Investigations into the pitting behaviour of different gear lubricants by using a rolling four ball test configuration

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.Godkänd; 2011; 20111212 (jenjoh