【正文】 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 bined bending and torsion , it should be tested under bined 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 pleted 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 plicated 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 crosssectional 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 coleworker. Such operations as bending and drawing are metalprocessing 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 hardnesstesting 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 putation, 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 one’s thoughts and as an aid in discussion with others,but particularly for muniaction with one’s own mind, as a stimulant for creative ideas. When the general shape and a few dimensions of the several ponents bee apparent, analysis can begin in earnest. The analy