SMALL SAWMILLS, THEIR EQUIPMENT, ETC. 23 



are also used in emergencies. Special endless belts with a diagonal 

 splice are supplied by some manufacturers. 



A belt should be capable of transmitting from 5 to 25 per cent more 

 power than is actually needed, the excess capacity being governed 

 by the type of drive, the smaller excess for heavy main drives, and the 

 greatest for machine belts. Never use belts the full width of the 

 pulley face, as a slight lack of alingment may cause part of the belt 

 to run beyond the edge of the pulley and perhaps against a shifting 

 finger or pulley flange. This is sure to result in loss of power and 

 perhaps in a badly damaged belt. On the other hand, a belt too 

 narrow necessitates high tension to transmit the required power, thus 

 causing excessive journal friction and the early destruction of the 

 belt. 



Generally speaking, single belts, if heavy enough to carry the load, 

 should be used on small pulleys. A single belt should never be wider 

 than 1^ times the diameter of the smallest pulley. Where small 

 diameter pulleys and the load would require an unusually wide single 

 belt, it is advisable to substitute narrower pulleys and a narrow 

 double belt. Thin wide belts give the best service; working verti- 

 cally, a thick narrow vertical belt will not grip the pulley well. 

 Double belts of medium or heavy weight should never be used on 

 pulleys less than 12 inches in diameter, or, even better, on pulleys 

 less than 20 inches in diameter. 



Belts too heavy for the load weave back and forth on the pulleys. 

 This is best illustrated by a belt working under intermittent loads, 

 which runs straight while carrying the maximum or proper load 

 but shows a tendency to weave when the load is reduced. 



The tightness with which belts are adjusted to the pulleys is of 

 prime importance. If they are put on too tightly, there is a large 

 unnecessary loss of power from excessive friction at the bearings, to 

 say nothing of the overstrain and injury to the belt itself. If, on the 

 other hand, the belt is too loose, it is likely to flop around and jump 

 from the pulleys, particularly when a load is suddenly thrown on 

 or off. The slacker a belt can be run up to a certain point while 

 doing its work satisfactorily, the greater the economy. In installing 

 belting and taking it up, it should be remembered that certain kinds 

 of belting are affected by weather conditions, lengthening and 

 shortening according to the amount of moisture in the air. Instances 

 have been known where the babbitt was melted out of the boxes and 

 even shafting pulled out of alignment as a result of belting being put 

 on too tight. 



Be sure to note carefully whether your shafting is properly in 

 line. More belting is ruined by improperly lined shafting and pul- 

 leys than in any other way. The belts under this condition are 



