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1、英文资料Limits and TolerancesThe breakage of the machine spare parts ,generally always from the surface layer beginning of .The function of the product ,particularly its credibility and durable ,be decided by the quantity of spare parts surface layer to a large extent. Purpose that studies the machine t
2、o process the surface quantity be for control the machine process medium various craft factor to process the surface quantity influence of regulation, in order to make use of these regulations to control to process the process, end attain to improve the surface quantity, the exaltation product use t
3、he function of purpose .The machine processes the surface quantity to use the influence of the function to the machine(A) The surface quantity to bear to whet the sexual influence 1.Rough degree of surface to bear to whet the sexual influenceA just process vice-of two contact surfaces of good fricti
4、on, the first stage is rough only in the surface of the peak department contact ,the actual contact area is much smaller than theoretical contact area, in contact with each other the peak of the units have very great stress, to produce actual contact with the surface area of plastic deformation, def
5、ormation and peak between the Department of shear failure, causing serious wear.Parts wear may generally be divided into three stages, the initial stage of wear and tear, normal wear and tear all of a sudden intense phase of stage wear.Parts of the surface roughness of the surface wear big impact. I
6、n general the smaller the value of surface roughness, wear better. However, surface roughness value is too small, lubricants difficult to store, contact between the adhesive-prone elements, wear it to increase. Therefore, the surface roughness of a best value, the value and parts of the work related
7、 to increased work load, the initial wear increased, the best rough surface is also increased.2.Cold Working hardening the surface of the wear resistance Processing the Cold Work hardening the surface of the friction surface layer of metal microhardness increase, it will generally improve the wear r
8、esistance. Cold Working but not a higher degree of hardening, wear resistance for the better, because too much will lead to hardening of the Cold Working excessive loose organization of metal, even a crack and peeling off the surface of the metal, declined to wear resistance.(B)The surface quality o
9、f the impact of fatigue strengthMetal hand alternating loads of fatigue after the damage occurred in parts often Chilled layer below the surface and, therefore parts of the surface quality of fatigue very influential.1. Surface roughness on the impact of fatigue strengthIn alternating load, the surf
10、ace roughness of the Au-site easily lead to stress concentration, a fatigue crack, the higher the value of surface roughness, surface traces of Yu Shen Wen, Wen at the end of the radius smaller, anti-fatigue damage at the end of the more capacity Worse.2. Residual stress, fatigue Cold Work hardening
11、 of the impact Residual stress on the impact of large parts fatigue. Surface layer of residual stress fatigue crack will expand and accelerate the fatigue damage the surface layer and the residual stress can prevent fatigue crack growth, delaying the formation of fatigue damage.(C)The surface qualit
12、y of the corrosion resistance of the impactParts of the corrosion resistance to a large extent depends on the surface roughness. The higher the value of surface roughness, Au Valley accumulate on the more corrosive substances. Corrosion resistance of the more worse.Surface layer of residual stress w
13、ill produce stress corrosion cracking, lower parts of the wear-resistance, and the residual stress is to prevent stress corrosion cracking.(D) The surface quality with qualityRough surface will affect the value of the size of the co-ordination with the surface quality. The gap with rough value will
14、increase wear and tear, increased space, with the requirements of the destruction of nature. For Fit, the assembly part of the process of convex surface-crowded peak times, the actual reduction of the surplus and reduce the support of the connection between the strength.DimensioningThe design of a m
15、achine includes many factors other than those of determining the loads and stresses and selecting the proper materials. 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 freque
16、ntly 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.Drawings should be carefully checked to see that the dimensioning is done in a m
17、anner that will be most convenient and understandable to the production departments. It is obvious that a drawing should be made in such a way that it has one and only one interpretation. In particular, shop personnel should not be required to make trigonometric or other involved calculations before
18、 the production machines can be set up.Dimensioning is an involved subject and long experience is required for its mastery.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 a given dimension.
