Sustainable production of quality bronze alloy bearings in under-resourced small scale casting facilities

Lungu, Chozi Vincent
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There are good reasons to keep the local economy of the Copper belt in Zambia growing. Mining consumables and spares that can be manufactured locally such as bronze sleeve bearings is one way of doing so and can benefit local people a great deal. Bronze sleeve bearings of a fit for purpose quality can to a large extent be cast in a small under-resourced foundry (SURF) sustainably. Sustainability implies survival requiring both consistency of output and cost effective selection and use of secondary materials and aiming for products suited to the market. There is therefore a combination of both technical and economic or business questions. The research examined challenges encountered in a typical SURF and sought to offer a solution for cost effectively producing near net size finished bronze sleeve bearings of a composition that achieves the required mechanical properties to replace directly machined varieties. It was aimed therefore to produce fit for purpose sleeve bearing that represented value for money and were cost effective. Within this thesis focus was on one selected candidate bronze foundry, Fox Foundry, situated in Kitwe on the Copper belt of Zambia. To establish the fit for purpose standard, the methodology employed sampling of used bronze bearings considered successful in service from producers and various users. These were tested for chemical, mechanical and metallurgical properties. Experimental castings were made using a permanent mould at Heroes Foundry and the University of Johannesburg. Factors examined for the experimental castings were; temperature, composition, feeding, pouring and mass. A matrix design approach was also employed to optimise on the number of experimental runs required as opposed to using conventional factorial method. The resulting microstructure from each experimental cast was the response variable determined. Test results of the bronze bearings considered successful in service (or OEM) showed irregular microstructure, inconsistent mechanical properties and unacceptable porosity levels. Chemical composition results indicated no control on levels of alloying elements and that had an impact on microstructure and properties dependent on it. Results from experimental castings on the other hand showed that a consistent and optimal microstructure was key in producing a successful bearing as it was the basis of all secondary properties. It can be concluded that a SURF such as Fox Foundry of Kitwe can successfully produce near net sleeve bearing requiring little or no machining providing basic elements of quality in materials and processes are adhered to during production. Whilst SURFs have a low fixed capital base the costs of energy use and metal procurement are an important contributor to competitiveness and should be monitored carefully.. A training programme developed for a typical SURF which when implemented can complement the important measures of material and process quality control and energy conservation to ensure a first time casting fit for purpose ivThe work undertaken in here is dedicated to enterprises and individuals serving in the foundry sector particularly in Zambia contributing tirelessly to improving local economic conditions where they are based.
A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020