On the running-in of gears

University dissertation from Stockholm : KTH

Abstract: The general trend in gear industry, today, is an increased focus on gear transmission efficiency. Gear transmission efficiency losses arise from loaded and unloaded gear contacts, seals, lubricant and bearings. One way of minimising the losses is to lower the lubricant viscosity. This will reduce the speed dependent losses. However, the load dependent losses might increase. To avoid this, the ratio between lubricant film thickness and surface roughness must be maintained, which can be fulfilled by producing smoother gear surfaces. As a starting point for this realisation process, the present manufacturing processes, the design tools and the characteristics of the gear flank interface must be further investigated and developed. This must be achieved with an emphasis on economic production.This thesis focuses on our understanding of how different gear manufacturing methods —particularly the contribution of the running-in process—affect the surface characteristics, with the view of increasing gearbox efficiency. The thesis consists of a summary and three appended papers.Paper A and paper B discuss the relationship between design parameters and real gear wheel surfaces manufactured with different manufacturing methods. The research hypothesis was that the contact area ratio is a descriptive parameter for the contact condition. Paper A deals with the influence of manufacturing method on the initial contact conditions and also serves as a validation of the simulation program used. The emphasis in Paper B is the changes that occur during running-in, and to correlate these changes to design requirements. Paper C approaches the influences of manganese phosphate-coating and lubricants with respect to friction and the risk of scuffing at the initial contact.The main conclusions of this thesis are that the contact area ratio presents a descriptive measure of how surface topography influences the contact, seen at both a global (form deviation) and local (roughness) level. The surface topography caused by the manufacturing method has a significant influence on the contact area ratio. This is an important result, since neither national standards nor commercially available gear evaluation programs handle surface topography on the local scale. Shaving was found to have the highest contact area ratio, and should therefore be the best choice if deviations from case hardening could be minimised. It is also confirmed that gear-like surfaces coated with manganese phosphate have a low coefficient of friction, and raise the limiting load for scuffing failure enormously compared to the ground equivalent.