· The mill normally operates at 3 150 rpm 1 Outboard Motor Bearing Legend 2 Inboard Motor Bearing 3 Inboard Mill Bearing 4 Outboard Mill Bearing Bearing that was failing The mill rotor bearings which are mounted in a steel housing are two 2 cylindrical roller bearings NU 316 ECM C3 and one 1 grooved ball bearing SKF 6316 C3