A New Basis for the Rating of Roller-Chain Drives

Abstract

Abstract Reference tables giving horsepower ratings for roller transmission chains are commonly based upon certain allowable working pressures per square inch of projected bearing area at the chain joints. The allowable working load is assumed to increase in proportion to the projected area of the pins and to decrease with an increase in chain velocity and an increase in the “centrifugal pull” of the chain. An analysis of the bending action of the links at both driving and driven wheels and the work of friction between pins and bushings yields an algebraic expression for the beat generated per minute, or for the rate of wear in the chain. This is proportional to the rate of elongation of the chain; and this rate of elongation is found to be directly proportional to the horsepower transmitted and inversely proportional to the pitch, the length of the bushing, the number of links in the chain, and the product of the number of teeth in the two sprockets (very nearly). A set of tables is developed based upon chain velocity and the number of teeth in both sprockets. The horsepowers taken from these tables are such as will produce approximately the same rate of elongation in all drives whose design and operation conform to average conditions. Similar tables are computed on the basis of the rpm of the smaller sprocket instead of the chain velocity. These tables also allow for the extra pull on the chain due to centrifugal force, and for the chain lengths and center distances necessary to keep the rate of elongation within the limits allowed. An examination of these tables shows that, with certain combinations of sprocket teeth and chain lengths, loads twice as great as those usually given in tables can be transmitted. And these figures seem to be supported by actual practice. The subject of limiting speeds for sprockets is treated and three sets of limiting conditions are investigated, namely: (a) the effect of a single impact between chain rollers and sprocket teeth, (b) the energy of impact per tooth per minute, and (c) the effect of centrifugal action. A table is compiled giving maximum chain velocities and sprocket speeds, based upon these investigations, for various chains and various numbers of teeth in the sprockets. This table shows the conditions under which certain chains can be operated at velocities above 3000 fpm and other conditions under which the same chains should not be operated at more than 240 fpm.