The breaking tensile strength of leather belting varies from 3,000 to 5,000 pounds per square inch. Joints are made by lacing, by metal fasteners, or by cementing. The strength of a laced joint may be about 7/10, of a metal-fastened joint, about ½, and of a cemented joint, about equal to the full strength of the belt cross-sectional area. The proper working strength of belting depends on the use to which the belt is put. A continuously running belt should have a low tension in order to have long life and a minimum loss of time for repairs. For double leather belting it has been shown that a working tension of 240 pounds per square inch of sectional area gives an annual cost - for repairs, maintenance, and renewals - of 14 per cent of first cost. At 400 pounds working tension, the annual expense becomes 37 per cent of first cost. These results apply to belts running continuously; larger values may be used where the full load comes on but a short time, as in the case of dynamos.
Good average values for working tensions of leather belts are:
Cemented joints, 400 pounds per square inch. Laced joints, 300 " " " "
Metal joints, 250 " " " "
If P is the driving force in pounds at the Tim of the pulley, and V is the velocity of the belt in feet per minute, the theoretical horse-power transmitted ii evidently:
H.P = P x V / 33,000. (9)
It is evident from the above that the horse-power of a belt depends upon two things, the driving force P and the velocity V. If either of these factors is increased, the horse-power is increased. Increasing P means a tight belt. Hence a tight belt and high speed together give maximum horse-power. But a tight belt means more side strain on shaft and journal. Therefore, from the standpoint of efficiency, use a narrow belt under low tension at as high a speed as possible.
Empirical rules for horse-power of belting, if used with judgment, give safe results when applied to very general cases. A common rule used by American engineers is:
H.P. = b x v / 1,000. (10).
For a double belt, assuming double strength, this becomes:
H.P. = b x v / 500. (11)
With large pulleys and moderate velocities, this may hold good. With small pulleys and high velocities, however, the uncertain stresses induced by the bending of the fibers of the belt around the pulley, and the relatively great loss due to centrifugal force, modify this relation* and a safer value for a double belt of the ordinary kind is:
H.P. = b x v / 540 ; (12) or, still safer, H. P. = b x v / 700. (13)
If we compare the theoretical value of equation 9 with the empirical value of equation 10 by putting them equal to each other, thus:
H.P. = PXV/ 33,000 = bxv/1,000.
And solve for P, we get :
P = 33b. (14)
This develops the fact that the empirical rule of equation 10 assumes a driving force of 33 pounds per inch of width of single belt.
Another way of expressing equation 10 is: A single belt will transmit one horse-power for every inch of width at a belt speed of 1,000 feet per minute.