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Mill Critical Speed Determination

The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shell's inside surface. This is the rotational speed where balls will not fall away from the mill's shell.

Mill Inside Diameter:

Enter the mill diameter inside the shell (excluding liners). Most mill sizes are reported by vendors are outside the shellliners, but inside the shell itself.

Mill Shell Liner Width:

Enter the width of a mill shell liner. Note this is not the width of a lifter! You may use the Mill Liner Effective Width calculation to determine this value. The mill critical speed will be calculated based on the diameter (above) less twice this shell liner width.

Mill Actual RPM:

Enter the measured mill rotation in revolutions per minute.

Result #1: This mill would need to spin at RPM to be at 100% critical speed.
Result #2: This mill's measured RPM is % of critical speed.

Calculation Backup:
the formula used for Critical Speed is:
Nc =76.6(D-0.5)
  • Nc is the critical speed,in revolutions per minute,
  • D is the mill effective inside diameter, in feet.
source: SME Mineral Processing Handbook, Page 3C-4

Neither the author nor the web host will accept any responsibility for loss or damage caused by use of this program. The program has not been tested under stringent conditions that allow it to be considered robust enough for Engineering use. People using this program must themselves accept responsibility to confirm that results are correct and applicable to the application being calculated.