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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。
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。
Tolerances may be either unilateral or bilateral. Unilateral tolerance means that any variation is made in only one direction from the nominal or basic dimension. Referring to the previous example, the hole is dimensioned 1.500+0.003, which represents a unilateral tolerance.
公差可以是单向的也可以是双向的。单向公差意味着任何变动都是只从名义或基本尺寸出发向一个方向变动的。引用前例,孔的尺寸标注为1.500+0.003,它表示了一个单向公差。
If the dimensions were given as 1.500±0.003, the tolerance would be bilateral; that is, it would vary both over and under the nominal dimension. The unilateral system permits changing the tolerance while still retaining the same allowance or type of fit.
如果尺寸标为1.500±0.003,就是双向公差即它可以在名义尺寸之上或之下变化。单向体系允许在依然保留相同允差或配合类型的情况下改变公差。
With the bilateral system, this is not possible without also changing the nominal size dimension of one or both of the two mating parts. In mass production, where mating parts must be interchangeable, unilateral tolerances are customary. To have an interference or force fit between mating parts, the tolerances must be such as to create a zero or negative allowance.
而双向体系在不同时改变一个或两个配合零件名义尺寸的情况下,这是不可能做到的。大规模生产中配合零件必须能互换,单向公差是经常遇到的。为了使配合零件之间具有过盈或强制配合,公差必须产生零或负允差。
