Mill Speed - Critical Speed

Mill Speed - Critical Speed

Mill Speed

No matter how large or small a mill, ball mill, ceramic lined mill, pebble mill, jar mill or laboratory jar rolling mill, its rotational speed is important to proper and efficient mill operation. Too low a speed and little energy is imparted on the product. Too fast and inefficient media movement (known as cataracting) will generate high impact but also greatly increase mill wear. Even faster speed will result in the media centrifuging inside the mill and virtually no milling or movement of media or product will occur. In most cases, the ideal mill speed will have the media tumbling from the top of the pile (the shoulder) to the bottom (the toe) with many impacts along the way. The ideal mill speed is usually somewhere between 55% to 75% of critical speed.

Critical Mill Speed

Critical Speed (left) is the speed at which the outer layer of media centrifuges against the wall.

Second Critical Speed (middle) is the speed at which the second layer of media centrifuges inside the first layer.

nth Critical speed (right) is the speed at which the nth layer of media centrifuges inside the n-1 layer

Calculating Jar and Jar Rolling Mill Speed

Calculating how fast a Jar needs to spin is a little tricky, but simple one you know the factors. We need to know the inside diameter of the mill, that determine what speed we want, the outside diameter of the mill and the diameter of the rollers. The last two diameters will tell us how fast the roller has to spin to achieve our Jar speed.

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