76 LEATHER BELTING 



from the driving pulley and should bear against the slack 

 side of the belt. Its effect is not only to increase the tension, 

 but also the arc of contact. The idler may be so constructed 

 that it can be moved against the belt at will, or it may act 

 automatically by means of a spring or gravity. Such an 

 arrangement is shown in Fig. 21. 



Quarter-twist Belts. If two shafts are at right angles 

 to each other, and in different planes, for instance an engine 

 shaft near the floor and a line shaft on the ceiling, and one is 

 to be driven from the other, a belt with a quarter twist in it 

 may be used, providing that the distance between the shafts 

 is not too short. The belt must run squarely on to each 

 pulley and the point at which the belt leaves each pulley 

 must lie in the straight line formed by the intersection of the 

 planes passing through the center line of the face of each 

 pulley and perpendicular to their axes. This is shown in 

 the illustration Fig. 22. The driving pulley is AD and the 

 belt runs in the direction shown by the arrows. The point 

 C at the end of the horizontal diameter of the driven pulley 

 at which the belt leaves it is set so that it is vertically above 

 the point A at the end of the horizontal diameter of the 

 driving pulley where the belt leaves that pulley. Should 

 the shafts in question be respectively a horizontal and a 

 vertical one the relative arrangement of the pulleys would 

 be the same as shown in the illustration. In fact, it makes 

 no difference what position the shafts take so long as the 

 leaving points of the belts on the pulleys are in the straight 

 line determined by the intersection of the two planes as 

 described. The quarter-twist belt can be used only with 

 narrow belts say up to 6 in. wide and where the distance 

 between the shafts is considerable. For heavy belts and 

 short center line distances, guide pulleys or mule pulley 

 stands should be substituted as shown below. A quarter- 

 twist belt will drive in one direction only. 



Where the quarter- twist belt drives through a floor the 



