There are many moving machine assemblies with conformal tribological contacts at very high contact pressures, e.g., sliding bearings, propeller shaft bearings and machine guideways. Furthermore, applications such as trunnion and guide vane bearing in Kaplan turbines have very low sliding speeds and oscillatory types of motion. Although there is a vast selection of tribology test rigs available, there is still a lack of test equipment to perform friction and wear tests under high contact pressure, reciprocatory sliding and large area contact. The aim of this work is thus to develop a novel reciprocating tribometer and test method that enables friction and wear tests under low-speed reciprocatory sliding with contact pressures up to 90 MPa in a flat-on-flat contact configuration. First, a thorough description of the test rig design is given. Secondly, the influence of contact pressure and stroke length on the tribological properties of a stainless steel and polymer composite material combination is studied. The significance of considering creep, friction during the stroke and contact temperature is specifically highlighted. The novel tribometer can be used to screen different bearing and shaft material combinations and to evaluate the friction and wear performance of self-lubricating bearings for the specific operating conditions found in Kaplan turbines. 

In this study, the friction and wear properties of six different new and used wind turbine gear oils (ISO VG 320), with different base oil formulations and additives packages, were investigated. For that purpose, a four-ball tribometer and an Optimol SRV were used. Moreover, the lubricants extreme pressure properties were also evaluated, using the same four-ball tribometer. The study also includes a characterization of the lubricants. The main objective was to compare the new and used gear oils in order to identify performance differences and predict oil change intervals. The results indicate that a use of 3 to 4 years is within the lifetime of the lubricant.

Increasing demands on the automotive industry to produce fuel-efficient vehicles has led the industry to explore different approaches, including reducing power losses and improving fuel efficiency of engines. The goal of this project has been to investigate the possibilities of reducing the frictional losses in drive axles and hypoid gears. In the literature, different ways of achieving low friction has been reported. However, before major changes can be made to commercial hypoid gear oils, a better understanding of their effect on the durability of transmission components has to be understood. The durability of gears includes many different failure modes. Macro-size contact fatigue (pitting) is one of the more commonly encountered and that ultimately limits the life of the components.In this work, experimental pitting life studies have been conducted using rolling four-ball and twin-disc test setups to analyse the impact of various gear oils’ physical and chemical properties on pitting during operation in the mixed lubrication regime. The results show that the frictional properties of the gear oils are the most significant in determining the pitting life. Enhancements in pitting life can be achieved in several ways, for example, by the choice of the base oil type, high viscosity oil, viscosity modifier type, and the tribofilms formed by anti-wear and extreme-pressure additives. Amongst these, all except high viscosity oils is compatible with the aim of reducing losses (load and load-independent losses). However, especially, the use of low friction type base oils or the low friction tribofilms formed by certain anti-wear and extreme-pressure additives have been found to be effective (with a slight preference for the latter) in improving pitting life. The results of these studies have been contained in five paper manuscripts and a brief gist of the work and salient results in each of these are briefly described below.Paper A: A range of different hypoid gear oils based on different base oils, viscosity levels, and friction modifying additives, were chosen for pitting studies. Each oil was characterized in terms of its physical properties and the pitting performance was analysed using a rolling four-ball test. The correlation between specific oil properties and pitting performance was analysed using multiple linear regression analysis.Paper B: In this, the used ball samples from tests with two of the oils tested in paper A were analysed to investigate the pit formation mechanisms. The worn surfaces and sub-surface materials revealed the differences in the behaviour of the two oils.Paper C. Based on the findings of paper A and B, a second batch of oils was prepared for investigations with a view to obtain low thin-film friction by the using different additives. The oils’ frictional behaviour was characterized in a ball-on-disc test rig. The pitting lives of two of these oils were measured using rolling four-ball tests and compared to two of the oils from paper A.Papers D and E: These two papers focussed on studies performed by using twin-disc rolling/sliding machine. The results confirmed that the trends and conclusions drawn from the rolling four-ball tests were relevant for gear contacts and enabled into optimising the gear oil formulation. Paper D deals with characterising of the frictional behaviour of several gear oils and identifying the two best performing formulations. In paper E, the pitting behaviour of the two oils has been analysed and compared to reference oil.

Vehicle manufacturers today face increasing demands to produce fuel efficient vehicles. If this is to be successfully achieved, the reduction of energy losses is vital [1]. One approach to loss reduction is to use lower viscosity trans mission fluids to reduce splash (churning) losses in the drive-train. However, this introduces potential problems in regard to the durability of machine components due to reduction in oil film thickness [2 ]. As regards gear transmission, the durability is mainly related to the formation of micropits on parts of gears where sliding is high, which later lead to pitting damage. The formation of micropits is due to surface stress which can be reduced by fluids that form thick EHD films or reduce sliding friction. In order to determine if these same lubricant properties as well as other parameters, known to influence the fuel efficiency of axle lubricants, also affect contact fatigue damage in rolling contacts, an extensive experimental study using a rolling four ball test was performed. The tests were performed with a series of fluids that form thin and thick EHD films and have low and high sliding friction. Additionally, these fluids have been formulated to hav e high and low hydrodynamic friction. All these fluids contai ned additive packages that meet the API GL-5 gear oil specifications. The different properties of the lubricating fluids were controlled by changes to the base oils as well as addition of friction modifiers. The results have shown that it is possible do distinguish the pitting properties of the different lubricants by using rolling four ball tests. A multiple linear regression statistical analysis was performed with the use of Matlab for evaluating the results obtained from rolling four ball tests. The statistical model developed, showed that some of the physical properties of fluids that a ffect fuel efficiency have an impact on pitting performance of the lubricants. The ball test specimens from rolling four ball tests have been analyzed by using SEM/ EDS and XPS in order to characterize the tribochemical films and understand the damage mechanisms.

