Whirl

Rotating shafts tend to bow out at certain speeds and whirl in a complicated manner. Whirling is defined as the rotation of the plane made by the bent shaft and the line of centres of the bearing.

The phenomenon results from various causes such as:

• mass unbalance
• hysteretic damping on the shaft
• gyroscopic forces
• fluid friction in the bearings

The whirling may take place in the same or opposite direction as shaft rotation and the whirling speed may or may not be equal to rotational speed. Synchronous whirl occurs when the rotational frequency coincides with a flexural natural frequency of the shaft.