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℃ .

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-VAR 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

hardness ,an advantageous strength/weight relationship and excellent resitance to rolling contact fatigue when there is adequate lubrication .Long term operation of this material using solid lubricants at temperatures above 500℃ was demonstrated by SKF in the U.S.A. during 1984.

However, there are drawbacks with silicon nitride, including tensile strength, low fracture toughness and extremely low coefficient of thermal expansion .Because of these properties ,considerable development work is needed for the manufacture and application of ceramic bearings .

Other ceramic materials such as silicon carbide silicon carbide and titanium carbide are being evaluated for their suitability as ball and ring materials .Although not as popular and mature as silicon nitride ,they do have certain popular and mature as silicon nitride ,they do have certain properties which make them candidates for high temperature rolling bearing materials for example, silicon carbide has been used as ball material in 40,000r/min bearing test , and although the temperature was not extreme ,it was above the liquid lubrication rage .The lubrication system consisted solely of a film of solid lubricant.

The positive properties of silicon carbide for use in high temperature bearing are its good thermal conductivity and thermal diffusivity ,its oxidation resistance and high purity (properties are little affected by impurities).One of the negative properties of the material is its high modulus of elasticity. This is about 50﹪ higher than hot pressed silicon nitride and has been cited as a potential problem because of the risk of high Hertzian contact stresses .These can be reduced by adjusting the osculation of the raceway grooves ,but this causes increased frictional heat generation .

NEW WORDS

turbine n. 涡轮机，透平机 hardness n. 硬度 aircraft n . 航空器，飞行器 oxidation n 氧化作用

mach n .马赫 temperature n. 温度 lubrication n .润滑油 aeroengine n. 航空发动机 carbide n . 硬质合金 bearing 轴承

excess n. 超过 ，过量 cobalt n. 钴 satellite n. 硬质合金 tungsten n. 钨 rolling v. 滚动 ceramic n. 陶瓷艺品 demonstrate v. 证明，示范 drawback n. 缺点，障碍 material n. 材料 steel n. 钢 ball n. 滚珠 friction n 摩擦 System n 系统 raceway n. 座圈

PHRASES AND EXPRESSIONS

liquid lubrication 液体润滑

thermo-mechanical properties 热力学性能 oxidation characteristic 氧化性能 bearing steel 轴承钢 hot pressed 热压的 silicon nitride 氮化硅 silicon carbide 碳化硅 titanium carbide 碳化钛 contact fatigue 接触疲劳 fracture toughness 断裂韧度 as well as 除……之外

coefficient of thermal expansion 热膨胀系数

thermal conductivity 热传导性 modulus of elasticity 弹性模量 Hertzian contact stress 赫兹接触应力

高温工况下的涡轮机轴承

要在下一世纪初用飞行器动力的先进的燃气轮机，目前已经处于设计阶段。这些发动机的功率很高，例如一些飞行器的速度将超过3马赫。因此弄清楚这些发动机的主轴轴承工作环境是非常必要的。据预测，传动轴的转速要超过300r/min，轴承温度要超过650℃。

在要求轴承使用寿命相对较长的应用情况中，润滑油润滑的钢轴承工作温度极限为200℃。使用寿命要求相对较短的轴承，其工作温度可达450℃。即使采用最先进的润滑油和金属合金，轴承在很多有限的使用寿命下工作温度也只能达到500℃。

要跨越常规的轴承设计极限需要新的思维并且利用最新研制的高温材料和固体润滑剂。根据目前的极限温度预测，在现有温度限制下，极限操作温度要达到800～900℃是不可能实现的。陶瓷轴承的运转温度可能超过650℃，但是轴承的各部分在高速运转下将会发生摩擦，高温陶瓷旋转轴承需要更加工复杂多变。轴承材料和润滑的选择取决于它们的热性能，物理性能，化学性能，力学性能以及运转的环境和发动机应用的约束情况。

滚珠和座圈的材料:

高温轴承的滚珠和座圈的材料选用的重要标准是材料的热强度，热力学性能和氧化性能。工具钢，比如 VIM-VAR M50，是目前用于飞机发动机轴承的常用材料，它的使用温度极限约为400℃，在这个温度下一般的轴承钢的强度将会迅速降低。高温轴承材料通常不采用高鈷基钨铬钴合金和高钨合金工具钢，因在温度约为500℃时，其硬度开始降低。

有希望用于高温轴承的材料是高性能的陶瓷系列。在温度高于1100℃时陶瓷材料比一般轴承钢有更高的硬度。在过去的十几年里出现了一种代替其他轴承材料的高速高温陶瓷材料：热压瓷和匀压硅。氮化硅是合乎要求的，因为它具有良好的高温强度和硬度，理想的强度/重量关系，并且当润滑充分时，它具有优良的抗滚动接触疲劳的性能。SKF公司于1984年在美国证实这种材料在使用固体润滑剂后可以在超过500℃的极限温度条件下长时间运转。

但是氮化硅也有自身的缺点，包括；低拉伸强度，低断裂韧度以及非常低的热膨胀系数。因此，陶瓷轴承的制造和使用还需要做大量的研发工作。

其他陶瓷材料，比如碳化硅和碳化钛也被认为合适作为座圈和滚珠的材料。尽管这两种材料不如氮化硅成熟和普遍，但它们却有某些特征合适做高温滚动轴承材料。如碳化硅已经被用于40000f/min 的轴承滚珠材料测试，尽管温度不是很高，但是也已经超过了液体润滑油的适用范围。这种材料采用的是固体润滑膜为基础的润滑系统。

碳化硅用于制作高温轴承的优点是具有良好的导热性，热扩散性以及它的抗氧化性和高纯度（性能不易受杂质影响）。该材料的负面特性之一是高弹性模量。它的弹性模量比热压的氮化硅高出50％，另外这种材料还存在一个潜在的问题，就是在高频率接触应力下工作危险。这一点可以通过调整轴承座圈槽的接触来降低，但由此却增加了摩擦产生的热量。