A scuffing load capacity test with the FZG gear test rig for gear lubricants with high EP performance

Methods for the evaluation of the scuffing load capacity of EP lubricants of API GL-4 and GL-5 performance levels have been developed for the standard FZG gear test rig, which has a centre distance of a = 91.5 mm. Procedure A10/16.6R/90 is capable of discriminating between industrial gear oils and automotive gear oils up to GL-4 performance level while procedure S-A10/16.6R/90 can be applied to GL-4 and GL-5 lubricants. The test methods were verified with reference oils and commercial gear lubricants. The results of the tests can be used to give a relative ranking of reference and candidate lubricants. They can also be used as limiting values in the scuffing load capacity rating method according to DIN 3990 (ISO DIS 6336).     

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.     

A review of scuffing models

The mechanism of wear known as scuffing has been a research topic of great interest for many years. However, the question of how scuffing is initiated and the factors that contribute to its occurrence are still poorly understood. In general, it can be said that scuffing manifests itself as the sudden failure of lubricating films in mechanical equipment operating under extreme conditions of load and/or speed. Components such as cams, tappets, gears and piston rings are all prone to scuffing failure. Understanding the mechanisms that initiate failure would enable the development of criteria for scuffing prediction. This paper reviews the numerous scuffing models and theories that exist today and, in doing so, defines the important factors involved in scuffing initiation. Emphasis is given to existing theoretical scuffing models which have been correlated by a wealth of research data obtained from experimental test rigs.     

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 temperatures at scuffing and seizure in a four‐ball contact

The temperature of surface asperities affects lubricant‐surface tribochemical interactions. It is important to know the nature of this to identify ways of preventing scuffing and seizure under extreme‐pressure (EP) conditions. A new model for the determination of the temperature of contacting asperities is presented in this paper. It assumes the superposition of thermal processes occurring on the macroscale and thermal phenomena in the contact of asperity tips (microscale). Numerical results have been obtained for conditions of four‐ball testing of various lubricating oils — a mineral base oil with and without antiwear and EP additives. To calculate the scuffing and seizure temperatures, knowledge of the mechanical and physical properties of the test ball material (bearing steel) and lubricants, as well as the parameters describing the surface topography of the balls, was necessary. Friction coefficient curves were also needed; they were determined during four‐ball tests with a continuously increasing load. For the base oil with lubricating additives, the temperature of contacting surface asperities at the moment of scuffing initiation was calculated to be about 230°C and increased to over 1000°C at the highest loading of the four‐ball tribosystem. This suggests the possibility of tribochemical reactions of the lubricating additives with the steel surface, and diffusion of some elements, a modified surface layer having good antiseizure properties being produced. Such a layer prevents seizure of the tribosystem. For the base oil without lubricating additives, scuffing initiated at about 150°C and the temperature exceeded 1200°C at seizure. The temperature values obtained agree with results in the literature.     

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.     

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.     

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.     

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.     

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.     

浅谈发动机拉缸产生的原因及防止措施

简述发动机拉缸的机理，分析造成发动机拉缸的原因，并提出了防止拉缸的措施。     

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.     

Antiseizure properties of aqueous solutions of compounds forming liquid crystalline structures

Amphiphilic compounds composed of a hydrophilic part and a hydrophobic alkyl chain were investigated. The compounds—sodium lauryl sulfate (SLS) and ethoxylated sodium lauryl sulfate (ESLS)—exhibit strong affinity for solid surfaces and form liquid crystalline structures in water. It is expected that they may become effective additives that could significantly modify antiseizure properties of water. They may extend the possibilities for applying water as an ecological base for lubricating substances. Rheological and structural X-ray studies as well as measurements of conductance and wetting angle were carried out. Their aim was to verify and validate the structures being formed in aqueous solutions of the compounds studied. Tribological studies were conducted to measure friction torque as a function of linearly increasing load by means of a four-ball apparatus. The tests were followed by measurement of the wear scar diameter on the balls. Based on the results obtained, antiwear properties were characterized by seizure and scuffing loads as well as by limiting pressure of seizure. The test results are highly surprising. The systems in which simple one-component aqueous solutions were the lubricating substance did not undergo seizure up to the maximum load of 8 kN designed for the tribological tester used. A number of typical antiwear additives in suitable oil bases do not exhibit such good characteristics. Also, the quantities characterizing seizure show relatively high values. Local maxima for the two compounds tested are formed in the area of low concentrations in the dependences of antiseizure properties as a function of concentration. It can be assumed that this is due to ordered structures being formed in the surface phase. The hexagonal phase of high viscosity of the order of several thousand Pa·s which forms in the bulk phase for ESLS and SLS solutions has no visible effect on antiseizure properties, whereas the existence of the lamellar phase for a 70% solution of ESLS affects an increase in the values being measured.     

