FARM POWER AND MACHINERY. 249 



Strength of Materials. 



The tensile strength of materials Is measured by the least weight in 

 pounds which will break a vertical rod one inch square, firmly and squarely 

 fixed at Its upper end, the load hanging from the lower end. 



Tensile Strength of Materials in lbs. per square inch. 



Elm 6,000 <^"'^t Iron 20,000 



Hickory 11,000 Wrought Iron 57,000 



Maple 10,000 Wrought Iron Wire rope. . 38,000 



Oak 10,000 Leather belts, good 3,000 



Poplar 7,000 Manilla rope, best 12,000 



White Pine 10,000 Hemp rope, best 15,000 



Ordinary Steel 85,000 Cast Steel 114.000 



If under compression Instead of tension cast iron will crush under a 

 weight of 105 lbs. to the square inch, cast steel 142, and the various woods 

 at about 3-4 of the tensile strength given. The crushing strength of stone Is 

 about 4,300 lbs. per square Inch. 



Factor of Safety. 



In actual practice we have to deal with the load that may be safely 

 borne, and for this purpose the breaking load Is divided by a factor of safety 

 to secure the desired safe load. The following table gives the usual factors 

 of safety. 



Steady Stress. Varying Stress. Shocks. 



Material. (Buildings) (Bridges) (Machines) 



Timber 8 10 15 



Stone 15 25 30 



Cast Iron 6 15 20 



Wrought Iron 4 6 10 



Steel 5 7 15 



Strength of C!omnion Ropes. 



The following table shows breaking weight and also safe weights which 

 may be borne by ordinary ropes: 



Rope Breaking Weight Borne with safety 



One-eighth inch diameter 78 lbs. 31 lbs. 



One-fourth Inch diameter 314 lbs. 125 lbs. 



One-half inch diameter 1,250 lbs. 500 lbs. 



One Inch diameter 5.000 lbs. 2.000 lbs. 



One and one-fourth Inch diameter 7.500 lbs. 3.000 lbs. 



One and one-half Inch diameter 12,500 lbs. 4,500 lbs. 



Hnman Force. 



The proportionate force between the hand of man on the tool used and 



the force exerted by the tool Is given respectively In the first and second 

 columns following: 



Hand Tool 



Draw knife 100 lbs. 100 lbs. 



Large auger 100 lbs. 800 lbs. 



Screw driver 84 lbs. 250 lbs. 



Wrench, vise handle 72 lbs. 1.000 lbs. 



Windlass, one hand fiO lbs. 180 to 700 lbs. 



Hand saw 36 lbs. 36 lbs. 



Brace and bit 16 lbs. 150 to 700 lbs. 



Button screw, thumb and finger 14 lbs. 14 to 70 lbs. 



