. Р相同的理论或方程可应用在一个一起的非常小的零件上,也可用在一个复杂的设备的大型相似件上,既然如此,毫无疑问,数学计算是绝对的和最终的。他们都符合不同的设想,这必须由工程量决定。有时,一台机器的零件全部计算仅仅是设计的一部分。零件的结构和尺寸通常根据实际考虑。另一方面,如果机器和昂贵,或者质量很重要,例如飞机,那麽每一个零件都要设计计算。Р4 The purpose of the design calculations is, of course, to attempt to predict the stress or deformation in the part in order 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, compression, torsion, and fatigue and apply them to all plicated and involved situations encountered in present-day machinery.
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