Articles liés à Calculations of Elements of Machine Design
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Synopsis
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Présentation de l'éditeur
It is the custom among most firms engaged in the designing of machinery to settle upon certain stressesf as proper for given materials in given classes of work. These stresses are chosen as the result of many years of experience on their own part, or of observation of the successful experience of others, and so long as the quality of the material remains unchanged, and the service does not vary in character, the method is eminently satisfactory. Progress, however, brings up new service, for which precedent is lacking, and materials of different qualities, either better or cheaper, for which the safe working stresses have not been determined, and the designer is compelled to determine the stress proper for the work in hand by using a so-called .factor of safety. The name factor of safety is misleading for several reasons. In the first place, it is not a factor at all, from a mathematical point of view, but is in its use a divisor, and in its derivation a product. In order to obtain the safe working stress, we divide the ultimate strength of the material by the proper factor of safety, and in order to obtain this factor of safety we multiply together several factors, which, in turn, depend upon the qualities of the material, and the conditions of service. So our factor of safety is both a product and a divisor, but it is not a factor. Then again, we infer, naturally, that with a factor of twelve, say, we could increase the load upon a machine member to twelve times its ordinary amount before rupture would occur, when, as a matter of fact, this is not so, at least not in a machine with moving parts, sometimes under load, and sometimes not subjected to working stresses. Still more dangerous conditions are met with when the parts are subjected to load first in one direction, and then in the other, or to shocks or sudden loading and unloading. The margin of safety
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