TECHNICAL NOTES 8 GRINDING R P KingThe critical speed of the mill c is defined as the speed at which a single ball will just remain against the wall for a full cycle At the top of the cycle =0 and Fc Fg mp 2 cDm 2 mpg c 2g Dm 1/2 The critical speed is usually expressed in terms of the number of revolutions per second Nc c 2 1 2 2g Dm 1/2 2× 1/2