DEVELOPMENT OF A LEAD-FREE BEARING MATERIAL FOR AEROSPACE APPLICATIONS
J. Thomson, R. Zavadil and M. Sahoo
CANMET-Materials Technology Laboratory, Ottawa, Ontario, Canada A. Dadouche, W. Dmochowski and M. Conlon
Gas Turbine Laboratory/Institute of Aerospace Research/National Research Council, Ottawa, Ontario, Canada Copyright © 2010 American Foundry Society
Abstract
Apart from providing excellent anti-friction and anti-wear properties of bearing alloys, lead is considered an environ- mentally hazardous material. Lead content in bearing bronze varies depending on the application and, in some cases, is fairly low. However, disposal of waste material from fabri- cation processes, disposal of used bearings, and contami- nation of lubricants can cause serious environmental and health concerns. In some countries, regulations prohibit the use of lead in plumbing applications, bearing materials, and electronic equipment.
The main objective of this project is to develop a lead-free Introduction
Recently, environmental issues related to the use of hazardous materials have become important considerations in the design and life-cycle management of aerospace systems. In bearing technology, these issues stem mainly from the presence of lead in bearing alloys which is used to achieve good frictional characteristics. Lead is a basic component of most commonly used bearing materials such as white metals, bronzes, and copper-lead, as well as in the materials for overlays in bimetal bearings. The environmental issues concerning lead content in bearings are related to the fabrication of parts with alloys containing lead. During fabrication, waste material from ma- chining processes and the used lubricants are contaminated with lead and need to be disposed of properly. Also, the dis- posal of used bearings that contain lead and other toxic metals such as nickel and cadmium are a concern.
Babbitts or white metal can be lead or tin based. Lead-based Babbitts may contain up to 80% lead. Tin-based Babbitts contain approximately 0.5% lead. Other additives in Bab- bitts are copper (as much as 6%) and antimony (as much as 8%). These alloys are used for light to medium loads in turbines, compressors and gears. Bearing manufacturers in North America are reluctant to switch to white metals be- cause of the relatively low lead contents in tin-based white metals and the small market for environmentally friendly bearing alloys. There is also concern about bond strength
International Journal of Metalcasting/Winter 10
environmentally friendly copper-based bearing material for high speed/load applications. In this study, bismuth was se- lected to replace lead in alloy C93700, and two different lead-free materials were cast in addition to the standard bearing material. This comparative study is focused on me- chanical properties, microstructure, and tribological char- acterization of the candidate replacements for the lead-con- taining bearing alloys.
Keywords: lead-free copper, bearing alloy, tribological properties, microstructure, mechanical properties, fric- tion force
between the layer of the new bearing material and its steel backing.
Bronze is one of the most popular materials for bearings. All bronzes can be used as bearing materials, but their bearing properties improve when lead is added. Compared with Bab- bitts, some bronzes have superior load-carrying capacity. Lead in bronzes contributes to good frictional characteristics but also to relatively low strength. High-tin alloys with little or no lead have high hardness but poor frictional characteristics. The composition of a no-lead bearing bronze alloy available on the market has 89% copper, 6% tin, 1% zinc and 4% bis- muth (alloy C89320); however none of the major manufac- turers of aerospace bearings offers lead-free bronze bearings. Bronze bushings are produced by machining cast materials and are therefore very economical and reliable. Bronze bear- ings are also widely used in automotive and electrical motors.
A mixture of copper and lead is used for higher loads than those carried by Babbitts. The bearings are made from a strip that consists of a steel backing, a layer of cast or sintered copper-lead powder, and a thin overlay of lead. Such bear- ings have high strength while the overlay provides conform- ability and embedability.
The C89320 alloy is a lead-free bearing material in which bismuth was substituted for lead. Its performance is very similar to that of the leaded material C93200.1
Sahu2 per- 19
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