Vibration in Rotating Equipment

Imbalance in Rotating Machinery

Examples of mass imbalance: unbalanced propeller and uneven distribution of electrical winding

The word imbalance, unbalanced, and unbalanced mass are interchangeable.

Vibration in rotating machinery is commonly caused by imbalance. Imbalance is a condition where a shaft’s rotational axis and weight distribution axis (mass of centerline) do not coincide due to uneven distribution of mass around a rotating shaft’s centerline.

They are three types of mass imbalance:

1. Static imbalance or forced imbalance
2. Couple imbalance
3. Dynamic imbalance or quasi-static imbalance

Static imbalance, which is also known as forced imbalance occurs when a heavy spot of on a rotor is located at the mid-point between the bearings that support the rotor. In practice, static imbalance is found in rotors that are short compared to their diameter. A static imbalance is identified by weight distribution axis parallels the rotational axis.

Couple imbalance occurs when the center point of weight distribution axis coincides with the center of point of rotational axis. This situation causes the rotor to develop an end to end rocking action as it turns. Couple imbalance differs from static imbalance: if supported between 2 frictionless centers will always come to rest with heavy spot at the lowest point, while a rotor with couple imbalance will come to rest at random points. A couple imbalance is identified by the rotational axis intersects the center of gravity on weight distribution axis.

Dynamic imbalance, which is also known as quasi-static imbalance occurs when the rotational axis and the weight distribution axis do not intersect at all. This is a combination of static and couple imbalances and is commonly observed imbalance in rotating machinery. Dynamic imbalance is identified by the weight distribution axis and the geometric axis does not intersect at all.

They are a number of reasons why a machine may not be balanced:

1. Corrosions or erosion of rotors
2. Uneven mass distribution in electrical windings
3. Deposit of dirt on rotors
4. Manufacturing defects
5. Material imperfections due to cavities, bubbles, foreign materials, etc
6. Eccentric rotors
7. Cracked fans/damaged fans
8. Incorrect key hole
9. Machining errors