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Design and Theory

A screw assembly has a maximum RPM which it should operate. The nut and screw both have individual maximum RPMs that should not be exceeded. But the limitations are dependent upon different factors for the screw than the nut. The maximum RPM the nut can operate and the maximum RPM the screw can operate at are usually different. Both maximums need to be considered when selecting a ball screw assembly.

The screw shaft can vibrate at high RPM

Screw Maximum RPM

Every ball screw, or any shaft, has a rotational speed at which the vibration and harmonics becomes excessive.
The screws tendency to vibrate is a function of:
  • Screw length
  • Screw diameter
  • Type of mounting (end fixity)
The RPM where this vibration becomes excessive is often referred to as the critical speed for the screw.  Exceeding this value may result in excessive vibration. For optimal performance, it is recommended to operate below 80% of the screw critical speed.

Ball nut maximum RPM

The ball nut maximum RPM or the RPM of the shaft inside the nut, is limited by the velocity of the ball bearings rotating through the ball nut. 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 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

Where 
n = rotation speed of the shaft
d0 = Nominal shaft diameter
DN = The speed rating for the nut

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 for actual values.

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