Grate Ball Mill. 1. Grate liner, 2. Bearing inner cover, 3. Hollow shaft 4.dustpan shape liner 5.central liner, 6. Rib 7. Wedge block. The grate ball mill is usually used in the first segment of the two stages, or place of discharging the rough ore. Overflow Type Ball Mill. 1. Cylinder 2,3. Cover end, 4. Main bearing 5,Hollow shaft 6. pinion 7.