"Now the question I ask myself is why we didn't reverse the bearings, so that as the wheel expands due to brake heat freeplay develops, but that most of the time we could have run with a bit of preload (that 1.4 Nm is just enough to pinch the bearing, it is not what I would call a real preload)."
Greg Locock
I think this one has some simple answers. It is obvious that having a high preload when cold will give an even higher one when hot. But the temp. range isn't very large, and the distance between the races is fairly short. Only if we start with a high cold preload do we have a problem. Also, the balls/rollers on car/light truck front wheels are small, so the skidding that will destroy a larger roller/ball, because of its inertia, doesn't have any effect at the slow wheel speeds-even at 100+ MPH.
I was involved in one case which lasted a few years. We had 4 large vertical-shaft fans in a $$$-critical environment (nuclear power). We only needed 1 of the 4, or we had to shut down (at ~$1 million /day). The motor mfg. had screwed up big-time, using back-to-back angular-contact ball bearings without much thought to preload. The bearings were about 8" OD, with balls ~1"+. We had many problems, always with the unloaded bearing.
We called in the bearing mfg. (one of the largest and best, IHO), and after studying the drawings they immediately saw the problem-no preload. But some of the dimwits decided that a *major* electric motor mfg. couldn't really be that stupid, so there must be something else. Yeah, there *was* something else-they had used bearings with NO seals, so when the grease got hot and thin it just ran down out of the bearings! That part was easy to fix, but getting some accurate preload wasn't. We also discovered that the bearings were approaching 300+ deg. F, so I came up with a fluoro-silicon grease which could take 450 Deg. F (at $100+ per tube, back in 1985!)-it's used in expensive "sealed-for life" bearings.
About 2 weeks later one of the bearings with *my* grease failed-and I mean completely. Looked like it had caught on fire, and someone tried to put it out with gasoline and a large grinder! I was dumbfounded. Much later I discovered the mechanics had packed it *full*, thinking that would help things.
It gets worse-when one of these bearings failed, it usually vibrated so much that a large ring of bolts holding the fan shroud together would fail via fatigue cracking, and we actually dropped one section of shroud right on the floor-which was a ways below, and I'll not elaborate on just what was struck on the way down. So part of the interim fix was (I am NOT making this up!) was to weld a piece of chain on the outside of the shroud to catch the lower piece on the way down! Yes, we also did the usual bolting things-went from the orig. soft bolts to grade 5, then grade 8 and raised the torques to match, but nothing was able to withstand the level of vibration when the bearing failed.
Naturally the fix that worked was to change the motors to a design which would allow preload to be set and maintained. At one point of this whole episode an engineer wrote an "official" report which blamed the *new* grease failure on the "fact" that "the new grease was so slippery it allowed the balls to skid instead of rotating". He was lucky I didn't see that until years later!
