Several types of ball mill drives can be furnished, made up of various combinations of gearing, motors and transmission equipment. The correct combination to be selected takes into consideration power requirements, gear ratings, floor space, interference from other plant equipment and motor characteristics. The main types are described and illustrated on these pages.
Motors considered are the squirrel cage motor, wound rotor motor, and synchronous motor. One important advantage of the synchronous motor is the possible correcting of power factor within your operation, through its use. When using a synchronous motor we recommend having 200% starting torque, 120% to 130% pull-in torque, and 225% pull-out torque. These assume no greater than a 10% voltage drop at the motor terminals.
V-belt or poly-V drives are generally used on grinding mills consuming 350 HP or less. Where power exceeds this, drives become so large and motors become special, resulting in an uneconomical application. Generally the driven mill sheave is of split hub construction to facilitate field assembly. V-belt drives allow use of higher speed motors, generally low in initial cost. Motor speeds should be between 580 and 1160 rpm. The lower speeds apply to large diameter mills, the higher speeds to the smaller.
When power exceeds 300 HP a direct connected drive is recommended. This drive is also applicable for lower power requirements if desired. Synchronous motors are preferred, but in some cases wound rotor motors may be used. Motor speeds will vary from 225 rpm to 450 rpm.
The motor and pinion shafts are connected together through a flexible coupling. This drive is the most compact, resulting in minimum floor space. In cases where motors must be placed away from the mill a pilot shaft extension with flexible couplings can be provided.
This is essentially a direct connected drive permitting the use of higher speed motors. The motor shaft is connected to the high speed shaft of a reducer by a flexible coupling and similarly the low speed shaft and pinion shaft are coupled.
Any direct connected drive necessitates perfect alignment of all rotating shafts. With Marcy Mills the pinion shaft is fixed in position. Any gear adjustments are made by moving mill gear into proper mesh with the pinion.