Knowledge

Welcome to our knowledge centre. Here you can find a selection of resources and articles on our products and industries we are involved with.

Paper

Ionic Liquids as a Neat Lubricant Applied to Steel–steel Contacts

This paper studies the use of 3 ionic liquids ([(NEMM)MOE][FAP], [BMP][FAP] and [BMP][NTf2]) as neat lubricant within steel–steel contact conditions. …

This paper studies the use of 3 ionic liquids ([(NEMM)MOE][FAP], [BMP][FAP] and [BMP][NTf2]) as neat lubricant within steel–steel contact conditions. Tribological tests (at 40 and 100 °C) were conducted in a HFRR tribometer and hence a complementary study was developed using a MTM tribometer. The wear surface on the discs was measured after the HFRR tests by confocal microscopy and also analyzed by SEM and XPS. The [BMP][NTf2] showed the lowest friction coefficient in the MTM and HFRR tests at 40 °C but at 100 °C its tribological behavior worsened due to its lowest viscosity. Similar results were found for wear behavior. Both antifriction and antiwear results were related to the tribofilms formation from the ECR and XPS measurements.

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Paper

Performance of Friction Modifiers on ZDDP-Generated Surfaces

Most modern engine oils contain a friction modifier additive to reduce boundary friction and thus contribute to improved fuel economy. …

Most modern engine oils contain a friction modifier additive to reduce boundary friction and thus contribute to improved fuel economy. Considerable research work has been carried out to measure the effectiveness of friction modifiers, both in base oil and in fully formulated engine oils. Most of this work has studied the behavior of friction modifiers on rubbing ferrous surfaces. However, engine oils generally also contain the antiwear additive zinc dialkyldithiophosphate (ZDDP) and it is well known that this additive reacts rapidly with rubbed ferrous surfaces to form a thick iron and zinc phosphate-based coating. Thus, to be effective, a friction modifier additive has to form a friction-reducing film not on a ferrous surface but on a zinc phosphate one.
In the current study, the friction-reducing properties of friction modifiers on steel surfaces have been compared with their performance on surfaces having a ZDDP reaction film coating. It has been found that additives that are effective in reducing friction on steel surfaces are not necessarily effective in reducing friction on a phosphate coating and vice versa. The reasons for these observed differences are discussed.

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Paper

An Investigation of Hydraulic Motor Efficiency and Tribological Surface Properties

Hydraulic motor efficiency does not depend upon viscosity alone. Under low-speed, high-torque conditions, hydraulic motors operate in the boundary regime …

Hydraulic motor efficiency does not depend upon viscosity alone. Under low-speed, high-torque conditions, hydraulic motors operate in the boundary regime and, therefore, surface interactions of lubricant additives can affect friction and efficiency. This article presents an investigation of boundary film formation, friction, and surface topography in benchtop tribometers and hydraulic motors. Fluids investigated included those with varied antiwear packages (zinc dialkyldithiophosphate [ZDDP], ashless) and friction modifiers (with and without) and base oil (Group I, Group III). The mechanical efficiencies of geroler, axial piston, bent-axis, and radial piston motors were measured under low-speed, high-torque conditions. The addition of a friction modifier to an ashless hydraulic fluid increased the efficiency of the motors at low speed. Energy-dispersive X-ray spectroscopy (EDX) analysis of motor surfaces after testing revealed the presence of tribochemical films from the hydraulic fluid additives. In benchtop tribometer testing, the friction modifier reduced friction significantly but also increased wear. This could be related to surface competition of the friction modifier and antiwear chemistries, as evidenced by the reduced concentration of phosphorus on the surface. These findings are significant because they provide insights toward the development of fluids that can enhance motor efficiency but also demonstrates the need for a well-balanced additive package so that improved motor efficiency can be achieved without affecting other important properties of the fluid.

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Paper

The Efficiency of a Hypoid Axle – A Thermally Coupled Lubrication Model

The final drive unit in road vehicles, such as medium and heavy trucks, and four-wheel-drive and rear-wheel-drive passenger cars, usually …

The final drive unit in road vehicles, such as medium and heavy trucks, and four-wheel-drive and rear-wheel-drive passenger cars, usually consists of a hypoid or spiral bevel geared transmission and differential, housed in a self-contained, dip-lubricated axle. Such units are subjected to very variable duty—including extreme combinations of speed, gradient, applied torque and external temperature—and are typically cooled by natural and forced convection on the exterior surface. On the other hand, there are appreciable internal power losses due to gear friction and churning and to bearing and seal losses. These losses are highly dependent upon the lubrication regime of the internal components and hence to the thermal behaviour of the entire axle.

In the present paper, we describe a thermally coupled model of axle lubrication. The torque and speed demand is first found from a specified duty (“drive cycle”) which includes terrain as well as speed-versus-time and external temperature data. The evolution of sump oil and component temperatures is followed, and increments of energy loss evaluated in each time-step. Elastohydrodynamic film thickness is determined for the hypoid gear set, using a development of Buckingham's method, and friction losses calculated using a simple oil rheological model based on tribometer (MTM) testing. Churning, seal and bearing (speed-dependent) losses are found using empirical algorithms. Energy losses over complete drive cycles for different lubricants are derived, enabling the relative fuel economy for different oils to be evaluated.

