HFRR

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

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