MACHINE

CHAPTER I
THE FACTOR OF SAFETY*
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 is, therefore, apparent, not real, and we will hereafter call the
quantity we are dealing with the "apparent factor of safety," for the
name factor is too firmly fixed in our minds to easily throw it off.
* MACHINERY, January, 1906.
t Throughout this chapter we will adhere to the following definitions :
A "stress" is a force acting within a material, resisting a deformation.
A "load" is a force applied to a body, from without. It tends to produce a
deformation, and is resisted by the stress which it creates within the body.
A "working load" is the maximum load occurring under ordinary working
conditions.
A "working stress" is the stress produced by the working load, statically
applied.
The "safe working stress" is the maximum permissible working stress under
the given conditions.
The "ultimate strength" of a material is its breaking strength in pounds per
square inch, in tension, compression, or shearing, as the case may be.
The "total stress" is the sum of all the stresses existing at any section of a
body.
Unless a stress is mentioned as a total stress, the number of pounds per
square inch of section, sometimes called "the intensity of stress," will be meant.
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