SIZE OF BELT 3 



If D, R and r are given 



, DR 



a = . 



r 



Example: A motor running at 1000 r.p.m. has a pulley 

 of 6 in. diameter, and is required to drive a line shaft at a 

 speed of 250 r.p.m. What size of pulley is required on the 

 line shaft? 



Substituting in the above formula, we have 



, 1000X6 

 a = = 24 m. 

 250 



Size of Belt Required. The pulley sizes being known 

 the question next to be answered is " How large a belt must 

 be used to transmit the horsepower needed? " The width 

 of belt that can be used is usually limited by the width of the 

 pulley on the machine or motor, in the case of a primary or 

 final drive, and only the thickness and tension in the belt 

 can be varied to suit the conditions imposed. In the case 

 of the drive from a line shaft to a jack shaft or to a counter- 

 shaft the designer, as a rule, has no such limitations, and can 

 adopt any combination of width, thickness and tension as 

 best suits his convenience or the pulleys and belts at his 

 disposal. A belt can, within the practical range of belt 

 velocities, be made to transmit almost any amount of power 

 desired, up to the limit of its strength, by increasing the ten- 

 sion under which it operates. High belt tensions, however, 

 mean high bearing and journal pressures, resulting in an 

 increased expenditure of power for overcoming line shaft 

 friction; they also mean high maintenance and repair charges 

 for the belts, frequent belt failures and short life of belts. 

 Thus while the adoption of a high tension will permit the use 

 of a thinner and lighter belt, the saving in first cost will be 

 more than counterbalanced by the high maintenance and 

 replacement charges and the generally unsatisfactory service 

 that will result. The use of heavy belts and comparatively 



