外文翻译---机械零件强度.docx

附录2 外文原文the strength of mechianical elementsone of the primary considerations in designing any machine of structure is that the strength must be sufficiently greater than the stress to assure both safety and reliability. to assure do fail. then we shall be able to relate the stresses with the strengths to achieve safety. ideally , in designing any machine clement, the engineer should have at his disposal the results of a great many strength tests of the particular material chosen. these tests shoule have been made on spccimens having the same heat treatment, surface roughness, and size as the element he proposes to design,and the tests should be made under exactly the same loading conditions as the part will experience in service. this means that, if the part is to experience a bending load, it should be tested with a bending load. if it is to be subjected to combined bending and torsion , it should be tested under combined bending and torsion. such tests will provide very useful and precise information. they tell the engineer what factor of safety to use and what the reliability is for a gicen service life. whenever such data are available for design purpses, the engineer can be assured that be is doing the best possible job of engineering. the cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life, or if the part ia manufactured in sufficiently large quantities. automobiles and refrigerators, for example,have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. the cost of making these tests is very low when it is divided the total number of parts manufactured. you can now appreciate the following four design categories:(1) failure of the part would endanger human life, or the part is made in extremely large quantities; consequently, an elaborate testing program is justified during design.(2) the part is made in large enough quantities so that a moderate series of tests is feasible.(3) the part is made in such small quantities that testing is not justified at all, or the design must be completed so rapidly that thert is not enough time for testing.(4) the part has already been designed, manufacturde, and tested and found to be unsatisfactory. analysis is required to understand why the part is unsatisfactory and what to do improce it. it is with the last three categories that we shall be mostly concerned. this means that the designer will usually have only published values of yield strength, ultimate strength, and percentage elongation .with this meager information the engineer is expected to design against static and dynamic loads, biaxial and tri axial stress states,high and low temperatures, and large and small parts! the data usually available for design have been obtained from the simple tension test, where the load was applied gradually and the strain given time to develop. yet these same must be was applied gradually and the strain given time to develop. yet these same data must be used in designing parts with complicated dynamic loads applied thousands of times per minute. no wonder machine parts sometimes fail. to sum up, the fundamental problem of the designer is to use the simple tension test data and relate them to the strength of the part, regardless of the stress state of the loading situation. it is possible for two metals to have exactly the same strength and hardness, yet one of these metals may have a superior ability to absorb overloads, because of the property called ductility. ductility is measured by the percentage elongation which occurs in the material at fracture. the usual dividing line between ductility and brittleness is 5 percent elongation. a material having less than 5 percent elongation at fracture is said to be brittle, while one having more is said to ductile. the elongaion of a material is usually measured over 50 mm gauge length. since this is not a measure of the actual strain, another method of detemining ductility is sometimes used. after the specimen has been fractured, measurements are made of the area of the cross-sectional area. the characteristic of aductile material which permits ti to absorb large overloads ia an additional safety factor in design. ductility is also important because it is a measure of that property of a material which permits it to be cole-worker. such operations as bending and drawing are metal-processing operations which require ductile materials. when a material is to be selected to tesist weat, erosion, or plastic deformation, hardness is generally the most important. sevetal