If a tire-mounted mill is ordered, it is essential to specify that the tires and rollers be made of steel either rolled or cast and not of cast iron. The inner ring should be rivetted, not bolted and there should be no sign of a leak, for the least bit of grit on the rollers soon ruins them. The roller shafts should be lubricated with a good grade of grease but the surface of the tires and rollers should be oiled with heavy black oil. Fig. 55 shows the Abbe style of tube mill bed plate with supporting rollers, thrust rollers and countershaft bearings for the drive end of the mill. The thrust rollers are for the purpose of preventing the mill from sliding off the supporting rollers, but a wellbalanced mill should float on the rollers and not touch the thrust rollers at all. Remembering, that the mill will travel toward the rollers that are spread apart the mill may be made to travel either way on the rollers by turning an adjusting screw a fraction of a turn. The rollers should be placed at an angle of 120, as shown in Fig. 56.

Fig. 57 illustrates a mill made by the Stearns Roger Co, the same being in use at the Liberty Bell mill, Colorado. In this mill the thrust rollers are absent and in their place we have a rim on the tires about 1 in. deep made of forged steel. This is an improvement because, if the shaft of a thrust bearing should break, there is nothing to prevent the mill from running off the tires, and such accidents may happen at any time.

With any type of bearing the mill should be revolved in the direction that will lift the gearing on the mill and press down that of the pinion shaft in order to avoid strains on the foundation bolts of the boxes.

Roller bearings have not generally been applied to tube mills because the usual mill is too heavy for this class of bearing but there is no reason why a medium-size mill run on tires should not be so provided. At the St. John del Key mill, Brazil roller bearings were put on the tube mills with an estimated saving of 7 hp. This is quite an item and it would be well for manufactures to pay more attention to this point.

The cost of grinding ores with a tube mill includes the three usual items of power, material and labor. The power as a rule being the greatest item of expense, every precaution should be taken to proportion the factors already discussed to obtain the maximum result with the minimum expenditure of fuel, and the type of transmission of power to the driving gears should be selected for efficiency with the first cost a secondary consideration. The cost for material should be only for the lining and pebbles, but renewal of tires and gears are often required due more to carelessness in operating than to actual wear of material. Clutches are often used for short periods and discarded being too weak to stand the starting strain of the mill, or belts are used too narrow for the work required and must be abused by stretching or " doping" until unfit for further work. A well-designed and well-operated tube mill will require but little expense outside of the pebbles and lining, but one that is neglected or abused will need frequent repairs and renewals of all the parts. The cost for labor outside of that required for relining the mill is a negligible quantity as the mill is usually run in connection with other crushers and the same labor may look after the tube mill, for outside of oiling and charging the new pebbles there is little work required until the mill is ready to be relined. At a certain mill in Nevada the labor cost amounted to over 30 cts. per ton due to poor operating and poor construction. The cost per ton of grinding in this mill is so abnormal that I have not included it in the list which follows.