Ball Screws Top Ball Screws Q&A What is Critical Speed?
Ball Screws and Ballscrew Actuators Q&A
Reasons for Preloading a ball screw What is Critical Speed?
What is Basic Dynamic Load Rating? What is the purpose of Lubrication?
What is Basic Static Load Rating? I want to know about Accuracy Grades
How is Ball Screw Life Calculated? What about Material and Heat-Treatment?
What is Backlash? I want to know about Ball Screw Noise
What is Ballscrew Actuator?
What is Critical Speed?

An elongated cylindrical object such as a ball screw shaft will always have a certain amount of sag at the middle due to its own mass (weight).  If such cylindrical object (a screw shaft) is held by rotary bearings at its ends and rotated, the sag would increase as the rotational speed is increased and may eventually reach a speed point that is destructive to the shaft.  This potentially destructive rotational speed is called the "Critical Speed".  It is also referred to as "Whirling Speed" or "Whipping Speed".
The Critical Speed parameter should be paid a high degree of attention when designing ball screw driven systems.  The Critical Speed can be obtained by calculating, or from "Allowable Speeds" nomogram provided in our catalog.  It can be observed from the nomogram that smaller the diameter and longer it is, the shaft will become critical at lower speeds (Critical Speed is low).   In addition, any imbalances in other rotating elements within the system will contribute in abnormal vibrations.  Careful design considerations ensuringto eliminate any axial concentricity errors in the support bearings, and to avoid any bending forces on the screw shaft.
Since the nuts travel in ball screws acting as moving supports, and the operation of ball screws involve reversing at their stroke ends, it is generally understood that the oscillation amplitudes of ball screws are smaller than that of common drive shafts.
<Note> When designing with ball screws, evaluation of "DmN value" is also required in addition to the Critical Speeds.
If ball screws are operated beyond their critical speeds, the accuracy of the machinery systems may be degraded due to the vibrations generated by the screw shaft.  It is recommended that all ball screws to be operated at 80% or below of their Critical Speeds.  Furthermore, the Critical Speeds may be improved by evaluating the shaft mounting methods (Fixed – Supported, etc.).  Selections in shaft-end bearings may also help to extend the high-speed capabilities.
As a general rule of thumb, please contact Kuroda Technical Support for consultations if the ball screws are to be operated at speeds higher than 2,000 rpm.