32 ENGINEERING ON THE FARM 



objects are being moved and lifted. The safe load is usu- 

 ally regarded as one-sixth of the breaking strength. The 

 breaking strength and safe load for old ropes must be 

 largely a matter of good judgment and experience. 



Calculation of strength. For new Manila rope the break- 

 ing strength in pounds may be found approximately by the 

 following rule : Square the diameter, measured in inches, 

 and multiply this product by 7,200. Results obtained from 

 this rule may vary as much as 15 per cent from actual tests. 

 The safe load can be found by dividing the breaking strength 



by 6. 



Suppose we wish to find the breaking strength and safe 

 load of a 24 -inch Manila rope. The square of y A is %%, 

 which, multiplied by 7,200, gives 4,050 pounds as its breaking 

 strength, and 4,050 divided by 6 gives 675 pounds as its 



safe load. 



Hemp rope is approximately three-fourths as strong as 

 Manila, so that we use the following rule for it : The breaking 

 strength of hemp rope in pounds is 5,400 times the square 

 of the diameter in inches. The safe load is found by divid- 

 ing the breaking strength by 6 as we did for the Manila 

 rope. Thus breaking strength for a ^-inch hemp rope 

 would be 



34 X Va X 5400 = 3037-5 pounds, 



and the estimated safe load would be 



3037-5 "5-6 = 506.25 pounds. 



Calculation of weight. One rule for calculating the 

 weight of rope is the following: To find the weight of a 

 piece of rope 1 foot long, square the diameter measured in 

 inches, and multiply this number by o . 3 2 . Results obtained 

 by this rule may vary as much as 10 per cent from the actual 

 weight of new rope. Rope will take up moisture if stored 

 in damp places, as basements, so that its weight will be 

 considerably increased. 



The actual diameter of rope is usually a little larger than 



