【正文】 der that it may sagely carry the loads, which will be imposed on it, and that it may last for the expected life of the machine. All calculations are, of course, dependent on the physical properties of the construction materials as determined by laboratory tests. A rational method of design attempts to take the results of relatively simple and fundamental tests such as tension, pression, torsion, and fatigue and apply them to all the plicated and involved situations encountered in presentday machinery. In addition, it has been amply proved that such details as surface condition, fillets, notches, manufacturing tolerances, and heat treatment have a market effect on the strength and useful life of a machine part. The design and drafting departments must specify pletely all such particulars, must specify pletely all such particulars, and thus exercise the necessary close control over the finished product. As mentioned above, machine design is a vast field of engineering technology. As such, it begins with the conception of an idea and follows through the various phases of design analysis, manufacturing, marketing and consumerism. The following is a list of the major areas of consideration in the general field of machine design: ① Initial design conception。 ② Strength analysis。 ③ Materials selection。 ④ Appearance。 ⑤ Manufacturing。 ⑥ Safety。 ⑦ Environment effects。 ⑨ Reliability and life。畢業(yè)設(shè)計(jì)（論文） 48 Strength is a measure of the ability to resist, without fails, forces which cause stresses and strains. The forces may be。 ① Gradually applied。 ② Suddenly applied。 ③ Applied under impact。 ④ Applied with continuous direction reversals。 ⑤ Applied at low or elevated temperatures. If a critical part of a machine fails, the whole machine must be shut down until a repair is made. Thus, when designing a new machine, it is extremely important that critical parts be made strong enough to prevent failure. The designer should determine as precisely as possible the nature, magnitude, direction and point of application of all forces. Machine design is mot, however, an exact science and it is, therefore, rarely possible to determine exactly all the applied forces. In addition, different samples of a specified material will exhibit somewhat different abilities to resist loads, temperatures and other environment conditions. In spite of this, design calculations based on appropriate assumptions are invaluable in the proper design of machine. Moreover, it is absolutely essential that a design engineer knows how and why parts fail so that reliable machines which require minimum maintenance can be designed. Sometimes, a failure can be serious, such as when a tire blows out on an automobile traveling at high speeds. On the other hand, a failure may be no more than a nuisance. An example is the loosening of the radiator hose in the automobile cooling system. The consequence of this latter failure is usually the loss of some radiator coolant, a condition which is readily detected and corrected. The type of load a part absorbs is just as significant as the magnitude. Generally speaking, dynamic loads with direction reversals cause greater difficulties than static loads and, therefore, fatigue strength must be considered. Another concern is whether the material is ductile or brittle. For example, brittle materials are 畢業(yè)設(shè)計(jì)（論文） 49 considered to be unacceptable where fatigue is involved. In general, the design engineer must consider all possible modes of failure, which include the following: ① Stress。 ② Deformation。 ③ Wear。 ④ Corrosion。 ⑤ Vibration。 ⑥ Environmental damage。 ⑦ Loosening of fastening devices. The part sizes and shapes selected must also take into account many dimensional factors which produce external load effects such as geometric discontinuities, residual stresses due to forming of desired contours, and the application of interference fit joint. Selected from” design of machine elements”, 6th edition, m. f. sports, prenticehall, inc., 1985 and “machine design”, Anthony Esposito, charles e., Merrill publishing pany, 1975.Mechanical properties of materials The material properties can be classified into three major headings: (1) physical, (2) chemical, (3) mechanicalPhysical properties Density or specific gravity, moisture content, etc., can be classified under this category. Chemical propertiesMany chemical properties e under this category. These include acidity or alkalinity, react6ivity and corrosion. The most important of these is corrosion which can be explained in layman’s terms as the resistance of the material to decay while 畢業(yè)設(shè)計(jì)（論文） 50 in continuous use in a particular atmosphere. Mechanical properties Mechanical properties include in the strength properties like tensile, pression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of crosssection of the specimen. This is a curve plotted between the stress along the This is a curve plotted between the stress along the Yaxis(ordinate) and the strain along the Xaxis (abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappears. For many materials this occurs op to a certain value of the stress called the elastic limit Ap. This is depicted by the straight line relationship and a small deviation thereafter, in the stressstrain curve (). Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality Ap. In this region, the metal obeys hookes’s law, which states that the stress