What are the Units of Unbalance?

Unbalance units are used to determine the amount and location of mass imbalance in a rotating part. In its most basic form, these units are obtained by multiplying the mass by the radius. This indicates not only the magnitude of the imbalance but also the distance from the center of rotation of the imbalance. For example, a 10-gram mass imbalance 50 millimeters from the center of rotation creates an imbalance of 500 grams-millimeters (gmm). This unit, gram-millimeter (gmm), is one of the most commonly used units of measurement and is generally preferred for small and medium-sized parts.

However, different units may be used depending on industrial applications and the measurement systems used. For example, units such as kilogram-meter (kgm) or meter-gram (mgm) may be preferred for larger parts. In the United States and some other countries, imperial units such as ounce-inch (oz-in) or pound-inch (lb-in) are used. All of these units are based on the same basic principle: multiplying the unbalanced mass by its distance from the axis of rotation. This allows us to precisely express the magnitude of the unbalance and plays a critical role in determining the extent of correction required for the balancing process. At MBS Balance, we support all of these units in our machines, in accordance with various industry standards and customer requirements.

In practice, there are two levels of expressing unbalance: absolute unbalance U (for example g·mm) and specific unbalance divided by rotor mass, e = U / m (in micrometers). The G quality grades in ISO 21940-11 are actually based on the product of this specific unbalance and the angular velocity (e·w); therefore the same U value means a large eccentricity on a light rotor and a small one on a heavy rotor. This distinction makes it possible to compare parts of different masses by the same quality criterion.

Another distinction is total unbalance versus unbalance per plane; in two-plane balancing, the permissible tolerance is usually distributed among the planes. Consistency in unit conversions (1 g·mm is approximately 0.00139 oz·in) is important. MBS Balance machines support both metric and imperial units and both absolute and specific unbalance display; when the operator enters the part's target G grade, the system automatically calculates the permissible residual unbalance.