S.S.Bearing

Tue Aug 5 00:53:15 AKDT 2003

If there is 0.1 mm difference in bearing OD it is significant.
Bearing with 36.9 mm OD in OS 1.40 crankcase is useless. It will rotate in case and will ruin it!
However Towerhobbies have also earlier messed with inch-metric conversions.

Originally 1.40 RX and FI called for the same rear bearing # 46030008
( see http://www.osengines.com/parts/xr19230.html )
Also the crankcase is the same for both RX and FI . So, if a bearing fits in FI , it will fit in RX.
The bearing needs to have a correct fit to be able to work properly. If it is loose in case do not run the engine.

The problem in OS 1.40 rear bearing is lubrication !
If the lubrication would be in order the bearing would outlast the engine. However with these slow running engines the axial force is very high for a ball bearing. The surface pressure between balls and races is far too high for any 2-stroke engine oil to cope. We would need a higher viscosity oil and EP additives.
If a bearing runs dry it has no chance, it will be ruined in a tiny fraction of intended running ours.
If the bearing runs hot the oil viscosity also decreases and at some point the lubrication film breaks.
Corrosion is also a fact but it is merely a spice in the soap. It is relatively easy to take care of (using no-nitro fuel after days last flight and, after-run oil etc.)

One possible solution is to use sealed bearings and EP grease (containing additives sustaining lubricating film under high surface pressure). This seems to be the case now for FI bearing.

When using sealed bearings one has to pay special attention to operating temperature.
Most EP greases can withstand constant temperature up to 100 degrees C and a bit more short time exposure. Going up to 130-140 (266-284 F) will decrease grease lifetime but is ok if bearings are replaced every two years. Also seals take 100 degrees but 120 seems to be limit as they are made of polymers.
Seal constructions vary. One of the best is NSK's that work even with small axial movement. Also the quality of original RX bearings, make NSK, is among the best. Most other Japanese makes are junk.
In most 2C engines air-fuel mixture would flush the bearing and penetrate into the grease regardless of seals. This of course ruins bearings. RX has a crankshaft that protects bearing against flushing. 

If crankcase temperature is kept at max 120 degrees C (248 F), grease contains EP additives and seals keep grease inside, the bearing will definitely last long enough. One has to make sure no oil redues nor after-run go into the grease. Putting a new bearing each season is recommended and still a lot less than required to maintain a YS 4C.

Using stainless steel bearings without change in lubrication does not change much. S/S bearings surface withstands a little higher temperatures and has a little advantage when running dry. This is not enough to cure the problem.
There are other solutions that would cure the problem but would require change in crankcase and and crankshaft.

If any of you replace sealed bearings, please check if the grease is in tact inside after use, and report to others. 
I am currently running RX with 52 flights, no bearing problems, so far. APC 17x12N, altitude 200 m, typical ambient temperature this summer 30 degrees C, fuel 10% nitro, 15 % Aeresave oil (http://www.aerosynth.de/pages/pdfs/ASaveEngl.pdf)

Regards,

Iiro Nikkila
Finland

PS.
Designing bearing solutions is part of my profession.
----- Original Message ----- 
  From: Dave Smith 
  To: discussion at nsrca.org 
  Sent: Tuesday, August 05, 2003 1:55 AM
  Subject: Re: S.S.Bearing

  Mike:  Also forgot to mention that Tower does not say this is stainless,but it is a few dollars more.  If I was to run a bearing that came with seals,I would remove them just on the chance the seals might let go.   The grease will come out anyway in a few runs.
  Here are the dimensions:  PN # 46030008,  NSK 6904: - from the 140RX parts list:- OD 37MM,   ID 20MM,  W 9MM.

  # 29430010 from the 140EFI Parts List:- 
  OD 36.9MM,   ID 20.0MM  H(W?) 8.8MM

  So far,I have no real complaints about the rear bearings in my 140RX,but when the time comes I will check a local bearing supply outlet for a stainless one.

  Verne,do you have any comments??

  Dave
    ----- Original Message ----- 
    From: mdarr00 at comcast.net 
    To: discussion at nsrca.org 
    Sent: Monday, August 04, 2003 6:59 PM
    Subject: Re: S.S.Bearing

        Dave, I was unaware of the differance in size, but I know for a fact that Verne Koester is running one in his O.S. 140 RX, and I got that p/n off of his spare bearing package.  What was the differance in the bearing size?  If it is slightly wider or narrower, it will most likely not make a differance, as long as the i.d. and o.d. is the same size.  Maybe Verne will chime in on this thread and let us know for sure.

    Mike
      ----- Original Message ----- 
      From: Dave Smith 
      To: discussion at nsrca.org 
      Sent: Monday, August 04, 2003 5:29 PM
      Subject: S.S.Bearing

      "  learned from other peoples experience that the best stainless steel bearing
      is the O.S. FI factory bearing p/n 29430010.  That is the bearing I will be
      using next, and I was told to leave the seals in the bearing to maintain the
      factory grease in the bearing."

      Mike:     On checking with Tower,the stainless bearing will not be available until October.

      Also,their specs show it to be very slightly different in size than the one they list for the 140RX.    Would this be a problem?

      Dave
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