Fluid film bearings operate on a hydrodynamic film, formed by a converging geometry, relative motion of a rotor and stator, and a viscous fluid. In fixed profile journal bearings, the converging geometry can be provided by the slight difference in bearing bore and shaft diameter. Additional stability can be provided through the machining of various lobe configurations. In fixed geometry thrust bearings, a converging geometry is machined into the bearing face.
The geometry is optimized to the application. For increased stability and to accommodate misalignment or varying operating conditions, tilt pad bearings are usually implemented.
TRI designs and manufactures large generator bearings that suppress vibrations caused by the unique conditions presented by generators. Our bearing designs account for the geometry of generator rotors and the magnetic forces that act on them. TRI uses proprietary software to analyze existing bearing designs and makes recommendations that improve reliability and operating range.
Generators typically have very long and relatively slender rotors (called &#;fields&#;) between the bearings. This long and slender design occurs because the electrical power that can be generated by a single generator is related to the length of the rotor and the stator as well as the diameter, whereas the cost of manufacturing a generator is related to the cost of the length and diameter of the field and the stator, along with a very significant cost for the end sections of the generator. In other words, the cost of an additional foot of length of the rotor and stator is relatively small compared to the cost of the entire generator with the ends included.
Journal Bearings can be designed to handle the rotor weight for rpm generators: the bearings are large diameter and they are long axially in order to keep the &#;specific loading&#; (psi) in a satisfactory range. However, there is a very serious issue that is related to the &#;first natural frequency&#; of the rotor, which may also be called the &#;first critical speed&#;: The first natural frequency for a large portion of the high-powered generator rotors is in the range from 600 rpm to rpm, and such a natural frequency can lead to sub-synchronous rotor vibration at this frequency when the bearing design does not suppress the sub-sync vibrations. If not suppressed, the vibration amplitudes can get large enough to seriously damage the generator rotor, bearings, and frame. Consequently, the vibration monitoring equipment is installed and set to trip the unit when such high amplitude vibrations start. It is definitely not good to trip a large steam turbine-generator at full power because numerous areas of the plant can be damaged.
While many generator manufacturers have figured out how to design journal bearings to suppress sub-synchronous vibrations, the designs are often marginal, and a slight misalignment or incorrect refurbished bore geometry can lead to sub-sync vibrations. There are a number of recorded cases of high amplitude synchronous vibrations that have led to sub-synchronous rotor vibrations.
TRI has mastered the art of designing these large generator bearings to suppress rotor vibrations. In many cases, the original bearing design, whether an elliptical bore or round bore, just does not work because there are compromises regarding bearing length. Make the bearing long enough to not require lift oil and encounter sub-sync vibrations, or shorten to where the bearing wears rapidly and needs lift oil even though none is available.
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TRI analyzes the design and makes recommendations to control the sub-synchronous vibrations and deal with the compromises regarding bearing length. TRI finds that there are many times when the original fixed bore bearing can be redesigned with a different bore geometry that will control the vibration and still have adequate length to support the weight. On the other hand, there are certain applications when it is most appropriate to change to a five-pad tilting pad bearing with two pads down to split the load carried by each bottom pad. Lift oil may or may not be optional. This is one of the design features to be factored into the overall design. If lift oil was previously used, it is almost certain that the bottom pads can be configured with lift oil pockets.
Another factor to consider in the design of generator bearings has to do with insulation. Early insulation used a NEMA Grade G-9 industrial laminate which originally consisted of linen fabric and a melamine resin. Today, the linen fabric has been replaced by fiber glass woven material.
In recent times, NEMA Grade G-10 industrial laminate which is made with fiberglass fabric and epoxy. This is stronger and has improved thermal and insulating properties compared to Grade G-9.
TRI uses only a G-10 form of insulation. Either we machine it from commercially available plates or rods or we make the insulating material by dipping fiberglass in the epoxy, applying it to the part to be insulated, and letting it cure.
TRI manufactures generator bearings that have either a single layer of insulation or are &#;doubled insulated&#; according to the needs of the application, including the space available. When TRI makes new generator bearings, TRI offers to include insulation in both the turbine end bearing and the exciter end bearing.
For existing generator bearings that have insulation, TRI is prepared to refurbish these bearings. Almost always the insulation characteristics are improved.
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