mmc205
Senior Heliman
Location: Pottstown,PA - USA
| It may have very little to do with lack of oil, and a lot to do with shock loadings and dare i say it...preloading among other conditions bearings don't lke. All of these things will kill bearings early. Oil will do very little to help you if you are pounding the bearing hard enough to leave divots in the race. My sugguestion to run a little richer was only aimed at those who are really running lean. Experience at root cause analysis of a problem has shown that when there is a failure, there is almost never one cause, its usually several factors all contributing. However, if time isn't taken to fully examine the situation, a layperson will see what seems to be a factor and say "thats it". In this case, and i'm learning more from everyone's input (even counterpoints are appreciated as long as there is some kind of substantial data that goes along with them) it seems that
-Some bearings last a long time, some do not. It seems somewhat indepenent of flying style and mixture, though i think we can all agree if someone is running extremlely lean it would be detrimental.
-Stock replacment bearings, as well as aftermarket bearings also go bad in the same engine. (has anyone replaced a stock bearing after a having it go bad quickly and had no problem for appreciably longer than the original, this would be a good data point)
-Some bearings have shown a failure mode at the bottom (opposite from head) of the outside race, as if it were being continously shocked at either TDC or BDC. The inner race shows failure all around as would be expected.
-corrosion doesn't seem to cause this as many that failed had no signs of oxidation, and others that seemed to last forever were very oxidated.
from this i have a few thoughts. We may be dealing with two different causes of failure.
1) One is probably bad bearings, plain and simple. Poorly selected balls, bad races, lots of slop leading to skidding and pitting.
2) Two is probably engines loaded to heavily. Imagine getting the piston at top dead center, and then detonating the fuel while holding the crank. All that energy is transferred through the pistion, through the connecting rod, and into the bearing. Although this isn't the exact case with our helis, if someone were to have pitch/throttle curves set up poorly, the engine could have a heavy load at lower rpm, resulting in heavy loads on the bearing. Imagine going 5 miles an hour in you manual tranmission car and shifting into 5th and slamming the gas, can you imagine that terrible sound it makes? thats the sound of the explosive energy of the combustion having nowhere to go and thus pushing the piston straight into the journal bearing. I have to admit i'd never thought of this second cause but it could be the reason some people who fly very casually and rich might still have a problem, and why some who fly hard 3D with proper rpm and pitch don't have any problem.
These two cases are not mutually exclusive however and if one had both going on at the same time it could really do damage quickly.
Any thoughts (after my long winded post)
Heli's don't fly, they beat the air into submission!! |