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.     

Transient thermo‐elastohydrodynamic lubrication of gear teeth

During a gear mesh cycle, load, rolling and sliding velocities, curvature, and temperature change rapidly. In this paper, a transient, thermo‐elastohydrodynamic lubrication model is presented that has been used to study the lubrication parameters at 250 contact points along the path of a contact. The working flanks were assumed to be smooth. The line load was calculated at every contact point, using the finite‐element method. Dynamic loading was not considered. The steady‐state temperature field of the working flanks was used to obtain the surface temperature at the inlet of each contact point, determined by an experimental‐analytical method presented previously. In this model, the lubricant is considered to follow the Ree‐Eyring constitutive equation. The influences of both pressure and temperature on density, thermal conductivity, and specific heat of the lubricant are also taken into account. Results are presented for an FZG type A gear pair lubricated with an FVA No. 4 reference oil; these include film shape, pressure and temperature distributions, as well as friction coefficient.     

Influence of low friction coatings on the scuffing load capacity and efficiency of gears

The influence of multilayer composite surface coatings on gear scuffing load carrying capacity, gear friction coefficient and gearbox efficiency is discussed in this work.The deposition procedures of molybdenum disulphide/titanium (MoS₂/Ti) and carbon/chromium (C/Cr) composite coatings are described.Tests reported in the literature, such as Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear, confirm the excellent adhesion to the substrate and the tribological performance of these coatings, suggesting they can be applied with success in heavy loaded rolling–sliding contacts, such as those found in gears.FZG gear scuffing tests were performed in order to evaluate the coatings anti-scuffing performance, which both improved very significantly in comparison to uncoated gears. These results in conjunction with the friction power intensity (FPI) scuffing criterion allowed the determination of a friction coefficient factor X_SC to include the coating influence on the friction coefficient expression.The composite coatings were also applied to the gears of a transfer gearbox and its efficiency was measured and compared at different input speeds and torques with the uncoated carburized steel gears. Significant efficiency improvement was found with the MoS₂/Ti coating.     

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.     

Tribological Properties of AW/EP Additives Under Different Thermal Conditions

The authors have investigated the action of compositions containing AW/EP additives under scuffing conditions and the influence of temperature on the wear of friction elements lubricated with selected compositions. It has been stated that postponing of scuffing initiation is not always sufficient to obtain very good strength of the surface layer and decreased wear. Furthermore, the results show the influence of temperature and load on the additive action. Extreme friction conditions result in greater wear intensity and change in the lubricating action of additives.     

Additive efficiency and micellar structure of lubricants

This paper reports on the influence of the micellar structure of additives, for example antioxidant additives, on their performance in lubricants. It is well known that hydrocarbon lubricants have a complex liquid structure depending on the composition of the base oils and the additive packages used. The authors have elsewhere suggested a micellar inhibition mechanism that functions in cases where lubricant compositions comprise both antioxidant and detergent additives. A similar situation is created in strongly degraded lubricants when the micellar‐like structure of the hydrocarbons is due to the interaction of oxidation and degradation products. The mechanism of reverse micelle formation and its influence on the oxidation mechanism and oxidation inhibition activity of different additive types are discussed. The influence of the hydrocarbon chain length of the additives on their activity is also discussed.     

Sulfate based coatings for use as high temperature lubricants

Films of CaSO₄, BaSO₄, and SrSO₄ were grown with the pulsed laser deposition technique. The high temperature lubricious properties of these films were evaluated and it was found that all three sulfates exhibited a low coefficient of friction (μ ≈ 0.15) at 600°C. X-ray diffraction studies showed the presence of carbonate crystal structures along with the sulfate crystal structures after testing. The carbonate crystal structure is composed of alternating planes of alkali earth atoms and carbonate ions. The layered structure of the carbonates may be important to the lubricating mechanism as the layered structure of the dichalcogenides contributes to their tribological properties. Various observations of the wear region suggest that the starting surface finish of the metal coupon plays a significant role in the tribological mechanism of these materials. Additional composite films containing CaSO₄ were grown with the plasma spray technique. The high temperature friction properties of these films were measured and compared to those of similar films containing CaF₂/BaF₂. Although both types of films exhibited low coefficients of friction at 600°C, analysis of the wear regions did not show the presence of CaSO₄ and CaF₂/BaF₂, respectively. Ca and/or Ba were present, though, suggesting that a new compound is formed in the wear track and that the sulfate and fluoride phases may not be providing the low friction in these composites. This revised version was published online in July 2006 with corrections to the Cover Date.