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Tales of the unexpected: “Whirling”

Machine components at a standstill may behave very differently when they are moving, even at relatively low speeds. A solid shaft able to support a hundred times its own weight plus the weight of the components mounted on it may, when rotating at certain speeds, bend and vibrate as though it were made of rubber. These speeds are called ‘critical speeds’ and the bending and vibration is known as ‘whirling’. It is often severe enough to cause a number of problems:

Damage to equipment near the rotating shaft.

Dramatic reduction of the fatigue life of the shaft.

Bearing damage.

Excessive machine noise and vibration.

Causes. Whirling is usually associated with fast-rotating or slender shafts. As the shaft rotates, there are always some radial forces due to unbalance or slight bends in the shaft. These forces increase as the shaft deflects. When a shaft deflects, however, there are restoring forces due to the stiffness of the shaft which tend to bring the shaft back to its undeflected shape. These restoring forces also increase as the shaft deflects. Below the critical speeds, the restoring forces increase with increasing shaft deflection faster than the radial forces, so the deflection is held in check. At the critical speeds, the restoring forces increase at the same rate as the unbalance forces, so they cancel each other out. Shaft deflection is therefore unchecked and the shaft behaves as though it is very flexible! Above the critical speeds the unbalance forces hold sway, and the shaft rotates about the centre of mass of the shaft assembly (which is usually very close to the centre of the shaft).

Remedies. It is recommended that the speed of the shaft or the critical speed be altered so that the shaft speed and the critical speed are widely separated. The critical speed of the shaft can be modified by changing:

The shaft stiffness.

The mass distribution along the shaft.

The location and number of bearings.

If a shaft must be run near a critical speed it is possible to provide stops to limit the vibration of the shaft. If a shaft is to be run above a critical speed, the shaft may be provided with vibration-limiting stops or be accelerated swiftly through the critical speeds so as not to allow time for vibrations to build up.
The key to avoiding trouble with whirling is awareness of the phenomenon. Once the risk of whirling has been established, appropriate steps can be taken to avoid premature failure