,机电技术专业英语,2,Lesson Fourteen,Text,Introduction The engineer in industry is constantly faced with that no two machine parts can ever be made exactly the same. He learns that the small variations that occur in repetitive production must be considered in the design so that the tolerances placed on the dimensions will restrict the variations to acceptable limits. The tolerances provide zones in which the outline of finished part must lie. Proper tolerance practice ensures that the finished product functions in its intended manner and operates for its expected life.,Engineering Tolerance,Lesson Fourteen,Component Tolerances Control of dimensions is necessary in order to ensure assembly and interchangeability of components. Tolerances are specified on critical dimensions that affect clearances and interferences fit. One method of specifying tolerances is to state the nominal dimension followed by the permissible variation, so a dimension could be stated as 40.000mm0.003mm. This means that the dimension should be machined so that it is between 39.997mm and 40.003mm1. Where the variation can vary either side of the nominal dimension, the tolerance is called a bilateral tolerance. For a unilateral tolerance, one tolerance is zero, e.g.40+0.0060.000.,Lesson Fourteen,Standard Fits for Holes and Shafts A standard engineering task is to determine tolerances for a cylindrical component, e.g. a shaft, fitting or rotating inside a corresponding cylindrical component or hole. The tightness of fit will depend on the application. For example, a gear located onto a shaft would require a“tight” interference fit, where the diameter of the shaft is actually slightly greater than the inside diameter of the gear hub in order to be able to transmit the desired torque3. Alternatively, the diameter of a journal bearing must be greater than the diameter of the shaft to allow rotation. Given that it is not economically possible to manufacture components to exact dimensions, some variability in sizes of both the shaft and hole dimension must be specified4. However, the range of variability should not be so large that the operation of the assembly is impaired. Rather than having an infinite variety of tolerance dimensions that could be,Lesson Fourteen,specified, national and international standards have been produced defining bands of tolerances, examples of which are listed in Table A.1, e.g. H11/c11. In order to use this information the following list and Figure A.7 give definitions used in conventional tolerance. Usually the holebased system is used, as this result in a reduction in the variety of drill, reamer, broach and gauge tooling required within a company. Size：a number expressing in a particular unit the numerical value of a dimension. Actual size：the size of a part as obtained by measurement. Limits of size：the maximum and minimum sizes permitted for a feature. Maximum limit of size：the greater of the two limits of size. Minimum limit of size：the smaller of the two limits of size. Basic size：the size by reference to which the limits of size are fixed.,Lesson Fourteen,Deviation：the algebraic difference between a size and the corresponding basic size. Actual deviation：the algebraic difference between the actual size and the corresponding basic size. Upper deviation：the algebraic difference between the maximum limit of size and the corresponding basic size. Lower deviation：the algebraic difference between the minimum limit of size and the corresponding basic size. Tolerance：the difference between the maximum limit of size and the minimum limit of size. Shaft：the term used by convention to designate all external features of a part. Hole：the term used by convention to designate all internal features of a part.,Lesson Fourteen,Figure A.7 The Definitions Used in Conventional Tolerance,Lesson Fourteen,New Words and Expressions,Lesson Fourteen,Lesson Fourteen,Notes,1. This means that the dimension should be machined so that it is between 39.997mm and 40.003mm. 这意味着该尺寸必须加工成39.99740.003mm之间。 句中mean 意为“意味着”。实际上，当 mean 为动词时需注意以下两个结构。 mean to do something：计划做某事、打算做某事，主语通常是人。 e.g. I meant to have seen you, but I was too busy at that time. 我原打算去看你的，但是当时实在太忙。 mean doing something：意味着做某事，主语通常是物。 e.g. What he said at the meeting meant his finishing the task given by himself。 他在会上的发言意味着他将独自地完成所分配的任务。 2. When specifying a tolerance for a component, reference can be made to previous drawings or general engineering practice. 在确定零件的公差时，人们可以参考以前的图纸或者一般的工程实践经验。,Lesson Fourteen,句中When specifying a tolerance for a component 相当于 When we are specifying a tolerance for a component, 由于 specify 的逻辑主语就是主句的主语，同时该时间状语的主要谓语动词是 be，按照英语语法规则，可将 we are 省去。 e.g. When walking across the street，I heard someone call me behind. 我在过马路的时候听见后面有人叫我。 3. For example, a gear located onto a shaft would require a “tight” interference fit, where the diameter of the shaft is actually slightly greater than the inside diameter of the gear hub in order to be able to transmit th