What is "critical speed"?

Round rod-shaped objects such as the screw shaft of a ball screw are always subject to deflection due to bending and its own weight. When such a round rod-shaped screw shaft is supported by bearings on both ends and rotated, and the rotation speed is increased, the deflection increases and the rotating shaft may eventually be damaged.
Rotation speeds that create a dangerous condition that could damage the rotating shaft are called "critical speeds." They are also called "whirling speeds" or "whipping speeds." Critical speeds require careful consideration when designing the screw shaft of a ball screw.

This can be found by calculation or from the allowable rotational speed diagram (see catalog). From this, we can see that the thinner and longer the ball screw, the lower the critical speed, as it can be in a dangerous state even at low rotation speeds. Also, imbalance in the rotating system can be a factor in generating abnormal vibration. Please take care to ensure that there is no eccentricity between the screw shaft and its support, and that no bending force is applied.

Unlike a general transmission shaft, a ball screw has a nut that strokes, so it acts as a moving bearing, and does not always rotate in the same direction, but stops and reverses the rotation at the stroke end. Therefore, the vibration amplitude of a ball screw is said to be smaller than that of a general transmission shaft under the same conditions.

<Reference> When designing a screw shaft, it is necessary to check the DmN value as well as the critical speed.

If a ball screw is used at a rotational speed above the critical speed, the vibrations it generates can cause problems with the accuracy of the machinery. It is recommended that Ball Screws be used at a rotational speed below 80% of the critical speed.
The critical speed can be increased by considering the installation method of the ball screw (fixed-supported, etc.). It is also possible to reach a higher rotational speed by changing the bearing combination when installing the ball screw. As a rough guide, please contact KURODA if the ball screw rotational speed setting exceeds approximately 2,000 min^-1.