Steady Rest Bearings (extension shaft bearings)
Many turbines have long extension shafts, often approximately 6 feet long or longer, that are attached to the HP Turbine rotor and extend through the front standard. In almost all cases, the overspeed bolt and the main oil pump are mounted on this shaft, and in some cases, the thrust bearing runner is included as well.
For many reasons, at 3600 rpm and generally due to unbalance of the rotors and marginal control of the resulting rotor vibrations, these extension shafts vibrates excessively, causing wear to the seal rings (aka "wear rings") of the main oil pumps.
A number of turbines have extension shafts that extend through the front standard and drive large high powered fluid drives through flexible couplings. Westinghouse turbines use steady rest bearings to support the end of the extension shafts for these applications. The Westinghouse steady rest bearing design uses four small tilting-pads with small diameter rod supports that often pound out within a short time from being installed, whether new or after being renewed.Another extension shaft application that occurs with increasing frequency today has to do with the collector extension shafts outboard of generators. This occurs because rotating exciters that supported these collector extension shafts are being removed and solid-state excitation systems are being installed. This leaves these extension shafts unsupported or inadequately supported at the ends so they experience high amplitude vibrations. Serious consequences are that the collector brushes wear rapidly and the collector rings wear in a non-uniform pattern, often with a 2 per rev wear pattern, due to the two-pole design of the generator rotors with the inherent 2 per rev bending stiffnesses.
- Assuring that there is a journal surface that is concentric to the extension shaft and collector rings. This may involve "in-place machining" of the existing rotor, or adding on a short extension shaft with a qualified round and concentric journal surface.
- A TRI Tilting pad bearing of heavy duty design.
- A pedestal that extends from the new TRI bearing down to a sole plate.
- A sole plate, if necessary.