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Study of parameters influencing surface distress of gears

Alanou, M.P. 2006. Study of parameters influencing surface distress of gears. PhD Thesis, Cardiff University.

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This thesis aims to provide a better understanding of the influence of various parameters on surface distress in gears. The types of gears of concern in this study are the ones found in gas turbine engines and helicopter gearboxes where tooth contacts are particularly heavily loaded. The work is concerned with the influence of surface roughness, gear material and hard coatings and their effect in reducing the occurrence of scuffing, which is a form of surface distress that can occur under severe conditions of sliding and high temperature. An extensive experimental programme has been carried out using a two-disc machine developed to simulate gas turbine gearing and micro-elastohydrodynamic lubrication (micro-EHL) contacts under realistic conditions of load, speed, surface finish and temperature. The project involved a large number of disc tests with the aim of quantifying the effects of: a significant improvement in surface finish the use of a different gear material and surface hardening and the use of different hard coatings on the resistance to scuffing. This work was conducted using an existing disc machine to extend the work carried out by Patching (1994) who used a single material. The scuffing performance of axially ground (roughness average, Ra = 0.4 urn) case-carburised steel discs was used as a benchmark against which the performance of alternative materials/surfaces could be measured. Different materials and surface conditions were investigated as follows. The ground discs were superfinished to better than Ra = 0.1 um using a proprietary polishing method. A nitriding steel was used. Two different super-hard coatings were investigated. The main conclusions to be drawn from the work are: the scuffing resistance of nitrided surfaces is superior to case-carburised surfaces, but it is essential to remove the compound layer ('white layer') to achieve durability at high sliding speeds. The benefits of superfmishing are clearly demonstrated better finish improves the scuffing performance and also gives lower friction and bulk temperatures during operation. Hard coatings show promise, and the triple combination of nitriding, superfmishing and hard coatings gave particularly impressive scuffing resistance in this investigation. A micro-EHL solver was used to simulate the conditions of the various tests. The solver, which was developed at Cardiff by Professor H P Evans and his associates, can be used to simulate the lubrication of rough surfaces in contact under realistic thin film conditions in which there is significant asperity interaction including momentary 'dry' contact. Surface profiles acquired from the discs used in the tests were used as input to the solver. Using the solver a number of objective performance parameters were obtained including high pressure cycling and occurrences of thin or zero film within the nominal contact between the discs. The results of the simulations confirmed the distinct advantages of using smoother surfaces and contributed to an understanding of the way in which hard coatings can lead to surface improvement. In addition to work on scuffing the project included the design and manufacture of a test rig, complete with data acquisition and control system, for research on contact fatigue and micropitting in particular.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
ISBN: 9781303204791
Funders: U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio
Date of First Compliant Deposit: 30 March 2016
Last Modified: 09 Jan 2018 17:55

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