【正文】 對有基本尺寸為 45mm 孔和軸的配合就可以寫成 45H8/g7。 在世界標(biāo)準(zhǔn)公差系統(tǒng)，每個(gè)零件有一個(gè)基本 尺寸。 公差在不同尺寸上不同的是為了允許在制造上不可避免的缺陷。這樣的尺寸不標(biāo)公差。在大批量生產(chǎn)中配合必須滿足互換性，單邊公差是常用的。如果一個(gè)軸尺寸為 ?? ,與它配合的孔的尺寸 ?? ,最小孔為 和最大軸的尺寸為 。 英尺的變化被允許，尺寸將標(biāo)注為 177。如果尺寸公差沒有特別注明，圖樣必須要一個(gè)給出這些尺寸的公差值的綜合注釋。 公差必須標(biāo)注在圖紙尺寸的后面以便限制在尺寸方面的變動(dòng)偏差，因?yàn)椴豢赡苤酪粋€(gè)在尺寸方面非常精確的零件。 尺寸標(biāo)注 一個(gè)機(jī)構(gòu)的設(shè)計(jì)除了要考慮載荷硬力和選擇合適的條件外還包括許多因素。 （二）表面質(zhì)量對疲勞強(qiáng)度的影響 金屬手交變載荷作用后產(chǎn)生的疲勞破壞往往發(fā)生在零件表面和冷硬層下面，因此零件表面質(zhì)量對 疲勞強(qiáng)度影響很大。 機(jī)械加工表面質(zhì)量對機(jī)器使用性能的影響 （一）表面質(zhì)量對耐磨性的影響 一個(gè)剛加工好的摩擦副的兩個(gè)接觸表面之間，最初階段只在表面粗糙度的峰部接觸，實(shí)際接觸面積遠(yuǎn)小于理論接觸面積，在相互接觸的峰部有非常大的單位應(yīng)力，使實(shí)際接觸表面積處產(chǎn)生塑性變形、彈性變形和峰部之間的剪切破壞，引起嚴(yán)重磨損。 in.(177。 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. 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 fore fit between mating parts, the tolerances must be such as to create a zero or negative allowance. Tolerances Limits and Fits The drawing must be a true and plete statement of the designer’s expressed in such a way that the part is convenient to manufacture. Every dimension necessary to define the product must be stated once and repeated in different views. Dimensions relating to one particular feature, such as the position and size of hole, where possible, appear on the same view. There should be no more dimensions than are absolutely necessary, and no feature should be located by more than one dimension in any direction. It may be necessary occasionally to give an auxiliary dimension for reference, possibly for inspection. When this is so, the dimension should be enclosed in a bracket and marked for reference. Such dimensions are not governed by general tolerances. Dimensions that affect the function of the part should always be specified and not left as the sum or other dimensions. If this is not done, the total permissible variation on that dimension will form the sum or difference of the other dimensions and their tolerance, and this with result in these tolerances having to be made unnecessarily tight. The overall dimension should always appear. All dimensions must be governed by the general tolerance on the drawing unless otherwise stated. Usually, such a tolerance will be governed by the magnitude of the dimension. Specific tolerances must always be stated on dimensions affecting or interchangeability. A system of tolerances is necessary to allow for the variations in accuracy that are bound to occur during manufacture, and still provide for interchangeability and correct function of the part. A tolerance is the difference in a dimension in order to allow for unavoidable imperfections in workmanship. The tolerance range will depend on the accuracy of the manufacturing organization, the machining process and the magnitude of the dimension. The greater the tolerance range is disposed on both sides of the nominal dimension. A unilateral tolerance is one where the tolerance zone is on one side only of the nominal dimension, in which case the nominal dimension may from one of the limits. Limits are the extreme dimensions of the tolerance zone. For example, nominal dimension 30mm tolerance ?? limits Fits depend on the relationship between the tolerance zones of two mating parts, and may be broadly classified into a clearance fit with positive allowance, a transition fit w