Influence of lubricant on gear failures — test methods and application to gearboxes in practice

Base oil type, oil viscosity, and additive type and content have a strong influence on typical gear failures. As it is not possible to quantify the influence of a lubricant on load‐carrying capacity simply from a knowledge of the physical or chemical oil data, many test methods have been developed for the evaluation of mechanical—technological lubricant properties. Simple low‐cost bench test methods often show poor correlation with practice. From both experience and systematic investigation, it can be seen that testing of gear lubricants can be performed adequately only in gear test rigs using specified test gear geometry. The standard FZG back‐to‐back gear test rig has been developed over many years and improved for different types of gear failure simulation. The standard FZG oil test A/8.3/90 is widely used for the evaluation of the scuffing properties of industrial gear oils. Automotive gear oils of GL4 level can be tested in the step test A10/16.6R/90, and axle oils of GL5 level in the shock test S‐A10/16.6R/90. For slow‐speed regimes, the C/0.05/90:120/12 wear test can be used. The influence of lubricants on the micropitting performance of gears can be evaluated in the GF‐C/8.3/90 micropitting test. Different pitting tests are available, as single‐stage (PT‐C/9:10/90) or load spectrum (PT‐C/LLS:HLS/90) tests. The aim of this paper is to describe the influence of the lubricant on the different failure modes in gears, how to quantify this effect in adequate test methods, and how to introduce the results of such tests as determining values of the lubricant into load‐carrying capacity rating methods.     

A laboratory technique for the evaluation of automotive gear oils of API GL-4 level

Automotive gear oils, as typically used in rear axles, are normally evaluated in terms of their performance in full-scale bench tests. One such test is the CRC L-19/CRC L-42 test, which is included in API and other specifications. The L-19 test method is now obsolete, and the L-42 is used basically to define GL-5 level oils. An indigenous test method, IIP Method VAV-382, has been developed at the Indian Institute of Petroleum on an Amsler machine, operating under sliding conditions between two discs, and is also included in the IS:1118-92 specification for multi-purpose gear oils for GL-4 level oils. This method has been re-examined to obtain better resolution of scuffing of the disc surfaces. The modified procedure has proved effective, and the procedure and results for various commercial oils are described in this paper, which shows the method to be effective for the evaluation of GL-4 level oils.     

Effect of ageing automotive gear oils on scuffing and pitting

The aim of this work is to compare automotive gear oils of different performance levels (API GL) from the point of view of their resistance to degradation due to ageing. The influence of the gear oil degradation on scuffing and rolling contact fatigue (pitting) has been assessed. Two automotive gear oils of API GL-3 and GL-5 performance levels were tested. They were contaminated with dust, water and oxidation residues.It has been concluded that from the point of view of the resistance to scuffing the most dangerous contaminant is water. For the resistance to rolling contact fatigue the most dangerous are dust and water. In general, GL-5 oils are less vulnerable to deterioration due to ageing than GL-3 oils.     

Pitting load capacity test on the FZG gear test rig with load-spectra and one-stage investigations

Lubricants in their combination of base oil and additive influence the pitting and micropitting load-carrying capacity of cylindrical gears. The aim of the FVA ‘Pitting test’ research project was to develop and establish a test method on the standard FZG gear test rig with a centre distance a = 91.5 mm. Two different test procedures were proposed, one using a load spectrum and the other a constant load. These tests can be used to determine the relative pitting load capacities of reference and candidate oils. From these results the pitting load-carrying capacity can be calculated.     

Scoring tests of aircraft transmission lubricants at high speeds and high tem peratures

Aircraft engines contain gears that have to be lubricated under conditions of high speeds and extremely high temperatures, In this field of application scoring damage can occur. In Europe and partly also in the USA the scoring load capacity of gear oils is expressed in terms of FZG Scoring Load Stage; the FZG Gear Test Rig is described. The normal test procedure A/8.3/90 as standardised in DIN 51 354 using A-type gears at a pitch line velocity of v = 8.3 m/s and a starting oil temperature of 90°C is presented. A modified procedure at double speed and increased oil temperature A/16.6/140 is discussed. The scoring load capacity of aircraft transmission lubricants is expressed worldwide in Ryder Gear Test results. Because of the high costs and problems with the availability of test gears a modified FZG Ryder Test was developed. The method is presented and comparative results of typical aircraft engine oils in the FZG, the FZG-Ryder and the original Ryder Gear Test are shown. From this experience it becomes obvious that alternative test methods for the evaluation of scoring load capacity of aircraft transmission lubricants can be available in the near future.     

