BELTS. Recent experiments on belting (see Trans. A. S. if. E., ii, 91 and 224; vii. 347 and 540; viii. 520; and x. 765) all tend to confirm the statement made in Vol. I of this work, that on the amount of power that can be transmitted by a belt of given size show many discrepancies, which seem to be due to the fact that belts of different quality were experimented upon ; and it is pretty well settled that, while rules can be constructed that will show what power a good belt may transmit under given conditions, they can not be implicitly relied upon to show how much power a particular belt does transmit.." An elaborate set of experiments on belts was made in 18::15 by William Sellers S.: Co., and reported by Mr. Wilfred Lewis (Trans. A. S. X. E., vol. vii, p. 349). These experiments seemed to show that the principal resistance to straight belts was journal-friction, except at very high speeds, when the resistance of the air began to he felt. The resistance from stiffness of belt was not apparent, and no marked difference could be detected in the power required to run a Wide il()Ilble belt or a narrow light one for the same tension at moderate speeds. With crossed and quarter-twist belts, the friction of the belt upon itself or upon the pulley in leaving it was frequently an item of more importance, as was shown by special experiments for that purpose. The experiments also showed that, in all probability, leather is more elastic under light tensions than it is under high ones. A piece of belting 1 sq. in.
in section and 1/2 in. long was found by experiment to elongate in. when the load was increased from 100 to 130 lbs., and only in. when the load was increased from 450 to 500 lbs. The total elongation from 50 to 300 lbs. was in., but this tvOnld vary with the time of suspension, and the measurements here given were taken as soon as possible fitter apply ing the loads. In all cases the coefficient of friction was shown to increase with the per centage of slip. An interesting feature of these experiments is the progressive increase in the sum of the licit tensions during an increase in load. This is contrary to the generally accepted theory that the sum of the tensions is constant. The highest coefficient obtained was 1-07, lint. of course, this was temporary. The diameter of the pulley also appears to affect the coefficient of friction to some extent. This is especially to be noticed at the very slow speed of 18 revolutions per minute on 10-in. and 20-in. pulleys, where the adhesion on the 20-in. pulleys is decidedly greater : but, on the other hand, at 160 revolutions per minute, the adhesion on the 10-in. pulleys is often as good as, and sometimes better than, appeals for the 20 in. at the same velocity of sliding. It might be possible to determine the effect of pulley diameter upon adhesions for a perfectly dry belt, where the condition of its surface remains uniform: but for belts m ordinarily used it would be very difficult, on account of the ever changing condition of surface produced by slip and temperature. It is generally admitted that the larger the diameter the greater the adhesion for ally given tension, but no definite relation has ever been established, nor, indeed, does it seem possible to do so, except by the most elaborate and extensive experiments. Theoretical formulal hitherto used in calcula
tions of belt-power have assumed the coefficient of friction as uniform around the are of con tact, but this can no longer be correct if the coefficient varies with the pressure. Nr. Lewis says the driving-power of a leather belt depends upon such a variety of conditions that it would be impracticable, if not impossible, to correlate them all • and it is thought better to admit the difficulties at once than to involve the subject in a labyrinth of formula: which life is too short to solve. Mr. Lewis estimates that under good working conditions the of belt transmission may be assumed to he 97 per cent. When a belt is too tight there is a constant waste in journal-friction, and when too loose there may be a much greater loss in efficiency from slip. The indications and conclusions drawn from his experiments are as follows: 1. That the coefficient of friction may vary miller practical working conditions from 2.5 to 100 per cent. 2. That its value depends upon the nature and condition of the leather, the velocity of sliding, temperature, and pressure. 3. That an excessive amount of slip has a tendeney to become greater and greater, until the belt finally leaves the pulley. 4. That a belt will seldom remain upon the pulley when the slip exceeds per cent. 5. That excessive shilling dries out the leather, and leads toward the condition of minimum adhesion. G. That rawhide has much greater adhesion than tanned leather, giving a coefficient of 100 per cent, at the moderate slip of 5 ft. per minute. 7. That a velocity of sliding equal to of the belt-sileed is not excessive. S. That the coefficients in general use are rather below the average results obtained. 9. That, when suddenly forced to slip, the coefficient of friction becomes inomentarily very high, but that it gradually decreases as the slip continues. 10. That the satin of the tensions is not constant, but increases with the load to the maximum extent of about 33 per cent with vertical belts. 11. That, with horizontal belts. the sum of the tensions [nay increase indefinitely as far as the breaking strength of the twit. 12. That the economy of belt transmission depends principally npon journal-friction and slip. 13. That it is important on this account to make the belt-speed as high as possible within the limits of 5,000 or 0,0110 ft. per minute. 14. That qnarta•-twist bolts should be avoided. 15. That it is preferable in all cases, from eonsiderations Of economy in wear on belt and power consumed, to use all intermediate guide-pulley, so phieed that the belt may be in either direction. 10. That the introduction of guide and carrying pulleys adds to the internal resistances an amount proportional to the friction of their journals. 17. Thant there is still need of more light on the subject.