302 STRENGTH OF MATERIALS 



STRENGTH OF ROPES AND CHAINS 



Ropes. If C is the circumference of a rope in inches and P 

 the working load in pounds, then, for hemp and manila rope, 



This formula gives a factor of safety of from 1\ for manila 

 or tarred hemp rope to about 11 for the best three-strand 

 hemp rope. 



For iron-wire rope of seven strands, nineteen wires to a strand, 



and for the best steel-wire rope of seven strands, nineteen wires 

 to the strand , P = 1 ,000 O 



The last two formulas are based on a factor of safety of 6. 



Chains. If P is the safe load in pounds and d the diameter 

 of link in inches, then, for open-link chains made from a good 

 quality of wrought iron, 



P= 12,000 <# 

 and for stud-link chains, 



P= 18,000 df 



Chain Cables. The strength of a chain link is less than 

 twice that of a straight bar of a sectional area equal to that of 

 one side of the link. A weld exists at one end and a bend at 

 the other, each requiring at least one heat, which produces a 

 decrease in the strength. The report of the committee of the 

 U. S. Testing Board, on tests of wrought-iron and chain cables, 

 contains the following conclusions: 



"That beyond doubt, when made of American bar iron, 

 with cast-iron studs, the studded link is inferior in strength 

 to the unstudded one. 



"That, when proper care is exercised in the selection of 

 material, the strength of chain cables will vary by about 5% 

 to 17% of the resistance of the strongest. Without this care 

 the variation may rise to 25%. 



"That with proper material and construction the ultimate 

 resistance of the chain may be expected to vary from 155% to 

 170% of that of the bar used in making the links, and show an 

 average of about 163%. 



"That the proof test of a chain cable should be about 50% 

 of the ultimate resistance of the weakest link." 



