This depends strongly on the specific mechanism of vibration. In some cases the vibrations observed are believed to be hazardous; undesired restrictions of operations may result. The following is a however incomplete list of examples.
Both axial and radial alternating forces may act on the rotating parts, with corresponding risk of damage to journal bearings, labyrinth seals, spiders etc. Low-frequency pressure pulsations may result in unacceptable power swings. Excessive pressure variations in the draft tube may results in fatigue cracking at draft tube flanges or inserts. In case of resonance, Von Kármán vortex streets can cause fatigue cracks at stay vanes, runner blades, trash rack bars and other components, often accompanied by annoying emission of noise. Periodic pulsations due to blade passage may cause runner blade damage and sometimes also excessive vibration and noise at the penstock or other components of the plant.