cjack
K Bikes Complex by Choice
nrpetersen said:Some thoughts from a retired mech engineer (having been there, done that, a few too many times....) -
It could be bad bearings - but there from what I read, there seem to be individual bad axle assemblies that won't stay fixed.
It could be underdesigned for the task - but then there would be many failures at very roughly (order of magnitude that is) the same mileages, because the likely loading spectrums can't be that different between drivers.
It could be a bad design - it is frankly unusual having a large ID ball bearing opposed by a tapered roller bearing, but I don't see that as necessarily bad providing the clearances & preloads are set up right.
It could be it needs a special lubricant - but then the primary variation would be miles-to-failure. The failures would generally be to the gears (when the anti-wear additives give out), not the bearings. The failure distances seem too varied for that & the benefits of exotic lubricants over more conventional antiwear oils are not that significant.
It could be oil breakdown or excessively viscous oils but the failures don't seem to be related to high (or low?) ambient temperature operations from what I can see of the poster locations.
It could be variations in assembly preloads - then manufacturing tolerances of the special parts would accumulate such that replacing the bearings (which are generally only fair at being identical) would have a marginal chance of success. The assembly includes shims to take care of this, but the shim selection would be critical. I sense that many shops just use the same shims assuming the bearings are identical. That is a mistake. The shim selection should be done using a dial indicator to measure clearance as individual bearings can and will vary in their thrust dimensions. The shim increment used (.002 inch?) does seem pretty coarse.
Bearings with an excessive preload will not roll noticeably harder. Forget about that check. Only if they are substantially overloaded will they even run hot, but it is the only preload indicator we have short of disassembly and measurement. Operating temperature rise might be another indicator though.
Gears are very critical on the locations of the operating centers and rotating axes alignment. This is probably why there is some preload involved. Get it wrong (i, e, loose) and they will get very noisy. But within limits, even noisy gears can last a long time.
It seems the initial failures are bearing related. The gears fail only when the bearing debris grinds the gears up (right?).
There has been some talk of "black stuff" that I can't explain except that fine metal debris can appear black. A magnet should establish what it is.
NRP
The fine black stuff looks and feels just like Moly additive. I have seen it in new BMW transmissions from the parts shelf as well as the drives. The new X1200X drives come with it inside from the factory and the ABS sensor and "drain" plug are installed. They are full of oil so I figure that's why all the holes are plugged. The black stuff is not magnetic and is the devil to get off your fingers if you rub it to see if it feels like moly. I guess it does. At least there is no perceptable grit to it at all. I have seen this same stuff in assembly lube as well when BMW was recommending it for splines in the late '70s.
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