In building a machine or a structure of any kind, care must be taken not to subject any part to a stress that would strain it beyond its elastic limit. The usual practice is to divide the ultimate strength of the material by some number depending upon the kind and quality of material and upon the nature of the stress. This quotient is called the factor of safety. The factor of safety of any material is the ratio of its ultimate strength to the actual stress to which it is to be subjected.

Suppose the actual tensile stress on a rod 1 in. square is to be 10,000 lbs., and we have found by testing that the ultimate tensile strength of a material of this kind is 70,000 lbs. Then the factor of safety for this material would be

70,000 / 10, 000 = 7

The rod when stressed 10,000 lbs. will then have a factor of safety of 7.

As has been stated, a force acting suddenly is called a shock and does more damage than the same force gradually applied. This rapidly applied force has been found by tests to be about twice as much as the slowly applied one. Therefore, in designing machinery it is necessary to consider whether the part will be subjected to a steady stress, a varying stress, or a shock, before deciding the proper factor of safety to use.

The table below gives the factors of safety generally used in American practice:

 Material Steady Stress Varying Stress Shock Timber....................... 8 10 15 Brick or Stone................ 15 25 30 Cast Iron..................... 6 10 15 Wrought Iron................. 4 6 10 Steel......................... 5 7 10