MECHANICS 69 



SOLUTION. Substituting in formula 1, 

 30X120 



i= ^T- 12ta ' 



EXAMPLE 2. A driving pulley 48 in. in diameter makes 

 175 R. P. M. and is connected by belt to a driven pulley 14 in. 

 in diameter. What is the speed of the driven pulley? 



SOLUTION. Substituting in formula 2, 



48X175 



= 600 R. P.M. 



14 



BELTING 



A belt is a flexible band by which motion is transmitted 

 from one pulley to another. The materials most commonly 

 used for belts are leather, cotton, and rubber, although thin, 

 flat bands of steel are coming into use. Leather belts are 

 usually made either single or double. A single belt is one 

 composed of a single thickness of leather, and a double 

 belt is one composed of two thicknesses of leather cemented 

 and riveted together throughout the whole length of the belt. 

 Still heavier belts, consisting of three or four thicknesses of 

 leather, and known as triple or quadruple belts, are sometimes 

 made for heavy drives. Cotton belts are made up of a number 

 of layers, or plies, sewed together and treated with a water- 

 proofing substance. They are termed two-ply, three-ply, etc., 

 according to the number of plies they contain. Four-ply cot- 

 ton belting is usually considered equal to single leather belting. 

 Rubber belts are particularly adapted for use in damp or 

 wet places. They withstand changes of temperature without 

 injury, are durable, and are claimed to be less liable to slip 

 than are leather belts. 



Sag of Belts. The distance between pulley centers depends 

 on the size of the pulleys and of the belt; it should be great 

 enough so that the belt will run with a slight sag and a gently 

 undulating motion, but not great enough to cause excessive 

 sag and an unsteady flapping motion of the belt. In general, 

 the centers of small pulleys carrying light narrow belts should 

 be about 15 ft. apart and the belt sag 1J to 2 in.; for large 

 pulleys and heavy belts the distance should be 20 to 30 ft. 

 and the sag 2 to 5 in. Loose-running belts will last much 



