Knowledge

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.

Keywords: Boundary Lubrication, ZDDP, Friction Modifying Additives, Friction, Molybdenum
DOI Link: https://doi.org/10.1080/10402000701413505

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.

Keywords: Boundary Lubrication, Friction Modifying Additives, Hydraulic Motor Efficiency

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.

Keywords: Hypoid, Friction, Elastohydrodynamic, Lubrication

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.

Keywords: Rolling–Contact Fatigue, Rolling Four-Ball, Gear Lubricants, Traction, Viscosity, Heat Capacity, Thermal Conductivity, Rheology

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.

Keywords: Fatigue, tribology, bearings, failure modes and effects analysis

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 (mini traction 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.

Keywords: Gear oils, Friction, Rolling–sliding, Pure sliding

Bearing rollers were coated with CN_x 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 CN_x coating prevents steel-to-steel contact of the counterparts, prior to the elastohydrodynamic lubrication, reducing their wear and increasing the lifetime expectancy.

Keywords: Coating, Pitting, Rolling, Lubrication

The growing demands for renewable energy production have recently resulted in a significant increase in wind plant installation. Field data from these plants show that wind turbines suffer from costly repair, maintenance and high failure rates. Often times the reliability issues are linked with tribological components used in wind turbine drivetrains. The primary failure modes in bearings and gears are associated with micropitting, wear, brinelling, scuffing, smearing and macropitting all of which occur at or near the surface. Accordingly, a variety of surface engineering approaches are currently being considered to alter the near surface properties of such bearings and gears to prevent these tribological failures. In the present work, we have evaluated the tribological performance of compliant highly hydrogenated diamond like carbon coating developed at Argonne National Laboratory, under mixed rolling/sliding contact conditions for wind turbine drivetrain components. The coating was deposited on AISI 52100 steel specimens using a magnetron sputter deposition system. The experiments were performed on a PCS Micro-Pitting-Rig (MPR) with four material pairs at 1.79 GPa contact stress, 40% slide to roll ratio and in polyalphaolefin (PAO4) basestock oil (to ensure extreme boundary conditions). The post-test analysis was performed using optical microscopy, surface profilometry, and Raman spectroscopy. The results obtained show a potential for these coatings in sliding/rolling contact applications as no failures were observed with coated specimens even after 100 million cycles compared to uncoated pair in which they failed after 32 million cycles, under the given test conditions.

Keywords: Fatigue, Micropitting, Coating, DLC

In this study, the capabilities of a WC/aC:H coating to affect the onset of micropitting in SAE 52100 bearing steel is examined. Experiments are conducted in which two pairs of contacts (steel on steel, and steel on WC/a-C:H) and three sliding-to-rolling ratios of -10%, 0% and +10% are tested at high contact stress and in a low h regime. Results reveal that the WC/a-C:H coating can dramatically increase the micropitting resistance of the coated part, achieving a much higher number of contact cycles and higher levels of contact stress than the steel-on-steel contact.

Keywords: Micropitting, Diamond like carbon, Abrasive wear, Bearing steel