英语作业

Turbine Engine Bearings for Ultra-High Temperatures The advanced gas turbine engines which will be used to power aircraft in the early part of the next century are already at the design stage .these engines will be extremely efficient and in many instances will produce aircraft speeds above mach 3.The operating conditions for the main shaft bearing of these engines can be considered as extremely demanding .Shaft speeds in excess of 30,000r/min and bearing temperatures greater than 650℃ are predicted.

In applications where relatively lives are required, the present temperature limit for liquid-lubricated steel bearings is 200℃ .For short-life bearing applications it is possible to operate up to 450℃.Even using the most technologically advanced liquid lubricants and metallic alloys ,the temperature capability of the bearings operating in very limited life application is 500℃

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New thinking is required which crosses the boundaries of conventional bearing design and exploits the latest developments of research into high temperature materials and solid lubricants. The predicted extreme operating temperatures(800℃to 900℃) appear impossible to attain ,given present temperature limitations. Ceramic bearings offer the hope of increasing operating temperature considerably 650℃, but the tribological reaction taking place between the components of a high speed ,high temperature ceramic rolling bearing are complex and varied .The selection of effective bearing and lubricant materials depends on their thermal ,physical chemical and mechanical properties ,as well as the operational and engineering constraints of the application.

The most important criteria for evaluating materials to be used for balls and rings of high temperature bearing are: high temperature strength (hardness), thermo-mechanical properties and oxidation characteristics. Tool steels ,such as VIM-V AR M50,are currently the most common materials used for aeroengine bearings with a practical temperature limit of approximately 400℃.At such temperature normal bearing steels rapidly lose their hardness .High cobalt-based stellites and high tungsten alloy tool steels ,not normally considered bearing steels ,have been used at high temperature ,but they begin to lose hardness at temperature around 500℃.

A group materials which appears promising for ultra-high temperature bearing operation is high performance ceramics .At temperature above 1,100℃these ceramic materials have a high hardness than normal bearing steels .For the past decade one ceramic material has been developed to the virtual exclusion of all others for high speed, high temperature rolling bearing :hot pressed or hot isostatically pressed silicon. Silicon nitride is desirable because it has good high temperature strength and