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The Cornell Reading-Courses 



Size of pulleys. — When used for hoisting, as on a hay carrier or in a 

 tackle, rope is continually bending and straightening as it goes around the 

 pulleys. This bending causes the strands to chafe one another at the 

 center of the rope. The smaller the pulley, the worse is the chafing; 

 therefore, in order to avoid serious wear on a hoisting rope from this cause, 

 it should be run over a pulley of a diameter not less than eight times the 

 diameter of the rope, as shown in column VII of Table i. Thus, a hay 

 car carrying a f-inch rope should have rope wheels 6 inches in diameter, 

 and for a i-inch rope 8-inch wheels should be used; the wheels in the pulley 

 blocks should be equally large or larger. In order to give economical 

 service, rope used for transmitting power should be run over pulleys not 

 smaller in diameter than forty times the diameter of the rope. 



As mentioned above, the chafing in a four-strand rope is less than that 

 in a three-strand rope, and for this reason if small pulleys are absolutely 

 necessary or are already on hand a foiir-strand rope should be used. 



Weakening effect of knots. — Knots weaken a rope because the rope is 

 bent in order to form a knot and the outside fibers take most of the strain 

 at the bend, with the result that they are overloaded and break; this 

 throws the strain on the fibers below, which later break, and soon the 

 entire rope is gone. The knot that weakens the rope least is the one re- 

 quiring the least abrupt bending. Thus, in making a timber hitch (Fig. 

 72) the rope that carries the load is bent very gradually in passing around 

 the timber, and is weakened but 35 per cent; while in making a weaver's 

 knot (Fig. 120) the bend in the loaded rope is abrupt and the rope is 

 weakened 50 per cent. The following table shows the percentage of 

 strength left in a straight rope of any size, after any one of the knots 

 mentioned has been tied in the rope. The strength of the rope when tied 

 in knots not given may be estimated by comparison. 



TABLE 2. Approximate Efficiency of Knots, Hitches, and Splices* 



Care of rope. — (i) Storing. Rope is sold by the pound, so it is usually 

 stored by merchants in their cellars. This is bad for the rope. Rope 



* From experiments by Professor E. F. Miller, Massachusetts Institute of Technology. 



