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Bearings that have insufficient loads are called lightly loaded. Many designs of lightly loaded, fixed bore bearings are susceptible to sub-synchronous vibrations called "oil whirl".

The “specific loading” of a bearing is a critical parameter for determining the load carry capabilities of a bearing. The specific loading of a bearing is the ratio of the downward force to the cross-sectional area of the bearing. In a simple case, the downward force is equal to the rotor weight that is supported by the bearing. The cross-sectional area of a bearing is the axial length of the active bearing surface multiplied by the diameter of the bearing bore.

Bearings with specific loads under 75 pounds per square inch can be considered very lightly loaded. Depending upon bearing design, journal diameter, and oil viscosity, bearings with specific loads up to perhaps 225 pounds per square inch can be sufficiently lightly loaded that they will also encounter sub-synchronous rotor vibration called oil whirl (or oil whip).

Figure A shows a Frequency Analysis of the vibration signal of a steam turbine rotor that is rotating at 3600 rpm, hence, a component of the vibration appears at 60 hz. The sub-synchronous vibration components appear in the neighborhood of 20 hz because the natural frequency of the rotor is excited by the sub-synchronous activity of the journal bearing oil film.

In this case, the oil film produces greater pressure than is necessary to support the rotor in a stable position, and the oil film lifts the rotor up to the point that it falls over, and then the oil film lifts it up again and again.

The pressure pulsations in the oil film occur at the same frequency as the vibrations of the rotor, and these pulsations often are high enough to cause the stresses within the Babbitt layer and the stress levels in the Babbitt to steel bond to exceed the endurance limit of the Babbitt, leading to breakage of the Babbitt, separation of the Babbitt layer from the steel backing, and ultimately, to bearing failure as shown in Figure B.

When the surfaces of the damaged area are examined closely, there may be some pockets with smooth surfaces but with nothing in the pockets. A first thought is that this must be electrolysis, because electrolysis does leave similarly shaped pockets with smooth surfaces. However, after examining enough of them, pockets are found with small pieces of Babbitt in them also with smooth surfaces. This then leads to the proper conclusion that after chunks of Babbitt are broken out, the turbulent oil flow of the film causes the chunks to polish the surfaces of the pockets and themselves, often eroding the chunks until they disappear leaving no trace of what happened.

Now, to address the electrolysis further: When a chunk of Babbitt of sufficient size breaks out, it can wedge between the surface of a pocket from which it came and the journal surface, thereby becoming an excellent grounding device for the shaft, and this can lead to electrolysis activity. Note, however, that the sequence is that the Babbitt broke out first and the electrolysis resulted, not the other way around.

The solutions to this lightly loaded circumstance depend upon many factors that start with the range of loading that the journal bearing experiences. In some cases, it is as simple as rebabbitting the bearing and changing the design of the bearing bore geometry. In other cases, a change to tilting pad bearing design is required for a satisfactory long term solution. Sometimes, the problems can be solved by modifying only one bearing, and other times, several bearings on the train must be redesigned. Having satisfactorily solved numerous problems of this nature in the past, TRI's expertise can be quite effective for solving similar problems in the future.

 

A Tech Note was released in June that explained how pressure dam and elliptical bearing designs help to control sub-synchronous vibration.

Figure A Figure B

 

   

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