why vibration occur in gear box of steam turbo generating set
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Uneven cooling of the rotor, particularly after shutdown, also causes the rotor to contact stationary parts. After a unit shuts down, the relatively high-temperature rotor may bend solely due to the mass of the rotor and the distance between bearing supports, if left in a stationary position to cool. This situation can cause permanent shaft bending.
The effect of a permanent shaft bend caused by uneven cooling will immediately appear as high rotor vibration at the next startup. The vibration is caused by insufficient clearance between stationary and rotating parts, as well as a shaft located off-center in its bearing. Even if the clearance change is small, there may be significant rubbing along the rotor to cause damage. Again, the rubbing causes friction between stationary and rotating parts, localized heating of the rotor metal at the contact point, and shaft bending.
Furthermore, uneven shaft warming caused by rubbing between rotating and stationary parts can cause further bending of the shaft in the same direction of the existing bow and cause additional contact with stationary parts, increasing temperatures and therefore causing more bending. The effect cascades if allowed to continue. If the bending is allowed to continue, it is possible that the yield strength of the metal could be exceeded, causing a permanent deformation of the shaft. The allowed bending in 3,000-rpm turbines is up to 0.02–0.03 mm in each section. When on turning gearing, the limit is 0.05 mm.
The effect of a permanent shaft bend caused by uneven cooling will immediately appear as high rotor vibration at the next startup. The vibration is caused by insufficient clearance between stationary and rotating parts, as well as a shaft located off-center in its bearing. Even if the clearance change is small, there may be significant rubbing along the rotor to cause damage. Again, the rubbing causes friction between stationary and rotating parts, localized heating of the rotor metal at the contact point, and shaft bending.
Furthermore, uneven shaft warming caused by rubbing between rotating and stationary parts can cause further bending of the shaft in the same direction of the existing bow and cause additional contact with stationary parts, increasing temperatures and therefore causing more bending. The effect cascades if allowed to continue. If the bending is allowed to continue, it is possible that the yield strength of the metal could be exceeded, causing a permanent deformation of the shaft. The allowed bending in 3,000-rpm turbines is up to 0.02–0.03 mm in each section. When on turning gearing, the limit is 0.05 mm.
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