244 APPLIED SCIENCE 



applied. This rapidly applied force has been found by tests 

 to be about twice as much as the slowly applied one. There- 

 fore, 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: 



Steady Varying 

 Material Stress 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 



292. Strength of Chains. Chains for hoisting weights 

 are made from a good grade of wrought iron, which has a 

 tensile strength of from 40,000 to 48,000 Ibs. per square inch. 

 Chains used for raising weights should never be made from 

 steel, as it is not so strong under shock as wrought iron, and 

 does not weld so readily. Because of the possibility of the 

 weld not being as strong as the balance of the link, the 

 strength of the chain is not reckoned as twice the strength 

 of the bar from which it is made. When buying chains in 

 the open market it is advisable to base the computation of 

 strength on the lowest tensile strength of iron used for the 

 purpose, i.e., 40,000 Ibs. to the square inch. 



The strength of a chain link is 1.63 times the strength of 

 the bar from which it is made. The strength referred to is 

 the breaking, or tensile, strength. It is never safe to strain 

 to anywhere near the breaking point, because every time a 