methods of hardness testing are available, depending upon which particular property is most desired. the four hardness numbers in greatest use are the brinell, rockwell, vickers, and koop. most hardness-testing systems employ a standard load which is applied to a ball or pyramid in contact with the material to be tested. the hardness is then expressed as a function of the size of the resulting indentation. this means that hardness is an easy property to measure, because the test is mondestructive and test specimens are not required usually the test be conducted directly on an actual machine element.some rules for mechanical design designing starts with a need, real or imagined. existing apparatus may need improvements in durability, weight, speed, or cost. new apparatus may be needed toperform a function previously done by men, such as computation, assembly, or servicing. with the objecive wholly or partly defined, the next step in design is the conception of mechanisms and their arrangements that will perform the needed functions. for this, freehand sketching is of great value, not only as a record of ones thoughts and as an aid in discussion with others,but particularly for communiaction with ones own mind, as a stimulant for creative ideas. when the general shape and a few dimensions of the several components become apparent, analysis can begin in earnest. the analysis will have as its objective satisfactory or superior perfromance, plus safety and durability with minimum weight, and a competitive cost. optimum proportions and dimensions will be sought for each critically loaded section, together with a balance between the strength of the several components. materials and their treatment will be chosen. these important objectives can be attained only by analysis based upon the principles of mechanics, such as those of static for reacion forces and for the optimum utilization of friction; of dynamics for inertia, accelertion, and energy; of elasticity and stength of materials for stress and deflection; and of fluid mechanics for lubrication and hydrodynamic drives. finally, a design based upon funtion and reliability will be completed, and a prototype may be built. if its tests are satisfactory, and if the device is to be produced in quantity, the initial design will undergo certain modifications that enable it to be manufactured in quantity at a lower cost. during subsequent years of manufacture and service, the design is likely to undergo changes as new ideas are conceived or as further analysis based upon tests and experience indicate altertions. sales appeal, customer satisfaction, and manufacture cost are all relaed to design, and ability in design is intimately involved in the success of an engineering venture. to stimulate creative thought, the following rules are suggested for the designer.1、apply ingenuity to utilize desired physical properties and to control undesired ones. the performance requirements of a machine are met by utilizing laws of nature or properties of matter(e.g, flexibility, strength, gravity, inertia, buoyancy, centrifugal force, principles of the lever and inclined plane, friction, viscosity, fluid pressure ,and thermal expansion), also the many electrical, optical, thermal, and chemical phenomena. however, what may be useful in one application may be detrimental in the next. flexibility is desired in valve camshaft, friction is desired at the clutch face but not in the clutch bearing. ingenuity in design should be applied to utilize and control the physical properties that are desired and to minimize those that are not desired.2、provide for favorable stress distribute and stiffness with minimum weight. on components subjected to fluctuating stress,particulat attention is given to a reduction in stress concentration, and to an increase of strength at fillets, threads, holes, and fits. stress reduction are made by mondification in shape, and strengtening may be done by pre stressing treatments such as surface rolling and shallow hardening. hollow shafts and tubing, and box sections give a facorable stress distribution, together with stiffness and minimum weigh. sufficient stiffness to maintain alignment and uniform pressure between contacting surfaces should de provided for crank, cam, and gear shafts, and for enclosures and frames containing bearing supports. the stiffness of shafts and other components must be suitable to avoid resonant vibrations.3、use basic equations to calculate and optimize dimensions. the fundamental