|April 2000 Tech Feature||www.virtualindian.org|
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The Duesenberg of motorcycling which derived from the Henderson Four, changed to Ace, and finally became the Indian Four: all have the same crankshaft outline. This crankshaft is supported in the earlier types by three main bearings and along the way changed to a five main bearing crankshaft.
The main bearing consists of a bronze shell lined with babbitt, in English, white-metal. In 1839 invented by the American Isaac Babbitt, the first types of babbitted plain-bearings were a composition of mainly tin and small percentages of antimony and copper. This does not differ much from the babbitt we have nowadays, still used on shells for turbines, petrol and diesel engines and airplanes.
The advantage of babbitt as a bearing metal in a stationary contact (the bearing is fixed and the shaft rotates) is that this type of bearing does not score the shaft.
The babbitt is relatively soft, but strong enough to resist the bearing load. Babbitt also has a tendency to hold the oil. Hard particles can become imbedded in the soft babbitt without scoring the shaft journal.
A journal made of relatively soft steel can be used, on the condition that the difference between the hardness of the journal and the babbitt is at least 20% (the hardness of babbitt is around 25 brinell at room temperature). If there is not enough lubrication, babbitt, because of its composition still has a border lubrication ability. If lubrication fails completely, the babbitt will partly melt and does not damage the shaft journal. Its good wear resistance is due to the fact that hard copper-tin crystals are imbedded into a softer tin composition so they can adjust to the journal surface, but also resist the high pressure.
In the 30s the high price encouraged the development of a babbitt-type in which tin is partly changed by lead. So now we have a high tin content babbitt (more than 80% tin) and a lead-tin babbitt (tin content less than 80%). The specifications of a lead-tin (usually given at room temperature) seems at first sight good, but at higher temperatures hardness is better by 60% in a high tin content babbitt. At these higher temperatures the static and dynamic properties of a lead-tin babbitt can worsen. The material distorts. Hardness of a lead-tin babbitt can, for high temps, be improved by adding nickel and bronze. However, these very hard crystals can score the journal. A high lead content always lowers the specs but can be chosen if the bearing load is low. This type of babbitt also sets fast by rough use of the engine, so wear is significantly higher. A lead based babbitt is also less resistent against fluctuating forces. Lead is more prone to corrosion and less elastic than tin. The combination of a steel base together with a lead babbitt is not as good because of the great difference in thermal expansion between both materials. A high tin content babbitt is the best choice for engines of cars, four-cylinder motorcycles and airplanes, because it is more elastic so there is less wear. This type of babbitt allows more distortion at higher temps so there is less change in developing of microcracks. Cracks always evolve in the breaking away of babbitt.
Every manufacturer has its own "recipe" for the different types, more or less copper, antimony or arsenic to get a small crystal structure and better specifications of the material.
For American four-cylinder engines I prefer to use a babbitt type which is also used in Rolls Royce airplane engines. It is the ultimate quality. When an engine is not equipped with an oil cooler, temperature can rise considerably which influences specifications a lot. An airplane quality babbitt keeps its specs also at a higher temp, so it is very advisable to use it also in vintage racing cars.
The pouring/casting of babbitt.
A babbitt bearing can be made by pouring
the babbitt into a mould (the shells) or by casting with centrifugal force
(special machine). This is done only after the shell has been thoroughly
tinned with a 100% tin and heated till the tinned surface just starts to
melt, this also has to be checked with a "thermostick".
This is the way our American four
bearings are made. When finishing them we have to be sure the faces of
shell and babbitt are flush so when bolted against each other there will
be no loss of oil pressure. Initially, after casting there has to be an
"overheight" of the babbitt which is made even. Because the bearing shell
has to be locked up by the bearing cap, there has to be a nip between the
shell and the bearing cap.
The rough casted bearings have to
be turned down in a lathe till almost the final dimension. The engine case
without the babbitted bearing shells is now adjusted into the align-boring
machine. This is a very precise job as the boring bar has to be exactly
in the center of the bearinghouses. If this is not done correctly, the
pinions, later to be mounted at both ends of the crankshaft, will not correctly
mesh up with their counter pinions.
Two Indian-4 conrods with original factory babbitt after 42.000 miles.Top and bottom part are usually affected because up and down movement of piston with change of inertia. The "decay" starts with microcracks, with finally a seizure of the conrod and hopefully no damage to the crankcase. The babbitt is nicely flush with the conrod body, so no loss of oil pressure at this spot.
Some corrosion of a "Four" main bearing shell because of time and (old) oil composition. At the edge microcracks and loss of babbitt. The tinned shell surface is showing: so no breaking away because of an incomplete adherence of the babbitt to a not well tinned shell. This is sometimes the case.
Corrosion and "dirt" embedded in the soft babbitt. One of the advantages of this bearing material is less scoring of the crankshaft hub. In this case it is not necessary to treat the crankshaft like hardchrome plating, metal spraying or teniferen etc.
Original Indian-4 transmission shaft bearing shell. An oil groove like this one breaks the oil film and can speed up wear.
A bevel is made at the oil entrance hole to ease the oil flow. Also the edges of the circular oil groove at the other side of the shell, in the babbitt, has to be eased off.
Left: a perfect bond between shell and babbitt. Right: questionable; so this shell can not be used.
The babbitt is not flush with the shell, can result in loss of oil pressure. The strip is a remainder, after turning down, of the separation strip used when pouring. It can also be considered not to rebabbitt old shells but replace them by completely new babbitt bearings.
Babbitt has not adhered to the well tinned shell. Possible causes:
? shell too cold when poured
? more likely: babbitt has shrunk away during the cooling process because of a wrong starting point of cooling or because the center started to cool off before the shell did.
Hammering in this way makes the curvature larger so there will be a good contact between shell and the groundboring in the crankcase. Heat can dissipate and together with the "nip" of the shell, the bearing will not become loose. Do not give blows at the rear side of the shell, one could induce microcracks.
Align-boring of the "Four" transmission bearing shells. Note the 2 sturdy boring bar support arms close to the bearing; so no vibration which results in a bearing of the correct diameter across its whole length. (top left: in front of the sprocket a special measuring device to measure the diameter at a 90 degree angle when the boring bar is still in position.
A conrod bearing cap with babbitt on both thrust faces ("overheight"). The babbitt stands proud over the bearing cap faces, so when planed, no loss of oil pressure is possible.
The correct nip only can be checked when torquing down both nuts, then loosening one nut and check the nip. This check on a surface plate is just to know if there is a nip and it will not be too much.
Boring the rebabbitted conrod; exact in the center of the big end and also parallel to the piston pin