19、 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 Fig 14.1. It is therefore important that the tolerances be specified at the largest values that the oper
20、ating or functional considerations permit.Tolerances may be either unilateral or bilateral. In unilateral dimensioning, one tolerance is zero, and all the variations are given by the other tolerance. In bilateral dimensioning, a mean dimension is used which extends to the midpoint of the tolerance z
21、one with equal plus and minus variations extending each way from this dimension.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
22、individual parts, 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. H
23、owever, some companies do not use blanket notes on the supposition that if each dimension is considered individually, wider tolerance 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 sing
24、le interpretation.Dimension and ToleranceIn 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. To specify a degree of accuracy, it is necessary to add tolerance
25、figures to the dimension. Tolerance is the amount of variation permitted in the part or the total variation allowed in a given dimension. A shaft might have a nominal size of 2.5 in. (63.5mm), but for practical reasons this figure could not be maintained in manufacturing without great cost. Hence, a
26、 certain tolerance would be added and , if a variation of 0.003 in.(0.08mm) could be permitted, the dimension would be stated 2.5000.003(63.50.008mm).Dimensions given close tolerances mean that the part must fit properly with some other part. Both must be given tolerances in keeping with the allowan
27、ce desired, the manufacturing processes available, and the minimum cost of production and assembly that will maximize profit. 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 dev
28、iation is allowed from the nominal size.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. If a shaft, size 1.498, is to fit a hole of siz
29、e 1.500, 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.Tolerances may be either unilateral or bilateral. Unilateral tolerance means that any variation is
30、 made in only one direction from the nominal or basic dimension. Referring to the previous example, the hole is dimensioned 1.500, which represents a unilateral tolerance. If the dimensions were given as 1.5000.003, the tolerance would be bilateral; that is , it would vary both over and under the no
31、minal 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 par
32、ts 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 FitsThe drawing must be a true and complete statement of the designers expressed in s
33、uch 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
34、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 mark
35、ed 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 t
36、he 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 gover
37、ned 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
38、 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 ra
39、nge 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 di
40、mension 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 where the allowance may be either positive or negative (clearance or interference) , an interfer
41、ence fit where the allowance is always negative.Type of Limits and FitsThe ISO system of Limits and Fits, widely used in a number of leading metric countries, is considerably more complex than the ANSI system.In this system, each part has a basic size. Each limit of part, high and sign being obtaine
42、d by subtracting the basic size form the limit in question. The difference between the two limits of size of a part is called the tolerance, an absolute without sign.There are three classes of fits: 1) clearance fits, 2) transition fits ( the assembly may have either clearance or interference ), and
43、 3) interference fits .Either a shaft-basis system or a hole-basis system may be used. For any given basic size, a range of tolerance and deviations may be specified with respect to be line of zero deviation, called the zero line. The tolerance is a function of the basic size and is designated by a
44、number symbol, called the grade-thus the tolerance grade. The position of the tolerance with respect to the zero line also a function of the basic size-is indicated by a letter symbol(or two letter), a capital letter for holes and a lowercase letter for shafts. Thus the specification for a hole and
45、shaft having a basic size of 45mm might be45H8/g7. Twenty standard grades of tolerance are provided, called IT 01,IT 0 ,IT 1-18, providing numerical values for each nominal diameter, in arbitrary steps up to 500mm (for example 0-3,3-6,6-10, 400-500mm). The value of the tolerance unit, I, for grades
46、5-16 is Where i is in microns and D in millimeters.Standard shaft and hole deviations similarly are provided by sets of formulas, However, for practical, both tolerances and deviations are provided in three sets of rather complex tables. Additional tables gives the values for basic sizes above 500mm
47、 and for “Commonly Used Shafts and Holes” in two categories -“General Purpose” and “Fine Mecbanisms and Horology”.中文翻译极限与误差机械零件的破坏，一般总是从表层开始的。产品的性能，尤其式它的可靠性和耐久性，在很在程度上取决于零件表层的质量。研究机械加工表面质量的目的就是为了掌握机械加工中各种工艺因素对加工表面质量影响的规律，以便运用这些规律来控制加工过程，最终达到改善表面质量、提高产品使用性能的目的。机械加工表面质量对机器使用性能的影响（一）表面质量对耐磨性的影响 1.表面粗糙度
48、对耐磨性的影响一个刚加工好的摩擦副的两个接触表面之间，最初阶段只在表面粗糙度的峰部接触，实际接触面积远小于理论接触面积，在相互接触的峰部有非常大的单位应力，使实际接触表面积处产生塑性变形、弹性变形和峰部之间的剪切破坏，引起严重磨损。零件磨损一般可分为三个阶段，初期磨损阶段、正常磨损阶段忽然剧烈磨损阶段。表面粗糙度对零件表面磨损的影响很大。一般说表面粗糙度值越小，其磨损性越好。但表面粗糙度值太小，润滑油不易储存，接触面之间容易发生分子粘接，磨损反而增加。因此，接触面的粗糙度有一个最佳值，其值与零件的工作情况有关，工作载荷加大时，初期磨损量增大，表面粗糙最佳也加大。2.表面冷作硬化对耐磨性的影响加
49、工表面的冷作硬化使摩擦副表面层金属的显微硬度提高，故一般可使耐磨性提高。但也不是冷作硬化程度愈高，耐磨性久愈好，这是因为过分的冷作硬化将引起金属组织过度疏松，甚至出现裂纹和表面金属的剥落，使耐磨性下降。（二）表面质量对疲劳强度的影响金属手交变载荷作用后产生的疲劳破坏往往发生在零件表面和冷硬层下面，因此零件表面质量对疲劳强度影响很大。1.表面粗糙度对疲劳强度的影响在交变载荷作用下，表面粗糙度的凹谷部位容易引起应力集中，产生疲劳裂纹，表面粗糙度值愈大，表面的纹痕愈深，纹底半径愈小，抗疲劳破坏底能力就愈差。2.残余应力、冷作硬化对疲劳强度的影响残余应力对零件疲劳强度的影响很大。表面层残余拉应力将使疲劳裂纹扩大，加速疲劳破坏；而表面层残余应力能够阻止疲劳裂纹的扩展，延缓疲劳破坏的产生。（三）表面质量对耐蚀
