In order to study simplicity, assume mill cylinder body just installed a ball, the ball as a particle to study, and no slippage between the ball and the cylinder wall. Under this assumption, the force acting on the ball only normal component N and centrifugal C. Due to centrifugal force so that the ball raised to a certain height, point A is reached when, C≤N, parabolic trajectory for the ball drop. A point is called off point, the angle α with the vertical axis of the radius of gyration OA folder called off the corner. If the mill speed increases, the ball will start Paola point increase. When the speed of the mill to a certain value nc, centrifugal force is greater than the quality of the ball, the ball is no longer up to the mill vertex Z whereabouts centrifugal operation occurs. Thus, the centrifugal operation is critical conditions: C≥G. Thus, the ball of the required critical speed centrifugation, depending on the distance from the mill center to the center of the sphere. The ball away from the center of the outermost layer farthest mill, make it happen Chi centrifugal operation required minimum speed; innermost ball away from the center of the nearest mill, making it the highest speed of centrifugation. If the result of the inner radius of the mill as a mill with a formula to calculate the rotational speed, even though the outermost ball centrifugal operation has started, but the ball still other layers can be thrown down, can still be milled ore. Only the mill speed is higher than the above calculated rotational speed in many cases, all of the ball layer will be centrifuged, thereby reducing the useful work, it is actually the critical speed of the mill so that the outermost ball barrel highest centrifugal operation does not occur speed (r / min). 

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