Wednesday, March 27, 2019


Dynamic misalignment arises during the sustained operation of machinery. Few reasons for dynamic misalignment are:

  1. Magnetic centring of electric motors: Magnetic centre or electrical centre of a motor is the neutral position in which the motor shaft settles during the operation. When there are no electrical forces, the motor rests at the mechanical centre. While starting, the magnetic force between stator and rotor tries to pull the latter to a neutral position. Hence the axial position of the motor at rest will be different from that during operation. This causes the shaft to move and lead to misalignment. In most cases, magnetic centre coincides with the mechanical centre. But in some motors, especially the ones with journal bearings, care to be taken to consider the magnetic centre. Always refer to the manuals provided by the manufacturer.
    To align correctly, determine the magnetic centre during the solo run of the motor. The same value to be considered and the coupling spaces to be adjusted accordingly during alignment.
  2. Thermal growth: Equipment in high-temperature environment suffers thermal expansion. During the operation, such equipment will get hot and have differential expansion across coupling, leading to misalignment. For proper alignment, let the equipment run for some time under the normal operating conditions and then stop and correct the alignment before the equipment cools down. For critical equipment like turbines and compressors, follow the alignment values provided by the manufacturer.
  3. Structural looseness: Loose structure leads to relative movement between the driver and driven components creating misalignment. Structural integrity is a must to reduce the vibrations and failure of the machinery.

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