Brush Up On Unbalance And Runout

“Unbalance” and “runout” are not pretty words when used in the context of rotating equipment.

They’re also not interchangeable. According to a pair of recent blog posts from VibrAlign Inc., Richmond, VA (vibralign.com), correcting unbalance and runout requires personnel to understand the differences in these conditions.

Unbalance and runout both cause an uneven distribution of mass around an axis of rotation. They also cause destructive forces, contribute to misalignment, and result in shortened machinery life. Unbalance can be caused by corrosion, erosion of a fan wheel, or removal of a correction weight. Runout occurs due to eccentricity, i.e., machining error or a bent shaft.

While the two conditions can reflect serious problems in a plant, runout can be viewed as more of a root problem that needs to be corrected by replacing parts. Unbalance, on the other hand, often can be field-remedied by adding or removing weight from an accessible fan wheel.

To correct unbalance, personnel must know two things:

• The force: How much vibration is being caused by rotational unbalance?

• Direction: Where is the heavy (or light) spot?

There also must be an accurate measurement of runout.

Know the 3 Types of Unbalance

There are three common types of unbalance. According to VibrAlign, each type will reveal itself in the machine’s vibration amplitude and phase angles.

Static unbalance occurs when the unbalance is at the center of gravity. The vibration amplitude will be the same on both bearings, and the phase angles will also be the same.

Couple unbalance occurs when the principal inertia axis is displaced from the rotational axis, and the axes intersect at the center of gravity. The vibration amplitude will be the same on both bearings, as with static unbalance, but the phase angles will differ by 180 deg.

Dynamic unbalance, which is a mix of static and couple unbalance, occurs when the principal inertia axis is displaced from the rotational axis and does not intersect at the center of gravity. The vibration amplitude and phase angles will be different on both bearings.