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.