According to the International Organization for Standardization (ISO), unbalance is “that condition which exists in a rotor when the vibratory force or motion is imparted to bearings as a result of centrifugal forces”. In other words, unbalance is an unequal weight distribution of a rotor around its rotating centerline.

The rotating centerline or rotational axis of a rotor is identified by the bearing centerlines. This is the expected rotational centerline if there is no unbalance.

The Center of Gravity (CG) is the point on a rotor where the weight is equally distributed.

The Principal Inertia Axis (PIA) or Central Principal Axis is the line upon which the rotor’s weight is equally distributed. If no unbalance exists the rotating centerline and Principal Inertia Axis will be the same. With unbalance added to the rotor it will attempt to rotate around the PIA which imparts force on the bearings and the need to balance the part. 

Static Unbalance

Static Unbalance occurs when the unbalance is distributed equally around the center of gravity and is on the same side of the rotor. The Principal Inertia Axis is parallel to the rotating centerline. If a rotor was put on two knife edges the unbalance would always fall to the bottom. Static unbalance can be corrected in one plane (Figure 1). 

Couple Unbalance

Couple Unbalance occurs if the unbalance occurs at an equal distance from the center of gravity (CG) but 180 degrees on the opposite side of the rotor. Here the Principal Inertia Axis and the Center of Gravity intersect. Typically you would balance the part in at least two planes (Figure 2).

Dynamic Unbalance

Dynamic Unbalance is the combination of the static and couple unbalance. The Center of Gravity and Principal Inertia Access do not Intersect and the Principal Inertia Axis and rotating centerline are not parallel. Typically this would require correction in at least two planes (Figure 3).

Want to learn more about unbalance? Take a look at some of our additional resources below.

A Handy Guide to Balancing Rotating Machinery

Video – Types of Unbalance

Video – Rotational Unbalance

Identifying Unbalance and Misalignment

Balancing with the OneProd Falcon

Single Degree of Freedom

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