4 LEATHER BELTING 



low tensions is in accord with the best belt practice of today, 

 and is recommended as giving in the end the lowest total 

 belting cost as well as the most satisfactory service. 



The horsepower that will be transmitted by a belt can 

 be expressed by the general formula 



33000 



in which p is the effective pull in the belt, or the difference 

 in tension between the tight and slack strands of the running 

 belt, in pounds per square inch; V is the velocity of the belt 

 in feet per minute; and A the cross-sectional area of the belt 

 in square inches. By assuming a value for p and for the 

 thickness of the belt, the formula can be made to read 



HP - 

 " C" 



w being the width of the belt in inches and C a constant whose 

 value depends on the thickness of the belt and the value 

 adopted for p. This formula, whose use as it stands is not 

 to be recommended, is the basis of the old rules of thumb for 

 determining the size of belt required for a given horsepower. 

 One such typical rule, the value of p being assumed as 360, 

 reads " A single belt, i in. wide, traveling at the rate of 550 

 ft. per minute, will transmit one horsepower." 



Faults of the Rules of Thumb. These rules of thumb 

 are all defective in that they neglect many of the factors 

 affecting the value of p. The effective pull depends on the 

 arc of contact of the belt on the pulley, on the coefficient of 

 friction between the belt and the surface of the pulley, on the 

 initial tension of the belt when at rest, and on its velocity. 

 As the velocity of the belt increases, the centrifugal force 

 developed by the belt in passing over the pulleys manifests 

 itself and tends to decrease the effective pull. This effect, 

 known as centrifugal tension, is serious at high velocities and 



