Vibration Caused by Misalignment

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Misalignment can occur when the centerline of coupled shafts are not coincident. Misaligned shafts cause overloading and vibration in a system; this condition can be diagnosed by examining spectral data and looking for specific patterns. Two types of misalignment conditions are possible:

• Parallel misalignment

• Angular misalignment

When the centerline of two shafts are parallel but not coincident, it is referred to as parallel misalignment. 

If two shafts meet at a point but their centerlines are not parallel, then this is referred to as angular misalignment.

Most incidences of misalignment are typically a combination of parallel and angular misalignment. Misalignment can be observed in spectral data when there is significant 2X forcing frequency, and greater levels at the 1X forcing frequency, in the axial axis and and one of the radial axes. The phase difference between measurements on either shaft also matters when identifying misalignment. 

With parallel misalignment, the 2X frequency is usually high in the radial directions and the  1X frequency is typically smaller. There is also a 180° phase difference between radial measurements on the coupled shafts. The type of coupling may cause higher order frequencies to occur, however there will be no raised noise floor like you would observe for mechanical looseness. Note that with purely parallel misalignment, the 1X and 2X frequencies will be low in the axial direction.

Axial vibration is usually stronger with angular misalignment; the 1X is usually the strongest frequency, but the 2X and 3X frequencies may also be present. In this scenario, the angular measurements on the coupled shafts will be 180° out of phase. 1X and 2X frequencies will be observed in the radial direction as well, but these will be in phase.

Misalignment usually occurs during installation of the coupling. It can be avoided by using precision techniques such as laser alignment. Coupling manufacturers’ will usually specify the alignment tolerances of their products to ensure that they are not overloaded; however, even within acceptable tolerances, the misalignment can cause vibrations that excite harmonic frequencies and lead to failure. Bearing life can also be impacted by misalignment, which causes overloading and premature failure.