Fixed bore bearings come with a wide variety of bore designs, alignment options and lubrication schemes. TRI has worked with Pressure Dam Bearings, Elliptical Bore Bearings, Circular Bore Bearings and Multi-Arc Bearings.
Many customers come to TRI with a bearing that has failed. We help them to determine what the proper course of action is. If time is limited, TRI will re-Babbitt and re-bore the bearing on a 24/7 schedule. If there are design flaws, TRI will recommend a fix. With the customers approval, we can implement the design change. For re-occurring problems or critical applications, TRI can analyze the rotor and bearing system with our proprietary software. Over the years, our analysis has solved many vibration problems.
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.
Bearing Design Considerations
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 3600 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 1000 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.
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.
Babbitted bearings for industrial and large utility fans have been made from very crude to very sophisticated designs and materials.
Crude, low cost bearing design frequently lead to trouble. TRI refurbishes and improves existing fan bearings for better performance, or when it is appropriate, manufactures new bearings with pressure fed lubrication systems.
Very crude fan bearings are made with a coarse cast iron casting with dovetails (or mechanical Babbitt anchors) machined in the bore. Sometimes the Babbitt, whether lead based or tin based, is cast into the bore without any tinning, so there is complete dependency on the mechanical Babbitt anchors for retention of the Babbitt layer.
In some cases, the surfaces of the horizontal joints are only partially machined and the halves of the bearings are doweled and held together by only two bolts. Many of these bearings have water jackets with NPT threads for water pipes to cool the bearings.
Bearings of this design usually include two oiling rings that bring oil from the bottom of the oil reservoir to the journal surface. There is no oil flow when the shaft is not turning.
These bearings almost always start “dry”. That is, when the shaft is not rotating, the lube oil drains out, so that when it starts to rotate, there is minimal lubrication. The result is that the Babbitt layer at the bottom of the bearing gets hot and expands causing the Babbitt to distort in the dovetail grooves. The surface also wears slightly with every start. There are two inspection ports for each bearing, and operators are supposed to put a small amount of oil in each hole to avoid dry starts, but today when the number of operators is continually reduced, there are no operators available to oil the bearings.
After repeated starts of this nature, the Babbitt cracks and the surface wears more, so that at some time the Babbitt smears or “wipes”, leading to more rapid bearing damage and ultimate failure.
It is difficult for these bearings to maintain a suitable geometry of the Babbitt bore and they have a high failure rate and need to be refurbished often.
There is no easy method to refurbish these bearings correctly. The joints can be machined and more screws can be installed. The NPT threads can be re-tapped, but the threads leak water into the oil. The cast iron surface cannot be properly cleaned to be able to install a proper chemical tinned surface.
The conclusion is that these low-cost cast iron bearings have been problems and they will continue to be problems without design changes.
The most sophisticated fan bearings have these features:
- an outer housing made from high quality cast iron or cast steel,
- a bearing liner machined from hot-worked steel, either rolled plate or a forging,
- a pressure-fed lubrication system that feeds conditioned oil into the bearing liner,
- an oil conditioning system with duplex pumps, filters, and oil to air coolers.
- oiling rings can be used to assure oiling on a rundown in the case of loss of AC power.
TRI works with all of the various types of bearings, from crude cast iron to sophisticated all steel bearings, refurbishing them without design change or making substantial improvements.
In many cases, customers have accepted TRI recommendations to upgrade fan bearings by going to improved materials and separate oil conditioning systems. For instance, TRI has designed and manufactured completely new fabricated steel housings, steel liners with chemically bonded tin-based Babbitt typical of the best turbine bearings, and suitable cooling means which depend upon the application, particularly the rotor weight, rotor speed, bearing size, and ambient temperature and air flow conditions for cooling.
Boiler Feed Pump Bearings
Boiler feed pumps are a critical components. A good bearing system will reduce vibrations in the pump and keep a unit running even in adverse conditions. There are many aspects to a good bearing design for pumps. TRI has experience with bearings, pumps and many other types of rotating equipment.
Weather you are dealing with bearing damage or pump vibration issues, TRI has a an experienced engineering staff that can analyze your system, recommend and refurbish bearing improvements or design a better pump bearing systems.
TRI has introduced a new line of tilt pad bearings specifically designed for boiler feed pumps. These bearings have a compact design so that they fit within the physical limitation typically required in bearing retrofit jobs.
The tilting pad bearing design offers advantages over more simplistic bearing designs. The main advantage is the ability of the bearing to adapt to movement caused by thermal expansion. As hot water enters the boiler feed pump, the center line will typically rise throwing your equipment out of alignment. This will lead to increasing bearing temperatures and vibration.
Our bearings can be used with Ingersoll pump, KSB pumps and many others