diac). The decomposition of the lubricants appeared to start from the end groups.     

Mixture of safflower oil and synthetic ester as a base stock for biodegradable lubricants

Biodegradable industrial lubricant applications are gaining popularity with growing environmental concern and stringent government regulations. Plant oils are major base stocks for eco‐friendly green lubricants. However, because of their poor oxidation stability, their applications are restricted to limited usages. Improvements of these plant oils can be made either by addition of functional additives or by chemical/genetic modifications. This paper evaluates the synergistic approach of phenolic and aminic antioxidants in safflower oil. Further, to enhance the oxidation properties of the plant oil, we incorporated and studied synthetic esters (SEs) for thermo‐oxidative stability by using rotating pressure vessel oxidation test and differential scanning calorimeter. The combination of phenolic and aminic antioxidants at a 2 : 1 ratio exhibited the best synergistic effect, when incorporated into a combination of plant oil and SE. In addition to thermo‐oxidative properties, the tribological properties of neat base stocks were also studied. A substitute of mineral oil‐based lubricants can be formulated cost effectively by using an appropriate mixture of plant oil and SE. Copyright © 2013 John Wiley & Sons, Ltd.     

Development and testing of nano particulate lubricant for worm gear application

Bio-degradable lubricants are an attractive alternative for the mineral based and synthetic based lubricants. Bio-degradable lubricants are environmental-friendly and non-toxic. The present study deals with the tribological investigation of bio-degradable nano lubricants for worm gear applications. Nano additives like CuO and TiO₂ were used. Bio-degradable oils like palm oil and sunflower oil were used as base oils. The nano lubricants were prepared by adding two nano additives and two bio-degradable oils each of 0.1 % and 0.2 % weight composition. Friction and wear characteristics were tested on pin-on-disc tribometer under varying load conditions. Extreme pressure tests for nano lubricants were carried out using four ball tester. The wear surface obtained was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy. From the tests conducted, it was found that the addition of nano additives in biodegradable oils reduced the friction co-efficient and wear rate to a considerable extent.

     

基于数据采样的弧面凸轮自适应直接插补算法

针对目前弧面凸轮加工存在拟合精度与数据传输之间的矛盾,以及线性拟合破坏轮廓表面光顺性等问题,提出了一种基于数据采样的弧面凸轮自适应直接插补算法。该算法既能保证加工过程大部分时间内进给速度的恒定,而且能在曲率变化大的地方对进给速度进行自适应调整,以保证弓高误差不超出最大允许值。同时定义了相应的弧面凸轮自适应直接插补指令格式。最后基于UG开发了弧面凸轮自适应直接插补实体仿真系统,通过插补仿真实例进一步验证了该插补算法的正确性。该方法有效弥补了目前弧面凸轮线性拟合加工的缺陷,不仅能提高弧面凸轮加工过程中系统的稳定性和表面加工质量,而且使弧面分度凸轮机构定位精度和使用寿命提高,从而促进其推广和应用。     

Tribological degradation of two vegetable‐based lubricants at elevated temperatures

Renewable‐based lubricants are being considered as potential alternatives to petroleum‐based lubricants for various reasons, mainly increased environmental sensitivity. However, understanding the tribological performance of such vegetable‐based lubricants under elevated temperatures is critical for their industrial implementation. This study focuses on the friction and abrasion rate characteristics of soybean and sunflower base oils in comparison to a base mineral oil under sliding wear at elevated temperatures. It was found that the abrasion rate and friction were less severe for the vegetable‐based lubricants up to temperatures around 100°C. The observed performance of the vegetable‐based lubricants was verified using a kinetic reaction mechanism model of lubricant degradation. Copyright © 2007 John Wiley & Sons, Ltd.     

High temperature lubricants — oils and greases

Several compounds which can withstand high temperatures and retain their tribological properties have been used as high temperature lubricants. Such lubricants may be divided into three types: solid, liquid and greases. Solid lubricants can be used in more severe environments where oils and greases do not perform very well. A review of various solid lubricants useful at high temperature has already been published and this article reviews the tribological properties of several fluids and greases useful for high temperature lubrication.     

Coated gearings and bearings optimised for environmentally acceptable lubricants

Gearings and bearings are machine elements that are subjected to heavy tribological stresses. The pressures are high in the rolling contacts of these machine elements, and this leads to slippage between the surfaces of the contact partners. Correspondingly, the demands on the lubricants are also quite high. Nowadays, additives are used to reduce the friction and wear. However, the biodegradability of additivated lubricants is often not known, although the use of such lubricants can be critical for the environment. The objective of the investigations described in the present paper is to find a way of using lubricants with as few additives as possible but without decreasing the capability, reliability, and life of the machine elements. Thin coatings seem to be a good solution to substitute for the additives. The wear protection of coatings is investigated here with respect to gearings and spindle bearings for machine tools.     