There is a connection between the efficiency of oils and their wear and/or surface damage protective properties, an area not so well described in the literature. One such damage mode is macroscale contact fatigue on gear tooth flank surfaces, also called pitting. The present study is aimed at investigating the correlation between gear oils' physical properties, important in terms of gear transmission losses, and pitting life. Eight gear oils were formulated giving different combinations of base oil, viscosity, and concentration of friction modifiers. All eight oils also contained an additive package designed to meet GL-5 specifications. This study consists of three parts. In the first, the oils' physical properties were measured using a set of bench tests. In the second, the pitting lives of the oils were evaluated using rolling four-ball tests. The third part deals with the correlation between the measured physical properties of the oils and their pitting lives. This is achieved through multiple linear regression, with a view to finding the salient properties that have a significant influence on pitting life. The results show that gear oils' physical properties do have a large influence on the pitting lives. Oil properties that lower interfacial tangential stresses are beneficial in enhancing pitting life.

This paper describes an investigation into possibilities of enhancement of pitting lives of rolling components by using additive combinations with low thin-film friction. Various viscosity index improvers, anti-wear and extreme-pressure additive combinations were analysed in terms of their frictional behaviour, which in turn was compared to the oils pitting lives. For the pitting studies, a rolling four-ball test was employed. Friction was measured using a ball on disc machine as well as indirectly through “near contact” temperature measurements performed during rolling four-ball tests. The results show that additive combinations that result in low friction at the specific running condition can enhance pitting performance

Pitting, a form of rolling contact fatigue, is a complex phenomenon and several factors influence its occurrence, particularly under lubricated conditions. In this work, studies have been conducted to observe the events that occur during lubricated rolling four-ball tests that may affect or eventually lead to the formation of pits. This is performed to form an understanding of the pit formation process. Included is tribofilm formation, surface degradation, wear mode, material changes and crack initiation sites. These investigations have been performed on the ball samples from rolling four-ball tests, conducted using two API GL-5 gear oils. The analyses revealed the formation of a low hardness region beneath the surface of the running track due to martensite decay. The formation rate and expansion of this region was found to differ for the two lubricating oils. The pitted balls also indicated that the initial fatigue cracks were initiated at or close to the surface

Pitting due to rolling contact fatigue is a complex phenomenon and several factors influence its occurrence, particularly under lubricated conditions. A deeper understanding of pit formation mechanisms is needed in order to effectively evaluate the pitting behaviour of different lubricants. The present work focuses on investigating the events that lead to pit formation in the lubricated rolling four -ball test by analysing surface degradation, wear mode, material changes and crack initiation sites etc. These investigations have been conducted using two API - GL5 gear oils. These analyses of pitted balls revealed the formation of a low hardness region beneath the surface of the rolling track due to martensite decay. The formation rate and expans ion of this region was found to differ for the two lubricating oils. Examination of the pitted ball has also indicated that the initial rolling contact fatigue cracks were initiated at or close to the surface.

Earlier studies have shown that the load-dependent friction behavior of various gear oils can affect their pitting performance; that is, low friction resulted in a long pitting life. These studies were limited, however, to test methods and running conditions quite different from those occurring in actual gear transmissions. In the present study, a more gear-like twin-disc machine with test specimens and running conditions relevant for gear contacts was used to investigate whether the same trends could be found. To analyze this possible correlation, the first step was to prepare a set of hypoid gear oils and to test their friction performance to compare various ways of improving friction behavior but also to form an understanding of why their friction performance varied. The second step was to test the pitting performance of the oils. The pitting results could then be compared to the friction properties of the oils to analyze the correlation. Other possible mechanisms behind the formation of pits are also discussed. The results show that for the oils included, the antiwear and extreme-pressure additive package and the base oil type affect friction. The results further show that additive combinations and/or base oils that result in low friction lead to enhanced pitting performance.

Polytetrafluoroethylene (PTFE) nanoparticles were coated with consecutive plasma deposited siliceous and methacrylate coatings. Secondary zinc dialkyldithiophosphate (ZDDP), phosphonium cation and phosphate anion ionic liquid (IL), and IL with phosphonium cation and dithiophosphate anion were mixed with the functionalized nanoparticles. Tribological studies were carried out for seven separate formulations including base oil, oils with only additives, and oils with additives and functionalized PTFE particles. Results indicate strong synergistic interactions of ZDDP and ILs with functionalized nanoparticles providing enhanced friction and wear performance. Chemical analysis of the tribofilms using X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy indicates functionalized PTFE nanoparticles interact synergistically with ZDDP and ILs to form silicon- and fluorine-doped tribofilms resulting in superior tribological performance.

One of the most attractive ways to tackle vehicle engine's inefficiencies is the use of Low Viscosity Engine Oils (LVEO). Adopted some decades ago for their use in the Ligh Duty segment, LVEO are now reaching the Heavy Duty segment.

In this study, a comparative fuel consumption test, where a LVEO performance is evaluated on an urban compressed natural gas buses fleet is portrayed. Then the friction performance of the same oils are studied on a Cameron-Plint tribometer, on an adapted twin disc tribometer to simulate journal bearing friction and on a Ball-on-Disc rig, using real engine parts in the former and the same set of engine oils used during the fleet test.

Results show a fuel consumption reduction in the fleet test and corresponding friction reduction in the tribometers when LVEO are used.