1、 附录 2 外文原文 THE Strength of Mechianical Elements One 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 st
2、rengths 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
3、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 tor
4、sion , 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
5、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 ve
6、ry 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: (
7、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 sma
8、ll 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
9、 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 ag
10、ainst 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 m
11、ust 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
12、 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 call
13、ed 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 s
14、aid 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. T
15、he 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-processi
16、ng operations which require ductile materials. When a material is to be selected to tesistweat, 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 nu
17、mbers 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
18、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 impro
19、vements 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 pe
20、rform 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 sever
21、al 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, to
22、gether 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 fricti
23、on； 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 a
24、re 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 conceive
25、d 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