Assessing the energy efficiency of gear oils via the FZG test machine

Energy efficient lubricants are becoming increasingly popular. This is due to a global increase in environmental awareness combined with the potential of reducing operating costs. A new test method of evaluating the energy efficiency of gear oils has been described in this report. The method involves measuring the power required by an FZG test rig to run while using a particular test lubricant. For each oil that was being evaluated, the rig was run for 10 minutes at a load stage of 10. Six extreme pressure (EP) industrial gear oils of mineral base were tested. The difference in power requirements between the best and the worst performing oils was 2.77 and 3.24 kW, respectively. This equates to a 14.6% reduction in power, a significant amount if considered in relation to a high powered industrial machine. The oils of superior performance were noticed to run at reduced temperatures. They were also more expensive than the other products of lesser performance.     

Influence of oil temperature on gear failures

Typical gear failures like wear, scuffing, micropitting and pitting are influenced by the oil temperature in the lubrication system. High temperatures lead to low viscosities and thus thin lubricant films in the gear mesh with generally detrimental influence on failure performance. On the other hand, for gear oils with additives higher temperatures correspond with higher chemical activity and, at least in some cases, with better failure performance of the lubricant. Last, but not least, at very high temperatures even metallurgical changes have been found with a reduction in material endurance limits. Examples for the influence of oil temperature on gear failure modes, as well as their introduction into load carrying capacity calculation methods are shown. With this background, the often-applied practice of increasing the severity of a gear oil test method by increasing the oil temperature has to be revised. Adequate solutions are discussed.     

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.     

Synthetic fluids for automotive gear oil applications: A survey of potential performance

This paper describes studies of transmission lubricants in an axle efficiency test rig. The test lubricants were evaluated over various temperature ranges, for each of five road load speed conditions. This was done for both truck and passenger car. Three synthetic gear oils were evaluated, based on various combinations of synthetic hydrocarbons, esters, and viscosity improvers, and were compared to conventional SAE 80W-90 lubricants. All three oils demonstrated improvements in axle eficiency. Also reported are evaluations of seven test lubricants in a high-temperature, high-torque test, and results of seal compatibility tests.     

Evaluation of rear axle extreme pressure (ep) lubricants by a disc machine

The performance evaluation of extreme pressure lubricants for hypoid rear axles is a time-consuming and expensive process. A method for performance evaluation which utilizes a disc machine is described. Initial work shows that by using a special technique CRC reference gear oils can be clearly classified under shock loading conditions. The importance of a critical rise in friction and film conditioning with regard to scuffing was briefly considered.     

Fuel economy of multigrade gear lubricants

A light‐duty axle efficiency test for evaluating gear lubricants for their fuel economy performance is described. Data collected for an internal reference oil highlight the repeatability of the test with different axles. Comparisons between single‐grade SAE 90 and multigrade gear lubricants were made under a variety of pinion torques and speeds to simulate highway and city driving conditions. Lubricant rheology and its importance in maintaining film strength for adequate bearing and gear lubrication for optimum torque efficiency and axle temperature are discussed.     

A screening test for anti-wear additives in a modified four-ball test

Modern engine tests that evaluate the anti-wear properties of automotive engine oils are increasingly sophisticated and expensive, and often have relatively poor precision. The development of a simple, inexpensive, and reliable bench test to screen the anti-wear properties of fully formulated engine oils prior to their testing in engines is therefore very attractive. Numerous methods already exist, but they typically measure wear only at the end of the test by measuring the consequences of wear. The present paper describes an alternative approach, its purpose being the comparative evaluation of the anti-wear performance of lubricants throughout the test under variable load. A four-ball machine was used as a test rig for this work, and was equipped with instruments allowing study of the oil bath temperature, load applied to the balls, and the displacement of the load lever arm. The work presented focuses on a test procedure containing the following important elements: pre-ageing of oils, test start-up at very low load, incremental increases in load, stepwise increase in load, with each step sufficiently long to allow system equilibrium. Recording and analysis of the temperature and arm displacement curves permit the recognition of two distinct forms of wear: slow and gradual abrasive wear, and sudden and intense adhesive wear (scuffing). The presence and the intensity of the latter were found to have a direct relation with the anti-wear performance of candidate oils in the API Sequence VE engine test. The procedure ranked oils correctly in relation to their dithiophosphate concentration, correctly distinguished secondary and primary zinc dithiophosphates and, more interestingly, predicted the positive effects of some ashless anti-wear additives in accordance with results obtained in the Sequence VE.     

