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Good bearing turns bad after installation??

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frank mccune:
     Hello:

     While examining a Fox .15 bb Sch., I noticed that the rear bearing was very rough sounding and noisy.  I dismantled the engine and cleaned all parts.  Both bearings run as if they are new.  After reassembling the engine, the rear bearing once again became rough and noisy. I once again stripped the engine into its basic parts and once again the bearings proved to be as new,

       Now the question becomes, why is the rear bearing so very noisy and rough when the engine is reassembled? I placed the crankshaft in the freezer, -10 deg.F and heated the crankcase to 350 deg.F and pulled the rear bearing into place with the crankshaft.  I did not detect a bent crankshaft.

        Why is causing the bearing to act badly when installed into the engine?

        Comments/Suggestions

        Tia,

        Frank


       

bob whitney:
the rear bearing could be pitted and when pressed into the case the outer housing is not able to expand .dose it feel bad in the case without the crank installed

Ken Culbertson:
The case housing for the bearing may not be perfectly round causing the bearing to distort when mounted.  Have you tried a new bearing?  Does the engine run normally with the noisy bearing?  I had an OS46FX with a noisy front bearing that apparently had no effect on how the motor ran.

Ken

Dave Hull:
Possibilities:

1. The bearings are contaminated:
(a). They still have hardened oil in them and need to be soaked and cleaned. (Not really damage, but poor maintenance.)
(b). They were clean and oiled, but got dirt in them from a crash over grass, or grit blew into things while the plane sat on the pavement during a wind gust

2. The bearings are damaged:
(a). They have rusted and balls and/or races are pitted, and should be replaced.
(b). They have Brinelled balls or spalled balls due to impact overload or fatigue. This can happen if the crank hits the ground in a crash, or, if the fits aren’t right and the bearing is drawn into the case with the threads, etc.  Or a lot of cycles under high load, like the main bearing in a diesel that has been run hard. Replace the bearings.

3. The assembly is poor:
(a) If it was gummed up, was it solvent soaked and flushed to check for free running before tearing it down? Solvent flushing never changes fits or alignment and should be your first choice.
(b) If all of these same parts were once free-running and now they are not, did it hit the ground? Are there vise marks on the case? (I have come across several of these….)
(c) During assembly, if the parts did not fully seat when you dropped them in using differential thermal expansion, and you “pull up” the assembly with the prop nut you are changing the contact angle of the bearing. As Bob said, check the freedom and noise of the bearing in the case with the crank removed. If it now feels very smooth, then something is not aligned. If they still feel rough, then the bearings are damaged, improperly installed, or do not have the correct internal clearance for the case fit that you actually have.
(d) There is still contamination on the bearing shoulder(s) and the bearings are not seating. For example, if the rear bearing is going in cocked, and it scrapes the case, pushing a burr or flake of metal in front of the outer race, you now have a problem with the symptoms you describe. If you look inside the case bore and see scrape marks, well….
(e) Overloaded bearings will “feel rough.”  What you are feeling is the lack of proper internal clearance and high spots hitting on high spots which periodically deform the balls/races. Do not use a bearing with tighter internal clearance than the press-fit was designed for. Paying big bucks for tighter tolerances and tighter internal clearances does not achieve what you want here….
(f) Don’t spin the bearings with the balls dry! This makes them prone to skidding, generating flat spots and making them noisy. Clean them with solvent, flush them, but oil them right away! Arguably the best bearings for our purposes are made from 52100 steel—not stainless. Rust ruins bearings quickly.

4. The parts are not to tolerance:
(a) Wrong bearing internal clearance
(b) Case fit is too tight. This would take up too much of the bearing internal clearance after shrink-fitting. If the bearing drops in at 300F or so, then this is not problem.
(c) The case is damaged--dinged or ovaled or the nose of the case bent out of line.
(d) The crank fit is too tight. This would take up too much of the bearing internal clearance after shrink-fitting. If the crank slid in with less than a couple hundred degrees F differential, this is not the problem
(e) The crank is bent. Engines are set up with back-to-back bearing preload. It is unforgiving of misalignment due to the higher moment stiffness. A slightly cocked bearing pair will cause ball noise.
(f) The crank journal is too short for the case. This can happen with poor tolerances. For example, it may have been fine before, but if you swapped some parts around (case and crank) and maybe the manufacturer really didn’t achieve full interchangeability. Things feel fine until you tighten the prop nut and now you feel the balls, and if you keep going, you Brinell the balls. You now need new bearings and you also need to shim the front of the crank.

Peter in Fairfax, VA:
Wow.  Some great analysis here.  My thoughts are to try another set of bearings and see if the problem follows the bearings.  If you had another entire engine, you could see if it the case, crank or bearings using the same method of progressive substitution.

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