(286) The means just described are those which are in use on a large number of mules constructed by Messrs. Platt, and worked satisfactorily until the speed of the rim shaft was largely increased. Up to about 750 revolutions the train of gearing driving the taking-in side shaft could be used, but as the rim is now run at speeds as high as 900 revolutions it is the practice to drive the taking-in side shaft by means of a grooved pulley fastened upon it, and driven by a separate band from the counter shaft. In this way much of the strain is taken from the rim shaft, and the use of gearing obviated for the taking-in and backing-off. When this method is adopted—as is now almost generally done—there are many advantages gained, and it is the most modern practice.
(287) Another method of driving, also largely employed by Messrs. Platt, is a patented system of duplex driving. This is shown in plan in Fig. 157. Instead of using one belt only, by means of which the power is transmitted, two narrower ones are employed, each of which is 2-3/4 inches wide. The fast pulleys =A= are also 2-3/4 inches on their face, while the loose pulleys =B= are 3 inches wide. The strap guide is made, as shown, double, and the distance which it has to traverse is only half that which is usual. The advantages of this arrangement are derived both from the smaller width of the belts, and the shorter distance they need moving. The diminished width causes the belt to be more pliable and less rigid, and in consequence the pressure applied is more readily responded to. The shortened traverse enables the change of the belts to be made more easily and in less time, and, in consequence of the latter fact, the time the edges of the belts are pressed upon by the guider is reduced. This reduction involves considerably less wear of the strap edges, which, alike on this account and because of their easier and less strained motion, are found to have a much longer life. The smooth action of the belts produces another effect. It enables the full speed of the rim shaft to be more readily reached, and so tends to increase the production of the machine. The makers have now constructed a large number of mules with this arrangement, and its use is steadily extending.