Unit14 Limits and Tolerances
Dimensioning 标注尺寸 The design of a machine includes many factors other than those of determining the loads and stresses and selecting the proper materials. 机械设计除了计算载荷和应力、选择合适的 材料外，还包括许多其它因素。
Dimensions given close tolerances mean that the part must fit properly with some other part. Both must be given tolerances in keeping with the allowance desired, the manufacturing processes available, and the minimum cost of production and assembly that will maximize profit. 具有紧密公差的尺寸表示该零件必须恰 当地与某些其它零件配合。所采用的制造 工艺和使利润最大化的最小生产及装配成 本都要求给定公差以保持所需允差。
Before construction or manufacture can begin, it is necessary to have complete assembly and detail drawings to convey all necessary information to the shop men. The designer frequently is called upon to check the drawings before they are sent to the shop. Much experience and familiarity with manufacturing processes are needed before one can become conversant with all phases of production drawings. 在建造或制造开始前，完成装配图和零件图以把 必要信息传达给车间工人是必须的。在送往车间 前设计者常常被召集来检查图纸。而在精通生产 图纸的所有情况之前，需要有许多经验并熟悉制 造工艺。
Generally speaking, the cost of a part goes up as the tolerance is decreased. If a part has several or more surfaces to be machined, the cost can be excessive when little deviation is allowed from the nominal size. 一般而言，零件的成本随着公差的减小而 上升。如果一个零件有若干或较多表面要 机加工，且几乎不允许偏离名义尺寸，则 成本会超过正常合理的界限。
A shaft might have a nominal size of 2.5in.(63.5mm), but for practical reasons this figure could not be maintained in manufacturing without great cost. Hence, a certain tolerance would be added and, if a variation of±0.003in.(±0.08mm) could be permitted, the dimension would be stated 2.500±0.003(63.5±0.08mm). 一根轴可能的名义尺寸为2.5in.(63.5mm)，但由 于实际原因不用大成本是不能在制造中保持这个 数字的，因此要增加确定的公差。如果允许有 ±0.003in.(±0.08mm)的变化，则此尺寸可表达 为2.500±0.003(63.5±0.08mm)。
Tolerances must be placed on the dimensions of a drawing to limit the permissible variations in size because it is impossible to manufacture a part exactly to加工到正好为给定尺寸是 不可能的，因此图纸的尺寸必须加上公差 以限制其可允许的变化。
Dimension and Tolerance 尺寸和公差 In dimensioning a drawing, the numbers placed in the dimension lines represent dimension that are only approximate and do not represent any degree of accuracy unless so stated by the designer. 在图纸标注尺寸时，除非设计者有意标 明，注在尺寸线上的数字代表的尺寸仅仅 是近似的，并不代表任何精度等级。
The manner of placing tolerances on drawings depends somewhat on the kind of product or type of manufacturing process. If the tolerance on a dimension is not specifically stated, the drawing should contain a blanket note which gives the value of the tolerance for such dimensions. 在图纸上标注公差的方法相当程度上依赖 于产品的性质或制造工艺的类型。如果尺 寸公差没有特别注明，图纸应该包含一个 给出这些尺寸公差值的普遍适用注释。
However, some companies do not use blanket notes on the supposition that if each dimension is considered individually, wider tolerances than those called for in the note could probably be specified. In any event it is very important that a drawing be free from ambiguities and be subject only to a single interpretation. 然而有些公司不采用普遍适用注释，假定 每个尺寸是单独被考虑的，可能会规定出 比注释中要求的更宽的公差。在任何情况 下图纸不模棱两可并只服从于单一的解释 是十分重要的。
The development of production processes for large-volume manufacture at low cost has been largely dependent upon interchangeability of component parts. Thus the designer must determine both the proper tolerances for the individual parts, and the correct amount of clearance or interference to permit assembly with the mating parts. 大规模低成本制造生产工艺的发展很大 程度取决于组成零件的互换性。因此设计 者必须确定单个零件的合适公差以及配合 零件装配允许的正确间隙或过盈量。
If a shaft, size 1.498-0.003, is to fit a hole of size 1.500+0.003, the minimum size hole is 1.500 and the maximum size shaft is 1.498. Thus the allowance is 0.002 and the maximum clearance is 0.008 as based on the minimum shaft size and maximum hole dimension. 如果一根尺寸为1.498-0.003的轴与尺寸为 1.500+0.003的孔配合，孔的最小尺寸为 1.500而轴的最大尺寸为1.498。这样允差 就是0.002，而由最小轴尺寸和最大孔尺寸 形成的最大间隙为0.008。
Allowance, which is sometimes confused with tolerance, has an altogether different meaning. It is the minimum clearance space intended between mating parts and represents the condition of tightest permissible fit. 允差，有时会跟公差混淆，但其具有完 全不同的含义。它是配合零件之间最小的 预期间隙空间，代表着允许的最紧配合条 件。
Although small tolerances give higher quality work and a better operating mechanism, the cost of manufacture increases rapidly as the tolerances are reduced, as indicated by the typical curve of Fig14.1. It is therefore important that the tolerances be specified at the largest values that the operating or functional considerations permit.