82 



In addition to this weight there is the unbalanoed belt-pull vs-hich increases 

 the load on the bearings. Although the tension on the tight side of the belt may 

 not ordinarily exceed abont twice the tension in the slack side necessary for ad- 

 hesion, yet it is probable that belts are frequently run with a ratio of tension 

 equal to one to three, and occasionally one to four. On the other hand, it is a 

 very common thing for belts, especially short ones, to be laced so taut that the 

 initial tension is greatly in excess of that required for adhesion, in which case the 

 sum of the tensions approaches twice that in the tight side of the belt. 



With ordinary shop-worn belting it will be safe to assume that the tension 

 Tj on the slack side of the belts is one-half the tension Ti on the tight or driving 



T 



side, that is T^ = — ,-, hence, since Tj — T. = P, the driving force, we have 



H P -T^X-^^ ^'^ 



2 ^ 33000' 



Under the conditions which obtain in machine shops the diameter of a shaft 



to safely transmit a given horse-power without undue deflection may be obtained 



from the formula 



Combining ( 7 ) and ( 8 ) we have 



H P -iLy^^^^d^N (9) 



2 '^ 33000 100 ' 



, ^ 660 d^ y 



and 1 1 = :r= , 



Therefore the sum of the tensions on the entire length of shaft 



„ 1000 ., ^ . 



or Bi = d-' iS very nearly. 



d' N ... 



Hence the belt-pull per foot of length of shaft = 1000 -y ^. The force of fric- 

 tion, F, acting at the circumference of shaft, is _ o W, as before, but in this 

 ^case W equals the weight of shaft, W.^ and its furniture, as well as the unbalanced 



^belt-pull. 



The belt tension may act in any direction perpendicular to the axis of the 

 sTiaft, and the intensity of pull in any given direction will vary from to the 

 maximum to total tension. Besides these tensions there will be an additional pull 

 due to the tensions in the belt from fly-wheel to main line shaft. 



