Crankshaft main bearings - operating temperature?
Posted: Fri Mar 04, 2016 11:30 am
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
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