70 MECHANICS 



longer than tight ones, and will be less likely to cause heating 

 and wear of bearings. 



Speed of Belts. The higher the speed of a belt, the less 

 may be its width to transmit a given horsepower; consequently, 

 it follows that a belt should be run at as high a speed as con- 

 ditions will permit. The greatest allowable speed for a belt 

 joined by lacing is about 3,500 ft. per min., for ordinary single 

 and double leather belts. For belts joined by cementing, 

 when the joint has about the same strength as the solid belt, 

 the velocity may be as high as 5,000 ft. per min. Higher 

 speeds than these have been used, but there is little to be gained 

 by exceeding about 4,800 ft. per min. In choosing a proper 

 belt speed, due regard must be paid to commercial conditions. 

 Although a high speed of the belt means a narrow and cheaper 

 belt, the increased cost of the larger pulleys that may be 

 required may offset the gain due to the high speed of the belt, 

 at least so far as the first cost is concerned. The speed of a 

 belt, in feet per minute, may be found by multiplying the 

 number of revolutions per minute of the pulley by 3.1416 

 times the diameter of the pulley, in inches, and dividing the 

 product by 12. 



Horsepower of Belts. The pull on a belt is greatest on the 

 tight, or driving, side, and least on the slack side. The dif- 

 ference between the tensions, or pulls, in these two sides is 

 called the effective pull. The effective pull that may be allowed 

 per inch of width for single leather belts with different arcs 

 of contact is given in the accompanying table. The arc of 

 contact is the portion of the circumference of the smaller pulley 

 that is covered by the belt. The horsepower that can be 

 transmitted by a single leather belt may be found by the 

 formula 



cwv 



~ 33, 000* 

 in which // = horsepower of belt; 



C = effective pull, taken from table; 

 H' = width of belt, in inches; 

 T' = speed of belt, in feet per minute. 

 If it is desired to find the width of single belt required 

 transmit a given horsepower, the formula becomes 





