EPSRC Portfolio Area: Performance and inspection of mechanical systems and structures (Maintain)

Description:
Micropitting is associated with roughness effects and surface fatigue and is a particular problem in wind turbine gearboxes. Damage begins in the form of small surface pits 10-30m in diameter and rapidly progresses ultimately leading to tooth failure. Heavily loaded gears operate under "mixed lubrication", where surfaces are separated by a hydrodynamic film of lubricant, but the most aggressive surface roughness features (asperities) penetrate the lubricant film to make direct metallic contact. This leads to high cyclic contact stresses, resulting in contact fatigue failure of the surface asperities.

Aim of Project:

The aim of the project will be to thoroughly investigate the link between running-in and micropitting. This will be achieved by developing a method of taking real, measured surface roughness geometry information from run-in surfaces, and comparing this with as-manufactured surface geometry to determine the level of plastic deformation occurring during run-in. Finite element techniques will be developed to assess the resulting level of residual stress. The student will learn to use the mixed-elastohydrodynamic lubrication solvers and will incorporate techniques for evaluation of cyclic stress as part of a fatigue assessment of the gear surfaces.

Furthermore, the student will develop strong experimental skills, via an experimental programme where representative surfaces will be run-in using a power-recirculating disc machine, under typical loads and sliding speeds. These surfaces will then be used in contact fatigue tests until they micropit. In-situ profilometry will be used to track surface roughness features to assess their level of initial plastic deformation and subsequent pitting failure. Using the surface roughness profiles, finite element models will be constructed to evaluate the residual stress fields developed during the running-in process. These results, plus mixed-elastohydrodynamic lubrication simulations to predict cyclic contact stresses, will enable prediction of fatigue life of the surface roughness features.

Novelty/Value of work:
The outcomes of this project could have significant impact on designers and users of power transmission gearing, particularly in the renewable energy and aerospace industries.