On the influence of gear oil properties on pitting life

Sammanfattning: Increasing demands on the automotive industry to produce fuel-efficient vehicles has led the industry to explore different approaches, including reducing power losses and improving fuel efficiency of engines. The goal of this project has been to investigate the possibilities of reducing the frictional losses in drive axles and hypoid gears. In the literature, different ways of achieving low friction has been reported. However, before major changes can be made to commercial hypoid gear oils, a better understanding of their effect on the durability of transmission components has to be understood. The durability of gears includes many different failure modes. Macro-size contact fatigue (pitting) is one of the more commonly encountered and that ultimately limits the life of the components.In this work, experimental pitting life studies have been conducted using rolling four-ball and twin-disc test setups to analyse the impact of various gear oils’ physical and chemical properties on pitting during operation in the mixed lubrication regime. The results show that the frictional properties of the gear oils are the most significant in determining the pitting life. Enhancements in pitting life can be achieved in several ways, for example, by the choice of the base oil type, high viscosity oil, viscosity modifier type, and the tribofilms formed by anti-wear and extreme-pressure additives. Amongst these, all except high viscosity oils is compatible with the aim of reducing losses (load and load-independent losses). However, especially, the use of low friction type base oils or the low friction tribofilms formed by certain anti-wear and extreme-pressure additives have been found to be effective (with a slight preference for the latter) in improving pitting life. The results of these studies have been contained in five paper manuscripts and a brief gist of the work and salient results in each of these are briefly described below.Paper A: A range of different hypoid gear oils based on different base oils, viscosity levels, and friction modifying additives, were chosen for pitting studies. Each oil was characterized in terms of its physical properties and the pitting performance was analysed using a rolling four-ball test. The correlation between specific oil properties and pitting performance was analysed using multiple linear regression analysis.Paper B: In this, the used ball samples from tests with two of the oils tested in paper A were analysed to investigate the pit formation mechanisms. The worn surfaces and sub-surface materials revealed the differences in the behaviour of the two oils.Paper C. Based on the findings of paper A and B, a second batch of oils was prepared for investigations with a view to obtain low thin-film friction by the using different additives. The oils’ frictional behaviour was characterized in a ball-on-disc test rig. The pitting lives of two of these oils were measured using rolling four-ball tests and compared to two of the oils from paper A.Papers D and E: These two papers focussed on studies performed by using twin-disc rolling/sliding machine. The results confirmed that the trends and conclusions drawn from the rolling four-ball tests were relevant for gear contacts and enabled into optimising the gear oil formulation. Paper D deals with characterising of the frictional behaviour of several gear oils and identifying the two best performing formulations. In paper E, the pitting behaviour of the two oils has been analysed and compared to reference oil.