机电技术专业英语 教学课件 作者 徐起贺 张庆良pata a lesson Fourteen.pptVIP

* 机电技术专业英语 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 40.000mm±0.003mm. 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 dim