Understanding micropitting in gears

Micropitting is a serious form of erosive wear, which can occur on the teeth of transmission gears. It is associated with roughness effects and surface fatigue and has become a particular problem in the speed-increasing gearboxes of wind turbines. This paper reviews the contributions which the authors have made towards an understanding of the basic mechanism of micropitting in gears based on analysis of the contact mechanics and elastohydrodynamic lubrication (EHL) of gear tooth surfaces under realistic operating conditions. Results are presented which demonstrate the crucial influence of EHL film thickness in relation to roughness (the ‘Λ ratio’) on predicted contact and near-surface fatigue. The important effect of plastic deformation, which takes place during the initial stage of running-in of gears, has also been investigated, and it has been shown that significant residual effects occur, which can contribute to the early formation of surface-initiated cracks

Comparison of fatigue model results for rough surface elastohydrodynamic lubrication

The paper compares the results of applying different fatigue failure models to surfaces that experience elastohydrodynamic lubrication (EHL) and have surface roughness features which are large compared with the equivalent smooth surface film thickness. The surface profiles used for the comparisons reported are taken from test gear surfaces used in an FZG gear test. The surface has a roughness of 0.31 μm Ra, with peak to valley dimensions of the order 1.5 μm. The tips of the asperities have been modified by running-in during the gear test. A transient EHL analysis was carried out using contacting components having this profile for a number of sliding speeds and nominal viscosity values. From these analyses, time-dependent pressure and shear stress distributions at the contacting surfaces were calculated. The resulting subsurface stress field was obtained relative to axes fixed in the moving surfaces using an elastic analysis so as to give the stress history at each point in a representative test section of the contacting component. A number of multi-axial fatigue criteria based on a critical plane approach were applied to the test section and the results compared. In addition, a varying amplitude multi-axial fatigue theory based on shear strain cycles was also applied to the section. The cycle counts were obtained using the rainflow counting method and the accumulated damage in a single pass through the contact area was calculated. In comparing the results obtained, the various fatigue models were found to identify the same asperity features as being those most prone to fatigue

Contact and elastohydrodynamic analysis of worm gears, Part 1: Theoretical formulation

The paper presents the theoretical basis for modelling the contact conditions and elastohydrodynamic lubrication (EHL) of worm gears, the results of which are presented in Part 2. The asymmetric elongated contact that typifies worm gears is non-Hertzian and is treated using a novel three-dimensional elastic contact simulation technique. The kinematic conditions at the EHL contact are such that the surfaces have a slide-roll ratio equal to almost 2, and the sliding direction varies over the contact area. These considerations require a non-Newtonian, thermal analysis, and the appropriate form of a novel Reynolds equation is developed that can incorporate any form of the non-Newtonian relationship between shear stress and strain rate. A form that incorporates both limiting shear stress and Eyring shear thinning is utilized in which the two effects can be included both singly or together

Surface fatigue lives of case-carburized gears with an improved surface finish

Previous research provides qualitative evidence that an improved surface finish can increase the surface fatigue lives of gears. To quantify the influence of surface roughness on life, a set of AISI 9310 steel gears was provided with a near-mirror finish by superfinishing. The effects of the superfinishing on the quality of the gear tooth surfaces were determined using data from metrology, profilometry, and interferometric microscope inspections. The superfinishing reduced the roughness average by about a factor of 5. The superfinished gears were subjected to surface fatigue testing at 1.71 GPa (248-ksi) Hertz contact stress, and the data were compared with the NASA Glenn gear fatigue data base. The lives of gears with superfinished teeth were about four times greater compared with the lives of gears with ground teeth but with otherwise similar quality.</jats:p