In the case of two pulleys of different diameters, made of the same material, connected by a belt, the belt will slip first on the smaller pulley, partly because the wrap of the belt is less upon that one, partly because the belt does not "hug" so tightly, owing to the smaller radius of curvature to which the belt must be bent. Sometimes the smaller pulley is lagged with rubber or leather so as to give it increased grip on the belt, thus making up for the tendency to slip, due to its small diameter.
On an inclined or horizontal drive the slack side should be on top, and the tight or pulling side underneath, as the weight and slackness of the belt will act together to cause it to sag and increase the wrap. It is a well known fact that the greater the arc of contact, the greater the driving force which may be obtained from the belt.
Vertical drives should be avoided as much as possible, as here the weight of the belt is always tending to decrease the "hug" on the lower pulley.
Increasing the diameter of pulleys, the same linear speed of belt being maintained, does not increase the power transmitted, except by permitting the belt to "hug" the pulleys more tightly; and the larger the pulleys, the better this condition becomes, providing we do not exceed a certain economic speed for the belt. Flexible link belting, made of small leather links joined together by steel wire, gives excellent results, especially when used on horizontal drives, but it is rather expensive to install. A pulley rim perforated with small holes, to prevent any air cushion beneath the belt, is another means of increasing the "hug".
Belt-adjusting devices are often provided for changing the distances between the pulleys, thus enabling the proper tightness to be always maintained. Motor and dynamo bases are provided with slides and set screws for such adjustment.
A tightener pulley is often used to increase the wrap of the belt or maintain the proper tightness. This is an idler pulley, which is weighted, or adjusted by screws against the belt. While such a pulley is a very ready and simple means of accomplishing the purpose, yet it should be remembered that the shaft carrying it is subjected to heavy pressure in its bearings, due to the belt tension; and the friction of the drive is considerably increased thereby. Tightener pulleys are used only when specific conditions prevent the results from being otherwise secured.
It is generally preferable to use belts of two or more thicknesses for the sake of side stiffness, and also in order that any local imperfections of the leather in one layer may be taken care of by the other. Where the belts are to be shifted laterally, stiffness is an important item. If too pliable a belt is used on cone pulleys, the edges are apt to curl up, and the belt tends to climb and chafe against the side of the step, twisting like a corkscrew, and sometimes jumping from one step to another.
A good distance between the centers of shafting for ordinary belt drives is from 20 to 25 feet. With greater distances, the belt is apt to flop and run in waves; while at a less distance, the necessary tightness of the belt results in undue stretching. In crossed belts, the above distances should be especially adhered to; for, with a wide, stiff belt and a short distance between centers, there is an excessive amount of rubbing on the sides of the belt, as well as strain caused by the twisting.
It is well to use as few crossed and quarter-twist belts as conditions will permit. In quarter-twist belts, the side angle, where the belt leaves the pulley, should be kept under 25°, as considerable power is lost in side slipping. For the least distance between the shafts a safe value is obtained if the distance is made not less than 2½ times the diameter of the larger pulley. A narrow belt gives better results than a wide one, on twisted belting.