Synthetic adipic complex tetraesters as eco‐friendly lubricants

The paper presents results concerning the synthesis and characterisation as lubricants with biodegradability potential of some complex tetraesters realised on the basis of adipic acid and different glycols such as (mono) ethylene, 1,3‐propylene, 1,4‐butylene, 1,5‐pentamethylene, 1,6‐hexamethylene, diethylene and triethylene glycol, respectively, along with oleic acid used, considered as an end, final segment or as a capping element. On the basis of a regular alternation or successive distribution principle of the polar and nonpolar chemical functions equally distributed, shared out on the length of a sufficient, satisfactorily long, large molecule, valuable synthetic complex tetraester lubricants considered as eco‐friendly base oils with biodegradability potential were performed. These products showed very good tribological properties, such as high viscosity indices and high flash points, and also very good lubricity features. Copyright © 2013 John Wiley & Sons, Ltd.     

High-performance base fluids for environmentally adapted lubricants

Future lubricants have to be more environmentally adapted, have a higher level of performance, and lower total life cycle cost (LCC) than presently used lubricants. To be able to formulate those lubricants, the properties of the base fluids have to be well known. Base fluid properties that influence the formulated lubricant performance could be divided into three different groups. These groups are: physical, chemical, and film formation properties. In this study, properties from all of these groups are investigated to improve the understanding on thier influence on base fluid overall performance.There are more or less environmentally adapted base fluids available for formulation of lubricants. They could be divided into different groups, mineral, semi–synthetic, and synthetic fluids. Synthetic fluids could be of different types: polyalpha olefins (PAO), synthetic ester, polyglycols, and others. The most interesting group for formulation of environmentally adapted lubricants are the synthetic esters. In this study, the properties for a large number of environmentally adapted ester base fluids are studied in detail. The tested properties relate to the macroscopic/molecular behavior and include: viscosity–temperature—pressure effects, η(p,T), thermal conductivity, λ(p,T), and heat capacity per unit volume, ρc_p(p,T). The film formation capability in elasto-hydrodynamic contacts is also studied. Different connections between the molecular structure and the performance of the fluids are discussed. As an example, it is found that a large number of carboxylate groups in the ester molecule improve the thermal properties, and thereby a thicker lubricating film could be maintained in highly loaded, high-slip contacts.     

Lubricant Effects on White Etching Cracking Failures in Thrust Bearing Rig Tests

Abstract

White etching cracking (WEC) is a subsurface bearing failure mechanism influenced by a number of factors, including lubricant composition. Certain metal-containing lubricants have been reported to promote WEC-induced failure; however, the exact mechanisms linking lubricant effects on WEC propensity are still not fully understood. An interesting field that has not been elucidated is the influence of additive concentration and tribofilm growth on WEC initiation, propagation, and failure. The investigations conducted in this work involved two series of oil formulations: one with additives that give rise to WEC (WEC oils) in different combinations and concentrations and another with additives that do not cause WEC (non-WEC oils). A mini traction machine (MTM) in combination with a spacer layer imaging machine (SLIM) was employed to study the growth of tribofilms and their influence on friction response. Insights from the MTM-SLIM study allowed for better interpretation of FE8 bearing tests. When using oils that contribute to WEC formation, the tribofilm-induced WEC mechanism was confirmed, with cracks initiating as early as after 20 h of FE8 testing. Metal-containing additives were found to favor the formation of WECs by generating a high-friction tribofilm and increasing the water content in the lubricant. Furthermore, the source of subsurface H associated with WEC failure is investigated using heavy water (D2O)-saturated oil. A mechanism of water dissociation induced in tribofilm growth (incubation period) is proposed in this article.     

Influence of fatty acid composition on the tribological performance of two vegetable‐based lubricants

In light of diminishing natural resources, global climatic change and increased environmental sensitivity, renewable‐based lubricants are being considered potential alternatives to petroleum‐based lubricants. Understanding the tribological performance of vegetable‐based lubricants in relation to their chemical composition is essential for their industrial implementation. This study focuses on the friction and abrasion rate characteristics of soybean and sunflower oils in comparison to a base mineral oil under sliding wear at ambient conditions for various applied loads. It was found that the abrasion rate and friction were the least severe for the soybean, followed by the sunflower oil. The observed trends were attributed to differences in their fatty acid compositions, in particular, a lower percentage of linoleic and oleic acids within the soybean oil. Copyright © 2007 John Wiley & Sons, Ltd.     

Study of the lubrication mechanism of two-stroke engine oils using a disc machine

A technique has been developed to evaluate scuffing characteristics of two-stroke engine oils on a high speed disc machine. This paper deals with the lubrication mechanisms involved in the test adopted. The influence of h/σ, surface conformity and boundary characteristics of lubricants were examined. It was observed that the lubrication mode changed from hydrodynamic to mixed to boundary conditions leading to eventual failure. This situation is similar to that observed in two-stroke piston tightening tests where the lubrication mode changes from hydrodynamic to mixed lubrication as the temperature increases.     