equations of mechanics and the other sciences are the accepted bases for calculations. they are sometimes rearranged in special forms to facilitate the determination or optimization of demensions, such as the beam and surtace stress equations for determining geat-tooth size. factors may be added to a fundamental equation for conditions not analytically determinable, e.g, on thin steel tubes, an allowance forcorrosion added to the thickness based on pressure. when it is necessary to apply a fundamental equation to shapes, materials, or conditions which only approximate the assumptions for its derivation, it is done in a manner which gives results “ on the safe side” in situations where data are incomplete, equations of the sciences may be used as proportiong guides to extend a staifactory design to new capatities.4、choose materials for a conbination of properties. materials should be chosen for a conbination of pertinent properties, not only for strengths, hardness, and weight, but sometimes for resistance to impact, corrosion, and low or high temperatures. cost and fabrication properties are factors, such as weld ability, machine ability, sensitivity to cariation in heat-treating temperatures, and required coating.5、select carefully between stock and integral components. a previously developed components is frequently selected by a designer and company from the stocks of parts manufacturers, if the component meet the performance and reliability requirements and is adaptable without additional development costs to the particular machine being designed. however, its selecion should be carefully made with a full knowledge of its properties, since the reputation and liability of the company suffer if there is a failure in any one of the machines parts, in other cases the strength, reliability, and cost requirements are better if the designer of the machine also designs thecomponent, with the particular advantage of compactness if it is designs the component, with the particular advantage of compactness if it is designs integral with other components, e.g, gears to be forged in clusters of integral with a shaft.6、provide for accurate location and non-interference of parts in assembly. a good design provides for the correct locating of parts and for easy assembly and repair. shoulders and pilot surfaces give sccurate location without measurement during assembly. shapes can be designed so that parts cannot be assembled backwards or in the wrong place. interferences, as between screws in tapped holes, and between linkages must be foreseen and interference. inaccurate alignment and positioning between detrimental displacements and stresses.外文译文机械零件强度在设计任何机器或者结构时，所考虑的主要事项之一是其强度应该比它所承受的应力要大得多，以保证安全与可靠性。要保证机械零件在使用过程中不发生失效，就必须知道它们在某些时候会失效的原因，然后，才能应力与强度联系起来，以保证其安全。设计任何机械零件的理想情况为：工程师可以利用大量的他所选择的这种材料的强度试验数据。这些试验应该采用与实际的零件有着相同的情况下进行。这表明，如果零件简要进行承受弯曲载荷，那么就应该进行弯曲载荷的试验。这些种类的试验可以提供非常有用和精度的数据。它们可以告诉工程师应该使用的安全系数和对于给定使用寿命时的可靠性。在设计中，只要能够获得这些数据，工程师就可以尽可能好地进行工程设计工作。如果零件的失效可能危害人的生命安全，或者零件有足够大的产量，则在设计前搜集的这些广泛的数据所花的费用是值得的。例如，汽车和冰箱的零件的产量非常大，可以在生产之前对它们进行的大量的试验，使用具有较高的可靠性。如果把进行这些试验的费用分摊到了所生产的零件上话，则分摊到所生产的每个零件上的费用非常低你可以对下列四种类型的设计做出评价：1、零件的失效可能危害人的生命安全，或者零件的产量非常的大，因此在设计时安排一个完善的试验程序会被认为是合理的。2、零件的产量足够大，可以进行适当的系列试验。3、零件的产量非常小，以至于进行试验根本不合算；或者要求很快地完成设计，以至于没有足够的时间进行试验。4、零件已经完成设计、制造和试验，但结果不令人满意。这时需要采用分析的方法来弄清不能令人满意的原因和应该如何进行改进。我们将主要对后三种类型进行讨论。这就是说，设计人员通常只能利用那些公开发表的屈服强度，极限强度和延伸率等数据资料。人们期望工程师利用这些不是很多的数据资料的基础上，对静载荷与动载荷，二维应力状态与三维应力状态，高温与低温以及大零件与小零件进行设计，而设计中所能利用的数据通常是从简单的拉伸试验中得到的，其载荷是逐渐加上去的，有充分的时间产生应变。到目前为止，还必须利用这些数据来设计每分钟承受几千次复杂的动载荷和作用的零件，因此机械零件有时会失效是不足为奇的。 概括的说，设计人员所遇到的基本问题是，不论对于哪一种应力状态或者载荷情况，都能利用简单的试验所获得的数据并将其零件的强度联系起来。可能会有两种情况具有完全相同的强度和硬度值的金属，其中一种由于本身的延展性而具有很好的承受超载荷的能力。延展性与脆性的分界线。断裂时延伸率小于5%的材料称为脆性材料，大约5%的称为延性材料。材料的伸长量通常是在50mm的计量长度上测量的。因为这并不是对实际应变量的测量，所以有时也采用另一种测量延展性的方法。这个方法是在试件断裂后，测量其断裂处的横截面的面积。因此，延展性可以表示为横截面的收缩率。延性材料能够承受较大的超载荷这个特性，是设计中的附加安全因素。延性材料的重要性在于它是材料冷变形性能的衡量尺度。诸如弯曲和拉伸这种金属加工过程需要采用延性材料。 在选用抗磨损、抗腐蚀或者抗变形的材料时，硬度通常是最主要的性能。有几种可供选择的硬度试验反复法，采用哪一种方法取决于最希望测量的材料特性。最常用的四种硬度数值是步氏硬度、洛氏硬度、维氏硬度和努氏硬度。大多数硬度试验系统是将一个标准的载荷加在与被试验材料相接触的小球或者棱锥上。因此，硬度可以表示为所产生的压痕尺寸的函数。这表明由于硬度是非破坏性的试验，而且不需要专门的试件，因而，硬度是一个容易测量的性能。通常可以直接在实际的机械零件上进行硬度试验。机械设计规则 设计是从实际或者假象的需要开始的，对于现有的设备可能需要在耐用性，效率进度成本等方面做进一步改进工作，也可能需要新的设备完成以前由人来做的工作，例如计算机或者装配。当目标完成或部分被确定以后，下一步设计步骤是对能够完成所需要的机构及其布局进行总体设计。对于此项工作，徒手画的草图是很有价值的，它不仅可以记录下我们的想法，而且还有助于与别人进行讨论，特别是和自己的大脑记性交流，从而促进创新想法的产生。 当一些零件的大致形状和几个尺寸被确定后，就可以开始认真的分析工作。分析工作的目的是要在重量最轻、成本最低的情况下，令人满意，即优良的工作性能，并且还要安全耐用。对于每个关键承载截面，应该寻求最佳的比例和尺寸，同时要对这几个零件的受力进行平衡。要对材料和处理方式进行选择。只有根据力学原理进行分析才能达到这些重要目的。这些分析包括根据静力学原理分析反作用力和充分利用摩擦力，根据动力学原理分析惯