2 LEATHER BELTING 



clear that a machine whose earning capacity is high will lose 

 for its owners a considerable sum of money if its production 

 is stopped during working hours. This loss of production, 

 coupled with the wages of the operator of the machine during 

 the period that it is idle, will often far exceed the cost of the 

 heavier belt and the expense of regular inspection and main- 

 tenance that would have avoided the belt failure. To obtain 

 the lowest possible total belting cost, therefore, not only is 

 good design necessary, but also provision for regular inspection 

 of all the belts in a shop and their maintenance in good order, 

 together with regulation of the tensions under which they are 

 required to do their work. 



Relation of Sizes and Speeds of Pulleys. In laying out 

 a belt drive, the diameter and speed of rotation of one of 

 the pulleys involved is usually a fixed quantity, and also the 

 speed of rotation of the other pulley. Thus, if a machine is 

 to be driven, the number of revolutions at which it is designed 

 to operate is as a rule fixed by the maker, and the driving 

 pulley cannot be varied in diameter. The speed of the line 

 or countershaft from which it is driven is also incapable of 

 variation. Or, in the case of a motor or engine driving a 

 line shaft the pulley or fly wheel on the motor or engine is 

 fixed in size, as is the number of revolutions per minute at 

 which these prime movers operate. The speed of the line 

 shaft is also fixed by shop conditions. The first step, there- 

 fore, in laying out the drive is the determination of the size 

 of the pulley whose diameter is unknown, so that the shaft or 

 machine will be driven at the desired speed. 



The speed of rotation of two shafts connected by a belt 

 varies directly as the diameters of the pulleys. If D and d 

 are the diameters of two pulleys, both in the same unit, as 

 feet or inches, and R and r their respective speeds of rotation 

 in revolutions per minute, then 



