The means by which a tube mill is made to revolve as well as the position of the driving mechanism in respect to the feed and discharge openings appears to be a matter of individual preference, as shown by the pinion shaft driven by belts, Fig. 49, silent chains, Figs. 50 and 51, and motors direct-connected to the pinion shaft, Fig. 52. Some operators have changed from belts to gears ostensibly because the belts have given trouble by slipping but more likely due to not having the belt wide enough; some have changed from gears to belts because the gears have given poor service. We see some machines driven from the feed end and some from the discharge end of the mill. For convenience in being out of the way of possible splash and dirt the latter method is preferred. Both the Hardinge mill illustrated, Fig. 52 and the cylindrical mill, Fig. 11, are driven from the discharge end. In some cases mills have been driven by two parallel belts from both ends. All the up-to-date mills are driven from the pinion shaft by herringbone gears which in the case of the Hardinge mills is said to effect a saving of at least 15 per cent, in power. The advantage of this type of gear is not only that it saves power but not being subject to the amount of vibration in the ordinary tooth gear, it allows a motor to be direct-connected to the pinion shaft. Care should be taken that the gear is properly installed in the first place or the advantages claimed will not materialize.

Mistakes are frequently made of using belts on the pinion shaft, which are entirely too narrow to pull the mill without stretching the belt to the point of ruin, for it must be remembered that the belt must be wide enough to start the mill from rest, an effort which, as we have seen, requires about twice the power necessary to keep it going. A 5 by 22-ft. mill with a 54-in. pulley on the pinion shaft requires a 14-in. belt, but designers will persist in recommending 10 and 12-in. belts for this work, and then blame the belt for slipping. There is no economy in narrow belts for this heavy work even if doubled; it is better to have a wide belt in the beginning with the pulleys of proper proportions and save trouble.

A tube mill must be started with a clutch on the countershaft, pinion shaft or engine. If the engine runs nothing but the tube mill, the engine shaft is a good place for the clutch; otherwise, it is better on the pinion shaft or countershaft. Wherever it is placed it should be of sufficient strength to start the mill from rest which, in the case of a 5 by 22-ft. mill, means a clutch rated at 100 hp. or over. Any clutch below this capacity for this size of mill will last but a short time.

The mill may be revolved on tires or on trunnions or both. For a long mill such as a 22-ft. mill, tires are preferred because the weight can be better distributed, resulting in less thickness of shell, and there being less friction on a 6-ft. diameter tire running on rollers than on trunnions, less power is required to rotate the mill. For a medium-size mill, such as those 18 ft. long, a combination of tire and trunnion illustrated in Fig. 53 may be used or a ball mill, Fig. 54 may be mounted in the same way.