The power required to rotate a tube mill depends upon the speed of rotation, the weight of the mill, the character of the lining, the load of pebbles, the consistency of the pulp and the character of the power transmission. The following table will serve as a guide, and although not complete in details it may be used for making estimates.

The list of 5 by 22 ft. is the most interesting as it shows that the increased tonnage obtained by carrying the load above the center involves but a slight increase of power: it also shows the increase of power required when the mill is revolved at 32 r.p.m. To compensate in power for rotating the mill at 32 instead of 27 r.p.m. the tonnage should be at least double that at the latter speed.

The mistake is too often made of underestimating the amount of power required to start and run a tube mill. The mill must be started from rest with the pebbles imbedded in mud and even if the mill is rocked back and forth with a clutch the power required to start the mill is far above that required to keep it running once it has attained its maximum velocity. The diagrams, Figs. 47 and 48, given to me by H. H. Seeber of the Winona Copper Co. show the starting current of the motors used to start two sizes of Hardinge conical mills. It will be observed from the curve that for the first 5 sec. the motors must bear a great overload. At the Liberty Bell mill a 75-hp. motor is used to start the 5 by 22-ft. mill while the running load varies from 45 to 48 hp. Probably the 75-hp. motor takes at least a 25 per cent, overload to start the mill from rest. At the Montana-Tonopah a 5 by 20-ft. mill requires 60 hp. to start and 42^ hp. to keep running.

We may consider that a tube mill requires twice the amount of power to start, that it requires to keep it running.

If the source of power is an electric motor this overload must be provided for in the rated capacity of the motor and in its guaranteed overload for a given time; if run by an explosive gas engine the flywheels must be of sufficient weight to store up the momentum necessary to start the. mill without killing the engine. The momentary overload capacity of an electric motor being high and that of an internal combustion engine being low, it is not necessary to have a flywheel with the former, as it is essential with the latter.

A. M. Merton1 gives the following formula for calculating the power required to revolve a tube mill when 60 per cent, of the volume of the mill is occupied with pebbles, moisture in feed 38 to 40 per cent, and the peripheral velocity of the mill 400 ft. per minute.

Horsepower = 0.25 X cu. contents in feet (of pebbles).

This is a near approximation and figures out, for a 5 by 22-ft. mill, 52 hp.

When stopping a tube mill it is well to thin out the pulp, as this will help to start the mill from rest, and when starting the mill it should be flooded with water or solution, as when at rest, the solution runs out of the mill leaving a thick mud which, with the pebbles, forms an almost solid load.