Results show that (i) the bulk temperature rise of the axle is highly dependent on the specified vehicle duty and (ii) the efficiency can be strongly influenced by choices available to the lubricant formulator. Taken together, these findings suggest that specialist axle lubricant formulations for particular vehicle types and applications will be attractive as a route to optimum fuel economy.

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Paper

Gear Oil Additive

Gear oil formulations comprising a gear oil and a film forming agent are disclosed. The film forming agent comprises a …

Gear oil formulations comprising a gear oil and a film forming agent are disclosed. The film forming agent comprises a polymeric ester which is the reaction product of at least one polyfunctional alcohol, a dimer fatty acid, an optional aliphatic dicarboxylic acid having 5 to 18 carbon atoms and one or more ingredients to reduce the acid value of the polymeric ester to below 5 mgKOH/g with the resultant polymeric ester having a kinematic viscosity at 100° C. ranging from 400 to 5000 mm2/s and a weight average molecular weight ranging from 5000 to 20000. When used as an automotive gear oil formulation the specifications for API GL-4 gear oils are at least satisfied. Use of the gear oil formulation in manual transmissions, transfer cases and differentials and use of the gear oil formulation in an industrial gear suitable for lubricating spur, helical, bevel, worm and hypoid gears are disclosed. Methods of lubrication are also disclosed.

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Paper

Effects of Gear Oil Properties on Pitting Life in Rolling Four-Ball Test Configuration

There is a connection between the efficiency of oils and their wear and/or surface damage protective properties, an area not …

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.

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Paper

The Rolling Contact Fatigue Behaviour of Motorcycle Lubricants

Rolling contact fatigue is a particular type of fatigue that occurs in heavily loaded, non-conformal contacts, such as gears and …

Rolling contact fatigue is a particular type of fatigue that occurs in heavily loaded, non-conformal contacts, such as gears and rolling element bearings. It is primarily a failure mode associated with repeated cyclic loading that generates high local Hertzian pressures, leading to local plastic deformation and substantial surface or sub surface stress. This in turn leads to crack formation and propagation. In some instances this results in sudden and often critical mechanical failure of contacting parts. This failure mode can, to a certain degree, be controlled by the appropriate choice of lubricant; in terms of both the physical and chemical properties of the films formed at the surface. A three contact disc machine has been used to examine the rolling contact fatigue of motorcycle lubricants in such heavily loaded contacts. Three counterface test rings of equal diameter (54mm) are mounted 120° apart with a smaller (12mm diameter) test roller in the centre. Using this configuration, a large number of contact cycles are possible in a short period of time (up to one million per hour), which greatly accelerates the testing test. Using appropriate test conditions, the rig is able to replicate the tribological conditions typically found in both gears and bearings, giving the macropitting failure mode prevalent in such components. The magnitude of the applied stress may well be below the yield strength of the material, but it is the repetitive application of load which is sufficient to cause fatigue failure. By its very nature, any form of fatigue is a statistically random process and any finite collection of similar specimens, run under a fixed set of conditions (load, speed and lubricant) will belong to a population defined by the manner in which the fatigue lives of individual members are distributed. The results are presented in the form of a single mode Weibull distribution. In this paper it is shown that lubricants within the same viscosity grade can give statistically significant differences in rolling contact fatigue resistance. This is shown to depend primarily on lubricant composition (additive/base oil combination), which give quite different friction performance, particularly in the mixed lubrication regime. The paper will conclude with a discussion of the statistical significance of the fatigue results and its applicability to real engines.

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Paper

Comparison of Frictional Properties of Gear Oils in Boundary and Mixed Lubricated Rolling–sliding and Pure Sliding Contacts

The friction responses of five fully formulated gear oils including mineral and synthetic oils were studied. This article examines the …

The friction responses of five fully formulated gear oils including mineral and synthetic oils were studied. This article examines the impact of contact motion types (rolling–sliding and pure sliding) and contact pressure on boundary and mixed friction properties of the selected gear oils in MTM (minitraction machine) and SRV (Schwing-Reib-Verschleiss tribometer). Mineral oils are found to be less affected by contact pressure compared to synthetic oils. Gear oils that show adsorption appear to be less sensitive to contact motion type in mixed lubrication while behave much more sensitive in boundary lubrication regimes. The ranking of gear oils for mixed friction was similar regardless of contact motion types at low contact pressures while differ at high contact pressures.

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Paper

Rolling Contact Fatigue of Bearing Components Coated with Carbon Nitride Thin Films

Bearing rollers were coated with CNx films using high power impulse magnetron sputtering deposition in order to reduce their rolling-contact …

Bearing rollers were coated with CNx films using high power impulse magnetron sputtering deposition in order to reduce their rolling-contact fatigue as investigated using a Micro-Pitting Rig tribometer under poly-alpha-olefin lubricated conditions. Coated rollers with a ~15 nm thick W adhesion layer to the substrate, exhibit the best performance, presenting mild wear and no fatigue after 700 kcycles. The steady-state friction coefficient was ~0.05 for both uncoated and coated rollers. Uncoated rollers show run-in friction in the first 50 kcycles, because of steel-to-steel contact, which is absent for coated rollers. Analytical transmission electron microscopy and X-ray photoelectron spectroscopy show that the presence of a CNx coating prevents steel-to-steel contact of the counterparts, prior to the elastohydrodynamic lubrication, reducing their wear and increasing the lifetime expectancy.

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