24 BULLETIN 718, U. S. DEPARTMENT OF AGRICULTURE. 
either kept on the pulleys by guides or rub against hangers and tear 
or stretch on one edge. Belts so stretched will not run straight and 
may slip the pulleys at any time. Remember that because your shaft- 
ing was once in line it does not follow that it will stay so indefinitely. 
It is a very simple matter to connect two pulleys by a band in such a 
way that when one pulley is turned the other will go round. It is 
not at all a simple matter properly to proportion a drive and select 
the size and quality of belting which will transmit power most effec- 
tively and economically. 
Shafts should not be located too close together. There should 
be distance enough between them to allow the belt to recover some- 
what from the strain applied on the tight or working side. This 
distance depends entirely upon the size of the belt and pulleys, but 
should be sufficient to allow some slight sag to the slack side of the 
belt. Neither should the shafts be too far apart, for in such cases 
the weight of the belts draws heavily on the shafting and increases 
the friction load in the bearings, at the same time causing the belt 
to swing from side to side and sometimes to run off the pulleys. 
This constant swing is sometimes the cause of crooked spots in the 
belt, which wear out rapidly. 
It is the best and most economical practice to use pulleys of large 
diameters, thus obtaining a high belt speed, which means a saving 
both in the transmission of power and in the wear and tear on the 
equipment. The speed of the belt being the same, pulleys of large 
diameters effect a slightly greater transmission of power than do 
those of small diameter. The speed of the shafts being the same, 
the advantage in favor of large pulleys over small ones is in pro- 
portion to their diameters. Wooden or leather-covered pulleys have 
a greater transmitting power than iron. Cold-rolled shafting is 
said to have 30 per cent greater strength than hot rolled. The usual 
diameters for shafting are from l| to 3J inches. The proper speed 
is from 300 to 400 revolutions per minute, and its transmitting power 
D 3 y^R 
is given as — ~~— =H. P. D is diameter of shafting in inches, R 
revolutions per minute. 
Shafts that are to be connected with each other by belts should be 
far enough apart to allow a gentle sag to the belt when in motion. 
"When narrow belts are to be run over small pulleys, 15 feet is a good 
average, the belt having a sag of from 1^ to 2 inches. For larger belts 
working on larger pulleys the distance should be from 20 to 25 feet, 
with a sag of from 2^ to 4 inches. For main belts working on very 
large pulleys the distance should be from 25 to 30. feet, with a sag 
of 4 or 5 inches. If the distance between pullies is too great the belt 
will have an unsteady flapping motion which will tend to destroy 
