278 The N.Z. Journal of Science and Technology. [Sept. 
compared with direct-current machines, make them particularly suitable 
under adverse conditions such as are met with in every mill. As to the 
type of alternn ting-current motor adopted, it may be said that for small 
sizes the squirrel-cage type is the most suitable, but it has disadvantages 
which are well known, and when larger sizes are reached it is preferable to 
use a slip-ring motor rather than a squirel-cage motor and clutch. The 
slip-ring motor can usually stand up to all mill requirements, and it is very 
rarely necessary to adopt a clutch with this type. 
The experience gained in the electrification of the South African mills 
is very valuable, and the various papers on this subject in the Transactions 
of the Institute of Electrical Engineers are of great assistance to any one 
interested in this subject. 
The various departments of a mill may be summarized thus :— 
Crushing.— The ore is dumped into crushers, usually of the gyratory 
type. The load is variable, but a considerable percentage of power is 
taken in driving the crushers themselves when no ore is being broken. It 
is usual to drive these machines by ropes or belts, the reduction in the 
crusher being through bevel gearing. The starting is fairly heavy, but 
a slip-ring motor is suitable, although sometimes this type of motor is 
provided with a clutch for this class of work. 
Stamps. —In laying down new plants it is customary for each motor 
to drive 10 heads of stamps, and with the heavy stamps now in use this 
requires about a 50-horse-power motor. The stamps make about 100 drops 
a minute, corresponding to a speed of 50 revolutions per minute of the 
cam shaft; the ratio between this speed and the motor-speed being large, 
an intermediate shaft is necessary, and this shaft runs at a speed of 100 
revolutions per minute. The low belt-speed necessitates large belts, and a 
jockey pulley is invariably employed. In the South African mills slip-ring 
motors, running at a speed of 600 revolutions, are employed, and in the 
Victoria mill at the Waihi Gold-mining Company one 300-horse-power 
synchronous and two 110-horse-power slip-ring motors are used, the speed 
of these motors being 300 revolutions per minute. It is necessary for 
efficient duty to keep the number of drops per minute constant. 
Tube Mills.— Tube mills are employed for the purpose of grinding the 
product received from the stamps, and the fineness of grinding required 
depends largely on the method subsequently adopted in the treatment plant. 
At the Victoria mill the tube mills employed are each 18 ft. long by 
4 ft. 9 in. diameter (inside shell measurements) ; they carry a load of about 
5|- tons of flints, and when running at a speed of 26 revolutions per minute 
require about 55 horse-powerA 
The mills in use in South Africa are considerably larger and take about 
120 horse-power, and are each driven by a separate motor. Direct driving 
through gearing has been tried, and even machine-cut double helical gearing 
has been used, but has not been very successful owing to the end move¬ 
ment of the mill in its main trunnions. It is customary to drive by belt, 
the final reduction being by spur gearing. 
Treatment Plant— Power is required in this department for driving 
elevator-wheels, vanners, air-compressors for agitating, vacuum pumps, and 
cranes in connection with the slimes plant, disintegrators, and sundry pumps 
for pumping slimes, solutions, and water. In most of these cases it is usual 
to drive by either belts or ropes, with the exception of the pumps, which, 
should they be of the centrifugal type, are generally direct-coupled to the 
motor. None of these drives calls for comment. The larger motors are 
