the majority of the ore 45 is stirred around by the rotation whereby ore particles are reduced in size by abrasion and shearing forces at higher rates of rotation the ore 45 begins to fall down in the drum 42 as in a waterfall u201c tumbling u201d i e the ore particles fly freely through the drum 42 and then hit against its walls or against ore particles remaining in front of the wall whereby the ore particles are broken up by the impact at intermediate rotation rates both of these scenarios can be present at the same time at particularly high rates of rotation the ore 45 is centrifuged i e is pressed against the drum wall with the result that the individual ore particles do not break up any more the bundling and the tumbling movement characteristics of the ore 45 each has specific advantages in relation to the size reduction where these advantages depend on the nature of the ore to be milled in principle most types of ore require at least a certain proportion of tumbling motion in the drum 42 so that larger and dense ore particles are broken up for this reason it is often desirable to rotate the drum 42 at a relatively high rate of rotation to ensure a tumbling movement of the ore 45 within the drum 42 typically the drive is at rotation rates above 80 of a critical rotation rate where this critical rotation rate specifies a value at which the outermost layer of the ore 45 is already being centrifuged in the recent past even higher rotation rates have.