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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 ]
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.