bob27s
Posts: 4924
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
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bottom line here.......
some engine manufacturers use bearings that were designed for
1) roller skates and/or conveyor belts
2) contant lube applications
3) machine use, where the RPM is generally below 5000 rpm.
Yes, they are adequate, durable and will function in the application.
If the engine is run hard, (beyond typical rated RPM) the bearings may not stand up.
However, over time, the matierals they are made of will simply fail either mechanically or through corrosion.
For some reason, bearings originating the far east have proven to be the least durable over time, in all respects.
Don't let the idea of stainless steel or ceramic fool you. Almost all bearings need good lubrication or they will wear. If turning at 10,000-15,000 rpm as most typical small RC engines turn....... they need constant lubrication from the fuel. A well designed engine has lubrication paths to help fuel/oil migrate to those locations.
Also, do not let the ABEC rating fool you either. That is a measure of tolerance, not directly realated to durability under heat and load.
A model engine is a nasty place for steel parts. Burnt fuel creates acidic materials, which, when left on an unprotected surface, will rapidly corrode a steel or iron part. Air/moisture/chemicals combined in any fashion will corrode steel, alumiunum, any metal.
Several fuels on the market include rust inhibitors and 'coating' enhancing additives. These help somewhat. As noted, Castor oil tends to provide a coating that clings to metal parts... provides good wear and anti-corrision properties. But it alone is not a fix-all for crappy bearings.
The best thing you can do with the stock bearings, run the engine dry of fuel after each flying session, and load the engine up with several good squirts of Marvel Air Tool Oil....... or a similar air tool oil product. The air tool oils are great at coating, AND displacing moisture. I avoid ATF since the wrong one can damage your engine (some contain friction additives). After-run proceedures are your best defence.
Although.. its not always enough. Back in the 1980's when we flew the OS-P .61 pattern engines, one of the best engines of its day, the bearings were a known weak link. Some of us even tried running the engine after a flying session with alcohol/castor only fuel... for 3-4 minutes at idle.... just to coat and clean out the engine. It was a monster hassle. It worked, but only in part. It prevented corrision a bit, but the rear bearing still wore (and sometimes failed) under load. It simply was not sufficient for the load. It came to the point for many of us where any new engine was immediately disassembled, and the bearings replaced before it was ever run.
Clearly, the optimal solution is to equip your engine with bearings properly suited for the job. For sport use, no need to go through the extreme outlined above. When the originals wear out, its easy to replace them with good products.
Symptoms of failing bearings:
1) phycical "Feel" while turning the engine over. Crankshaft should rotate freely with no grinding, resistance or noticable flat spots.
2) suddenly the engine is behaving 'weird' and changing the glow plug does not correct the situation. A worn bearing can create an air leak, creating unstable crankcase pressure and fuel draw... resulting in inconsistant performance.
3) In flight 'sound'. IF you hear a raspy or rattling sound while in flight..... worth checking the bearings. Bearings on their way out will sound very unique.
4) If the engine has been "in the dirt", there is a very good chance there is dirt in the front bearing. Usually even if its a shielded bearing. Do not assume it will clear up.
At some point during a season its worth while pulling the backplate off of the engine to take a look at the rear bearing.
When should you change bearings??
1) you detect one of the above conditions.
2) If the engine has been hard in the ground, its usually worth changing the bearings. Its not hard to do or expensive.
If you maintain and store your engines properly, the original bearings will tend to last a long time. This tends to be more true for "older and vintage" engines (maybe they just had better bearings to start with). Even on my racing engines, I have the original bearings in several of them....even with them running very hard each time out.
But again, if the engine came with conveyer belt bearings in the first place, keep an eye on them for that first season. With those, its just a matter of time before they wear or corrode to death. If they survive that first season, you are caring for them fairly well.
Boca offers very good replacement bearings. Other good sources available too.
Bob
< Message edited by bob27s -- 11/9/2004 4:11:35 PM >
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Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> All this talk about bearings? 
All this talk about bearings? - 
) at full throttle from lack of fuel. I'm not saying that RC engines should be stopped like this every time but they should be at the end of the last flight for the day. Full throttle and either pinch the fuel line or remove it until the engine stops turning. 
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