In the early mills, the material was ground in batches, the mills being charged and discharged through doors in the shell. The shell was supported by means of solid trunnion shafts resting in plain bearings and the drive was accomplished through one of these shafts. In some later batch mills, the mill shell was supported on rollers which were also used to drive the mill. At an early stage, continuous operation was made possible by feeding and discharging the mill through hollow trunnions, the method still in most widespread use.
The effect of size on the design of grinding mills can be Illustrated by two very simple excursions into dimensional analysis and strength of materials.
In practice, the unit loads on the bearings of small mills are relatively low in magnitude. However the effect of increasing size is such that in mills which were considered large 5 to 10 years ago the bearing pressures (in conjunction with other factors to be discussed later) were approaching a critical level. Unfortunately, the requirements for material flow through the trunnions dictate their geometry to some extent so that the only methods available for relief of the situation are either to increase the