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Types of unbalance

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Categories: Rotor balancing basics

The location of the mass center and the principal inertia axes are determined by the distribution of mass
within the part. Unbalance exists when the axis of rotation is not coincident with a principal inertia axis.

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 a means to alter the mass properties 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. This section discusses unbalance as a mass property.

STATIC UNBALANCE

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. As defined, 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
unbalance.

Static unbalance can be corrected with a single weight. Ideally the correction is made in the plane of the
mass center and is sufficient to shift the mass center onto the axis of rotation. It is important to align the
correction with the initial unbalance to move the mass center directly towards the axis of rotation.

Static unbalance can be detected on rotating or non-rotating balancers.

COUPLE UNBALANCE

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. It carries the additional
condition that the mass center lie on the axis of rotation – 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
rotating balancer.

DYNAMIC UNBALANCE

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 unbalance, indicating that correction is required in
two planes to fully eliminate dynamic unbalance. A two plane balance specification must include the
axial location of the correction planes to be complete. Dynamic unbalance captures all the unbalance
which exists in a rotor.

This type of unbalance can only be measured on a rotating balancer since it includes couple unbalance.

QUASI-STATIC UNBALANCE

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. 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 ounces
or occasionally pounds and 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. 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? At one extreme, if the rotor mounts are rigid, the
forces exerted at the bearing supports can be very high and potentially damaging. The forces are a
function of the unbalance. They are the centrifugal forces described earlier. At the other extreme, 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. Most applications are a combination of both.