Analysis of helical gear performance under elastohydrodynamic lubrication

In this thesis an elastohydrodynamic lubrication (EHL) solution method has been developed for helical Gears. Helical gears mesh with each other and develop contact areas under load that are approximately elliptical in shape. The contact ellipses have aspect ratios which are large and lubricant entrainment takes place in the rolling / sliding direction which is along the minor axis of the contact ellipse. The contact between helical gear teeth is therefore considered as a point contact EHL problem and the EHL analysis has been developed to include all aspects of the correct gear geometry. This includes the variation in radius of relative curvature at the contact over the meshing cycle, the introduction of tooth tip relief to prevent premature tooth engagement under load, and axial profile relief to prevent edge contact at the face boundaries of the teeth. The EHL solution is first obtained as a quasi-steady state analysis at different positions in the meshing cycle and then developed into a transient analysis for the whole meshing cycle. The software developed has been used to assess the effects of geometrical modifications such as tip relief and axial crowning on the EHL performance of a gear, and different forms of these profile modifications are studied. The analysis shows that the transient squeeze film effect becomes significant when the contact reaches the tip relief zone. Thinning of the film thickness occurs in this region and is associated with high values of pressure which depend on the form of tip relief considered. A transient EHL analysis for helical gears having faceted tooth surfaces has also been developed. Such surface features arise from the manufacturing process and can have a significant effect on the predicted transient EHL behaviour. The EHL results have been found to depend significantly on the facet spacing and thus on the manufacturing process. The important effect of surface roughness is also considered by developing a three dimensional line contact model to include real surface roughness information by considering a finite length of the nominal contact in the transverse direction of the tooth. This model is based on the use of the fast Fourier transform method to provide the repetition of the solution space along the nominal contact line between the helical teeth with the inclusion of cyclic boundary conditions at the transverse boundaries of the solution space. In helical gears the lay of tooth roughness (direction of finishing) is generally inclined to the direction in which rolling (entrainment) and sliding take place, and this is found to have a significant effect on both film thickness and pressure distribution.