The location of the mass center and the principal inertia axes are determined by the distribution of mass
within the part. When the axis of rotation is not coincident with a principal inertia axis, it causes various types of unbalance.
It is important to draw a distinction between unbalance and balance correction. Unbalance is a mass
property. It becomes a characteristic of the part when an axis of rotation is defined. Balance correction
is used both to alter the mass properties and to improve the alignment of the axis of rotation with the mass
center and / or the central principal axis. Both can be expressed as weights and radii and have shared
terminology. Also, this section discusses unbalance as a mass property and its types.
A condition of static unbalance exists when the mass center does not lie on the axis of rotation. Static
unbalance is also known as Force Unbalance. Basically, static unbalance is an ideal condition, it has the
additional condition that the axis of rotation be parallel to the central principal axis – no couple
Generally, static unbalance can be corrected with a single weight. So, the correction is made in the plane of the mass center and is sufficient to shift the mass center onto the axis of rotation. To conclude, it is important to align the correction with the initial unbalance to move the mass center directly towards the axis of rotation.
To summarize, static unbalance can be detected on rotating or non-rotating balancers.
Firstly, there is a specific condition that exists when the central principal axis of inertia is not parallel with the axis of rotation. Couple unbalance is often presented as dynamic unbalance in engineering classes, however
this term is defined otherwise by ISO 1925 and is reserved for the more general case of combined static
and couple unbalance. As defined, couple unbalance is an ideal condition. Markedly, it carries the additional
condition that if mass center lies on the axis of rotation, there is no static unbalance.
Couple correction requires that two equal weights be added to the workpiece 180 degrees apart in two
correction planes. The distance between these planes is called the couple arm. Whereas static
unbalance can be measured with a non-rotating balancer, couple unbalance can only be measured on a
The most general case of unbalance in which the central principal axis is not parallel to and does not
intersect the axis of rotation.
Dynamic unbalance is also referred to as two plane disbalance, indicating that correction is required in
two planes to fully eliminate dynamic unbalance. In this situation, a two plane balancing specification must include the axial location of the correction planes to be complete. Dynamic unbalance captures all the unbalance, whenever it exists in a rotor.
This type of unbalance can only be measured on a rotating balancer, since it includes couple unbalance.
In short, it’s a special form of dynamic unbalance in which the static and couple unbalance vectors lie in the same plane. The central principal axis intersects the axis of rotation, but the mass center does not lie on the
axis of rotation.
This is the case where an otherwise balanced rotor is altered (weight added or removed) in a plane
some distance from the mass center. For that reason, the alteration creates a static unbalance as well as a couple
unbalance. Conversely, a rotor with quasi-static unbalance can be balanced with a single correction of
the right magnitude in the appropriate plane.
UNITS OF UNBALANCE
Balance corrections are normally specified as a weight added or removed at a radius. The weight or
mass units can be any convenient units of measure. The most commonly used weight units are both ounces
and, occasionally, pounds, but the most common mass units are grams (g) or kilograms (kg).
The capacity and accuracy of the weighing equipment available must be taken into account to ensure that weight precision is sufficient to the task.
So, the most common combinations used to specify unbalance are ounce inches, gram inches, gram millimeters, gram centimeters, and kilogram meters.
MOTION OF UNBALANCED PARTS
What is the effect of unbalance on a rotating part? On one hand, if the rotor mounts are rigid, the
forces exerted on the bearing supports can be very high and potentially damaging. These forces are the cause of the unbalance. Markedly, they are the centrifugal forces described earlier.
On the other hand, with flexible mounts, the part is loosely constrained and may exhibit large amplitudes of displacement. The amplitude of vibration is proportional to unbalance and limited by the distance between the mass center and the axis of rotation. So, most applications are a combination of both.