Seal rubs can cause shaft deflections
Seals in hydroelectric machines should prevent the escape of oil and oil vapor from bearings and water from the turbine case along the rotating shaft of the turbine/generator set. However, in cases where seals are placed or move too close to the shaft, they may be responsible for unacceptable shaft runout. During runout, the shaft connecting the turbine to the generator moves laterally, subjecting bearings and structural supports to large pulsating forces. This can lead to a breakdown of lubricating oil film, which, in turn, can damage the shaft. No shaft rotates in a perfectly concentric manner. For that reason, all hydroelectric machines exhibit some degree of shaft runout. In cases where unacceptable levels of vibration occur, station and maintenance staff may consider such possible causes as mechanical unbalance, excessive bearing clearances, hydraulic effects, and electrical unbalance. Seals rubbing against the shaft, are also a cause of runout. Maintenance staff often can determine whether seal rub is the cause of shaft runout by observing machine performance. Such machines will make smooth cold starts but as the machine continues to operate, runout becomes more and more pronounced. This characteristic points to a seal rub as a likely cause of the vibration. When localized heat is generated by seals rubbing against a rotating shaft, the heat bends the shaft in the direction of the original runout. The seal rub that produced the temperature rise then worsens, producing additional heat and more deflection. This snowballing effect can continue until the seal rub becomes destructive.
- OSTI ID:
- 5498478
- Journal Information:
- Hydro Review; (United States), Vol. 10:6; ISSN 0884-0385
- Country of Publication:
- United States
- Language:
- English
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