Crankshaft main bearings - operating temperature?

[James P]

Anyone care to offer an opinion on the normal or maximum expected operating temperatures of crankshaft main bearings, if the engine is running "correctly"?

The reason I want to know is: For many years I have been using crankshaft main bearings with glass-fibre-reinforced polyamide 66 cages. These cages are a little lighter than the usual pressed steel cages, which seems like a good thing. Also, there have been perhaps three or four occasions when I have stripped engines of unknown history to find cracks in the pressed steel cages of the main bearings (although I'm not suggesting it is a regular occurrence).
I have so far used the polyamide cages only where the bearing is lubricated by gearbox oil or packed with grease in a sealed chamber - I have not yet used them where the bearing is lubricated by the two-stroke fuel-oil mixture.

I contacted the manufacturer of the bearings I have used (although several manufacturers offer similar polyamide cages), asking whether the polyamide cages are suitable for lubrication by fuel-oil mixture. The reply stated "The cage can tolerate the medium, however will degrade more rapidly at temperatures exceeding 100 degC".
The various manufacturers' data sheets show that polyamide cages are otherwise suitable for operation up to 120 degC (if lubricated by straight mineral oil only) - allowable temperature is a little lower if lubricated by some greases, by mineral oils with extreme pressure additives or by some synthetic oils.
Without further data, it would not likely be easy to determine exactly how much more rapidly the cage would degrade with fuel-oil lubrication for temperatures in the range of 100-120 degC.

Since I have had no apparent trouble where the bearings are lubricated by gearbox oil or grease, I infer that the operating temperature of the bearings in those situations does not greatly exceed 120 degC. Will bearings lubricated by fuel-oil mixture run hotter, cooler, or about the same? Naturally, the ease with which the fuel-oil mixture flows through the bearings will have an effect, but I'm only looking for a ballpark figure to start with.

If I was to guess at the operating temperature of crankshaft main bearings in a two-stroke engine, I would say somewhere in the range of about 80-120 degC. However, the truth is that I simply don't know.
The imposed maximum of 100 degC for polyamide cages (where lubricated by fuel-oil mixture) is right in the middle of my guessed range, so my guessing doesn't really help. If anyone knows anything which may be useful here, or can offer a much more informed estimate, please let me know!

Thanks & regards,
James

[hybrid]

You could point a temperature gun at the surrounding area and get an idea.
I can't imagine it getting any hotter than coolant temp down there.

[James P]

You could point a temperature gun at the surrounding area and get an idea.
I can't imagine it getting any hotter than coolant temp down there.

That is certainly an idea, but there would still be some guesswork involved. Based on nothing more than a "touch test", I would estimate the temperature of the exterior of the crank case to reach about 70-80 degC. I would guess that the interior of the crank case would be hotter than the exterior, but I have no idea how much hotter. How much does the incoming fuel-oil mixture cool things down? How good is the crankcase as a heat-sink? I concede that there are a few variables here !

The nearest temperature reference I have so far is from thermocouples under the spark plugs on the bikes I ride with any regularity. The air-cooled bikes operate in the approximate range 125-215 degC (depending on riding conditions). The water-cooled ones are much more stable; about 75-100 degC or so.

I would guess that the temperature of the crankshaft main bearings would not exceed that of the spark plug bodies (whether air-cooled or water-cooled), but I have little idea of the difference. I may be able to attach a thermocouple on the crank case somewhere near one of the main bearings, but I don't think I could get all that close (too many moving parts get in the way).

To approach the issue from another angle; Did any two-stroke bikes come with polyamide bearing cages in the main bearings as standard (if the bearings are lubricated by fuel-oil mixture)? Has anyone used bearings with polyamide cages for this application?

Pertinent to this topic; AG Bell recommends the use of main bearings with "plastic or fibre" cages, but notes that they should be well lubricated. He states that a 5/32" (4mm) hole from the transfer cutaway to the bearing should be adequate (my engine in question already has a 5mm hole in each crank case half as standard for this purpose). Bell does not elaborate much more on the bearing cages, so I'm not sure whether he knows that running temperatures will not be a problem, or just that he has used non-metallic cages without problems...or perhaps he is just theorising.
I have several other books about two-stroke engines; some of them deal with main bearings in some detail, but none mention bearing running temperature or cage materials.

Research continues...

Regards,
James

[hybrid]

I would have thought the air/fuel would mean the inside was potentially cooler than the outside. What would make it hotter? Aluminium is a pretty good conductor of heat.

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[BRG1200]

The AG Bell book is pretty old now, I wonder what advances were made in the dying days of the 125/250/500GP days.

[oldjohnno]

I think you can be pretty certain that the temps downstairs will be under 100c given typical operating temperatures and their flow paths.

Fatigue failure of the steel cages isn't all that rare. I first came across it many years ago (not on an engine); if you catch it in time it isn't terminal but if you don't it will take out the entire bearing. Since then I've encountered it again several times, and every time it's been in a situation where there is a combination of light or intermittent radial loading along with torsional vibration. The unsteady rotational speed and/or the slip/stick cycling of the balls mean that the loaded ball repeatedly tries to accelerate/deccelerate the other balls via the cage. The constant tapping away of the balls on the cage will crack and eventually separate the cage into two halves and sometimes the pieces of cage will get eaten by the bearing, and failure follows soon after. I've seen this happen most often in the input shaft of vertical shaft gearboxes that have very light radial loads that drive a vibrating or chattery load. Resilient plastic cages last much longer or you can just change the bearings out more frequently. Fully submerged bearings seem to be less troublesome though I think this is more due to the damping effect of the oil than anything to do with lubrication. Often you can eliminate the problem by simply applying a light axial load to the bearing - with a wave washer or similar - that keeps all the balls firmly in contact with the races all the time.

[2TInstitute]

Case temps are about 80*C. I've had far less failures with poly cages than steel. The rivet heads fall off first.

[James P]

Thanks for the replies everyone (and thanks John for the additional benefit of your observations relating to steel cages). I had thought that the polyamide cages would be OK, but lacked any specific experience/data/test results with fuel-oil lubrication to confirm it. I'm a bit more confident now .

Thanks again & regards,
James