反推齿轮胶合临界温度可靠性的试验研究

本文提出了用闪温准则反推齿轮胶合临界温度的计算方法，进行了大量齿轮抗胶合承载能力试验，根据试验结果对５０号机械油的临界温度进行了反推计算，分析和对比结果表明，该计算方法是可靠的，试验方案设计是合理的，为确定各类油品润滑时齿轮胶合临界温度奠定了基础。     

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.     

A method for testing lubricants under conditions of scuffing. Part I. Presentation of the method

A method for the tribological assessment of lubricants under conditions of scuffing is presented. The method uses a four‐ball tester, and allows one to assess the effect of lubricant on scuffing intensity through an analysis of changes in the friction torque and wear of the stationary balls, at continuously increasing load. The behaviour of a lubricant under scuffing conditions can be characterised using the so‐called limiting pressure of seizure p_oz, which depends on the load at which the balls seize and the average value of the wear area calculated from the wear‐scar diameters measured on the stationary balls. A comparison is made ‐ from the point of view of the resolution, time consumption, and cost ‐ of the new method with the existing, standard tests, using a four‐ball tester and a gear test rig (FZG). It is concluded that the proposed method, unlike standard FZG and certain four‐ball tests, enables one to differentiate between gear oils, in agreement with their API GL performance level. The very short run‐time of the new method enables one to perform more tests and obtain a low standard deviation. The new method is much cheaper than the standard four‐ball and FZG methods.     

The Effect of Lubricant Traction on Scuffing

This paper presents the results of a disc machine gear simulation investigating the influence of lubricant traction characteristics and formulation on the load at which scuffing occurs. Scuffing theories in general link the onset of scuffing to the amount of heat generated in the contact and the authors hypothesized that reduced heat generation with low traction lubricants should lead to an increase in scuffing load. The study compared low traction PAO-based lubricants with mineral oils in basestock, antiwear and EP formulations and at both high (>6) and moderate (approximately 1.2) specific film thickness, λ. At λ > 6, the benefits of the synthetics over their mineral counterparts ranged from 25 percent to 220 percent and at λ ? 1.2, the benefits were a uniform 40 percent. It was particularly interesting to observe that the antiwear PAO-based oil gave a similar scuff load per unit contact width to an EP mineral gear oil. In addition, it was shown that scuffing load decreased with increasing traction coefficient to the power of approximately ? 1.85, close to the ?2.00 power predicted by the frictional power intensity concept. The agreement with flash temperature theory, with a predicted power of ?1.33, was less close.     

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.     

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.

     

几种防滑差速器油摩擦特性的比较

介绍了防滑差速器油的性能特点,分析了几种防滑差速器油的基本理化性能,并将防滑差速器油与GL-5齿轮油、自动传动液(ATF)的摩擦因数进行了比较。结果表明防滑差速器油性能与GL-5齿轮油有较大差异;昆仑LSD90车辆防滑差速器齿轮油的基本理化性能和摩擦性能优于参比的2种国内外同类油样。     

The design of fuel efficient automotive hypoid gear lubricants

Synthesised lubricant base stocks and the performance additives used in hypoid gear lubricants can both be chemically designed to give improved hypoid gear efficiency and, therefore, better vehicle fuel economy than provided by conventional lubricants In this paper, the authors review, relative to a widely used SAE 90 grade mineral oil-based lubricant, the changes in gear efficiency which have been documented using certain classes of synthesised lubricants. These advantages are described in terms of (a) EPA City and Highway Cycle efficiencies, (b) absolute data over a wide temperature range under four selected speed/torque combinations, and (c) absolute changes in inefficiency compared to the mineral oil product. Newtonian and non-Newtonian fluids have been studied. The bulk oil temperature at which a specific lubricant gives maximum efficiency is dependent on both gear operating conditions and on the chemical composition of the lubricant.