Evaluation of Gear Oils Lubrication Performance in a Rolling/Sliding Contact

Abstract

Lubrication in gears is a complex process affected by a wide range of variables. Today's design demands enhanced performance, which causes a more challenging operational environment for machine elements. Failures of machine elements may be minimized via design modification or the use of a superior lubricant. Lubrication may be enhanced by using a higher-quality base oil to reduce friction and temperature, and/or by introducing better additives that protect the surface from failures such as wear and scuffing. There exist standards that provide test methods for measuring the load-carrying capacity of lubricants, but these tests do not provide fundamental information on what occurs at a gear tooth in different locations. While real-world component tests such as the FZG are critical for assessing a lubricant's performance, they do not offer information on the lubrication conditions in various contact locations. In addition, these tests are costly and time-consuming. Alternatively, laboratory tests such as the ball-on-disc test may be utilized for initial screening to provide scientifically evaluated results that can be applied to real components. There are still many engineering problems in the field of tribology which are usually dealt with through estimation methods which do not have a solid theoretical background. Therefore, it is critical to provide a scientific and fundamental understanding of such engineering problems in the field.

On the other hand, as a consequence of environmental concerns, lubricant compositions are changing. Reduced oil reserves, increasing oil prices, and especially environmental regulations have all contributed to an increasing interest in Environmentally Accepted Lubricants (EALs). These oils were first introduced to the market in the 1970s, and manufacturers are continuously enhancing their tribological performance via the formulation of new base oils and additives. By examining the performance of these oils, engineers can gain a better understanding of the risks and advantages linked with their use in machine components.

The main objectives of this thesis are to study the lubrication in a rolling/sliding contact simulating the gear contact along the line of action. The lubrication factors that are targeted include friction, temperature, tribofilm formation, and its influence on scuffing and lubrication regimes. Friction measurements were made using a mini-traction machine that simulated the gear contact by a rolling/sliding ball-on-disc test. Friction was measured in a wide range of slide-to-roll ratios (SRRs), entrainment speeds, and contact pressures. Then, a methodology was used to generate temperature maps according to the Archard model.

To study the tribofilm evolution in an experimentally simulated gear contact, the Spacer Layer Imaging Method (SLIM) was employed in a ball-on-disc test. The tribofilm was examined in different working conditions, mimicking the locations along the line of action. In another experiment, the SLIM technique was used together with the Electrical Contact Resistance (ECR) technique to investigate the tribofilm evolution and its influence on metal-metal contact. In the scuffing test, the ball is replaced with a barrel specimen capable of generating contact pressures of up to 3 GPa. The contra-rotating technique was used to determine the scuffing capacity. A novel scuffing testing strategy was developed that is a combination of sliding speed steps and load steps. The SLIM method was used to monitor the tribofilm during these scuffing stages.

A comparison was carried out between commercial EALs and mineral oil regarding the EHL friction and contact temperature. The results indicate that EALs decreased friction in the EHL regime by up to 60%, and temperature by up to 20 °C depending on the SRR, entrainment speed, and pressure.

Regarding the development of the tribofilm and its impact on metal-metal contact, it was found that a very thick tribofilm may have a detrimental effect on the EHL film by blocking the inlet. This may result in an increased amount of metal-metal contact. Thus, when there is no risk of excessive wear or scuffing, a thin tribofilm is recommended. Additionally, it was shown that the rate of tribofilm formation in a simulated gear contact is strongly dependent on the pressure along the line of action. Observations indicated that there is a tribofilm threshold pressure at which the growth rate of the tribofilm is maximum. This threshold pressure is linked to the pressure in the asperity level and is very sensitive to the roughness of the surface.

Regarding the scuffing experiment, a new test method was devised that is able to accurately differentiate the scuffing performance of similar industrial lubricants. The test parameters were carefully selected to distinguish between the scuffing properties of the industrial oils with high accuracy. The running-in and test conditions were critical for obtaining repeatability and avoiding excessive wear. Tribofilm images shed light on the evolution of the surface during the scuffing test, emphasizing the contribution of the tribofilm in the micro-scuffing healing process. Among the tested industrial gear oils, EALs showed a higher scuffing capacity compared to mineral gear oils.