The critical speed of a ball screw is the rotational speed at which the screw starts to resonate and “whip" like a jump rope. It depends mainly on the screw diameter, unsupported length, and end support condition.

You can evaluate it in three steps:

As a practical example, take a common SFU1605 screw with standard fixed–supported (BK12/BF12) mounts:

• Up to ~1000 mm: Usually safe to run in the 2000–3000 rpm range.

• 1000–1500 mm: Becomes a warning zone. It is safer to limit the top speed to 800–1000 rpm.

• Longer than 1500 mm: The risk of whipping increases significantly. You must calculate the specific limit or upgrade the design.

The critical speed is inversely proportional to the square of the unsupported length.

• Physics: If you double the length, the allowable speed drops to one quarter.

• This is why long, thin screws are so difficult to spin fast. Even a small reduction in unsupported length (minimizing overhang) can produce a big improvement in permitted RPM.

• Option A – Larger Diameter (Most Direct):

  Stiffness grows with diameter. Upgrading from 16 mm to 20 mm or 25 mm significantly increases stiffness, allowing higher speeds for the same length.

• Option B – Improve End Supports (Most Economical):

  Standard mounts are usually Fixed–Supported. If you upgrade to Fixed–Fixed (fixing both ends with BK units and applying tension/stretching to the screw), the critical speed can increase by ~50%.

  Note: This requires precise mounting alignment.

• Option C – Rotating Nut (Ultimate Solution for Long Axes):

  Once travel exceeds 2–3 meters, spinning the screw becomes impractical.

  The solution is to keep the screw stationary and rotate the nut (using a rotating nut assembly). Since the screw doesn't spin, there is no whipping, allowing for high speeds over very long distances.