TABLE OF CONTENTS Many engineers fail to realize that the number of teeth are not the only consideration in computing gear trains until, on attempting to mount them, the gears either interfere with each other or with a part of the machine, or else they do not connect the distance between the shafts. Such troubles are almost invariably avoidable because they are based on insufficient considerations in specifying a gear train.
Another aspect often overlooked is that change-gears are subject to considerations of strength. Although ignoring this factor seldom leads to destruction of the gears, it is obvious that an arrangement of the gears which reduces the loads on the teeth guarantees a longer useful life of the gears and a more exact transmission of the motion.
Additional problems arise if the change-gears are to be used for the purpose of indexing. Indexing depends on the number of teeth and the locations in which the gears are mounted. Fortunately, the same simple rule can be applied to mountability and considerations of strength. However, the requirements for indexing are independent of, and in some cases even contrary to, those for mountability and for reducing the loads on the teeth, so that compromises are sometimes necessary.
If, in the case of a two-gear train, the same torque is transmitted by gears of different diameters, the loads on their teeth are inversely proportional to their diameters. In other words, the larger the gear, the smaller the load on its teeth. As the...
