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What is static unbalance?

Static unbalance is a condition where the center of gravity of a rotating body is not coincident with the axis of rotation. The resulting centripetal force produces a synchronous (1xTS) vibration which is in-phase across the two ends of the rotor. Assuming a symmetrical center-hung configuration, this becomes amplified at the first bending mode of the rotor (first balance critical) as the mid-span of the rotor deflects away from the axis of rotation forming a “U” shape (standing half wave).

This uneven distribution of mass on the rotor can be the result of imperfections from manufacturing, wear, product build-up, damage the rotor, and so on. At a stand-still, for a horizontally oriented rotor, the heavy spot will tend to be pulled to the bottom due to the force of gravity (assuming the torque generated exceeds the static friction in the bearings). The optimal solution for a purely static unbalance would be a single appropriately sized weight at the axial position of the center of gravity of the rotor which produces a force that is equal in magnitude but in opposite phase to the unbalance force.

For rotors with a relatively short axial length relative to their diameter, especially at low turning speeds, such as many process fans this can typically be done with a single plane procedure. For longer cylindrical rotors such as electric motors which may exhibit the same in-phase vibration behavior across the two ends of the rotor, a modified single plane procedure may be employed by utilizing a static couple correction – two weights equal in magnitude at the same angle on the two ends of the rotor.

What is dynamic unbalance?

Dynamic unbalance is a condition where the primary axis of inertia of the rotor is not parallel with the axis of rotation. The resulting centripetal force produces a synchronous (1xTS) vibration which is out-of-phase across the two ends of the rotor.  Assuming a symmetrical center-hung configuration, this becomes amplified at the second bending mode of the rotor (second balance critical) as the two ends of the rotor deflect away from the axis of rotation in opposite directions forming an “S” shape (standing full wave)

As with static unbalance, this is the result of uneven mass distribution.  However, unlike static unbalance, the heavy spots will not be pulled to the bottom in a horizontally oriented rotor due to gravity as they are equal in magnitude but in opposite phase and therefore cancel each other out statically. The optimal solution for a purely dynamic unbalance would be a pair of identical appropriately sized weights placed at precisely opposite angles with respect to one-another at axial positions which are far from the center of gravity of the rotor and produce forces which are equal in magnitude but in opposite phase to the unbalance forces.

This will always require a two-plane solution and is of primary importance for rotors with a relatively long axial length relative to their diameter, especially at high turning speeds, such as centrifugal compressors and steam turbines. For a purely dynamic unbalance a modified single plane procedure may be employed by utilizing a dynamic couple correction – two weights equal in magnitude at opposite angles on the two ends of the rotor.

In reality a rotor will always have some combination of static and dynamic unbalances, it is therefore the responsibility of the analyst to identify the most appropriate location (or locations) for unbalance correction.

Want to learn more about balancing? We offer industry training at ourNorthPoint Training Centre.

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Posted on: 15th Apr, 2019