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Ball nut critical speed (maximum RPM)
The ball nut critical speed, or maximum RPM of the shaft inside of the nut, is limited by the design of the nut and is a function of the velocity of the ball bearings.  Exceeding the ball nut maximum RPM may result in permanent damage to the ball re-circulation components. The ball bearings may break out of or jam the return system.  If that happens complete failure of the unit will occur.  Never exceed the ball nut maximum RPM.

The ball nut RPM limit due to ball bearing velocity is often expressed in the form of a DN number.  Given the DN number for a nut the maximum RPM can be calculated using the following formula:

  n = DN/d0

n = rotation speed of the shaft [RPM]
d0 = Nominal shaft diameter [in or mm]
DN = The speed rating for the nut [in-RPM or mm-RPM ]

The DN calculation is less accurate as it approaches smaller screw diameters and can give some unrealistic answers.  For RPM's over 4,500 RPM a detailed design evaluation is recommended.

Some typical DN values for several ball return designs are:
  • 76,200 mm-RPM (3,000 in-rpm) for Standard external return systems
  • 133,300 mm-RPM (5,250 in-rpm) for external returns systems with solid deflectors
  • 140,000 mm-RPM (5,510 in-rpm) for internal transfer designs
See product specifications to confirm values.  Some products simply list the maximum RPM instead of supplying the DN value.
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