Calculating actual profiles of conjugate disk cams by means of conjugate variation measurement

For an assembled conjugate cam mechanism with two separate oscillating roller followers serving as a conjugation measuring fixture, when the cams rotate, the variation of the subtending angle between the two follower arms can indicate the variation of the cam profile errors. The profile errors of the assembled conjugate disk cams can be indirectly calculated by means of the conjugate variation measurement and the inverse conjugate variation analysis. That is, if a pair of master conjugate cams with known profile errors is available, by means of the measured subtending angle variations of the follower arms induced by a pair of assembled conjugate cams that consists of one master cam and the other being the inspected cam, then the actual profile of the inspected cam can be calculated by applying the inverse conjugate variation analysis. In order to verify the presented concept, experiments meant to calculate the profile errors of a pair of machined conjugate cams were conducted to compare with the measuring results obtained by using a coordinate measuring machine. The experimental results showed that the calculated cam profiles were well consistent with those of the measured ones. The presented method can be an effective alternative means for examining profile errors of assembled conjugate disk cams and could be useful for the quality control in mass production of conjugate cams.     

The tribological behavior of Ni–Cu–Ag-based PVD coatings for hybrid bearings under different lubrication conditions

In a cryogenic environment, components like bearings with interacting surfaces in relative motion (tribosystems) often generate undesired heat and experience high wear. Because the properties of conventional bearing materials like stainless steel cannot be applied to this temperature range, the PVD coating based on metal–metal pairs with better frictional properties must be employed. To test the suitability of the Ni–Cu–Ag-based PVD coatings of hybrid bearings for liquid rocket engine turbopumps and to obtain reliable coating material data in the extreme environment, the tribological behaviors of coatings under the cryogenic fluid (liquid oxygen and liquid nitrogen) and water lubricated conditions are studied, respectively. In the paper, the specimens are in a vibrocryotribometer with the ball-on-plane contact type, and various running conditions in terms of lubricants, contacting loading, and contacting velocity are examined. The simulated experiment for testing the actual tribological performance of Ni–Cu–Ag-based PVD coatings for hybrid bearings is tested. The results of the tests indicate that the coatings can be suitable for cryogenic tribosystems of turbopumps. In the cryogenic environment, the volume wear rate of coatings increases rapidly with the contacting loading, but 15 min later, the volume wear volume of coatings turns into 2.5–15×10⁻⁴ mm³. Besides, under the liquid oxygen condition in simulating the liquid rocket engine turbopumps environment, the friction coefficients are 0.03–0.1.     

A new focus on the Walther equation for lubricant viscosity determination

Lubricants are widely used in industrial machinery in order to separate solid tribological surfaces and support high loads under severe conditions. In tribological contacts, viscosity plays an important role in the film‐forming abilities of the lubricant, but this property is strongly dependent on temperature. Consequently, small variations in temperature cause appreciable variations in the viscosity of lubricating oils. For this reason it is of practical value to be able to predict viscosity changes with temperature. This paper presents a new focus on the Walther equation to determine the viscosity of commercial lubricants at different temperatures. This new approach provides very good correlation with experimental measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

Novel method of measuring tappet rotation and the effect of lubricant rheology

One of the main drivers for developing lubricant technology is engine durability. Researchers and scientists are using new technologies, materials and advanced lubricant formulations to reduce overall engine friction and wear. One of the main engine tribological components is the valve train. This is one of the most challenging components to lubricate effectively because of the higher contact loadings and accounts for 10–20% of the total engine friction loss. The two main factors affecting the performance of engine valve trains are wear and friction, and a wide range of mechanical configurations are used to improve these. For example, direct-acting overhead camshaft valve train configurations use a rotating tappet design. Normally, the tappet is slightly offset from the cams and the cam is slightly conical to match the domed tappet to facilitate tappet rotation for even wear and to reduce slippage. In this paper, a novel innovative technique has been described to monitor tappet rotation in a real production engine having a direct overhead cam–tappet arrangement. The monitoring technique was applied to a VW Tdi engine head, and tests were carried out under different operating conditions. Lubricant compositions, oil temperature, pressure and camshaft speeds on tappet rotation were measured and all are shown to have an effect. The balance of forces between the cam–tappet and tappet-bore was found to be interlinked and the design of the hydraulic lash adjuster had a significant effect.This unique tappet rotation monitoring system can be used on most of the direct overhead camshaft engines, with minor engine modifications, to measure lubricant and hardware effects under both motored and fired conditions.     

Frictional properties of some additives for sliding guide way lubricants in a sliding speed range between 0.002 and 1.5 m/s with a thrust collar friction tester

Frictional properties of some additives used in sliding guide way lubricants were studied for a pair of mild steels under sliding speeds ranging from 0.002 to 1.5 m/s with a thrust collar type friction tester. As the sliding speed was decreased, the coefficient of friction first decreased, then after reaching the minimum value, it increased. The minimum friction coefficient μ_min and the transition sliding speed V_min showing μ_min depended upon the additive. That is, μ_min for the oil containing sulfurized lard was lower than those for the other oils tested and its V_min was as low as 0.005–0.1 m/s compared with the transition speeds of 0.2–0.5 m/s for the other oils tested. Observations of the surface profile and surface analyses of the test specimen after the tests indicated that the unique phenomena for sulfurized lard were due to the formation of a boundary film containing a long carbon chain with a chemical bond between the sulfur and carbon.