I find that surprising, why do they over heat? So on many trailers axles it is now common to have ez-lube hubs which does exactly that...fill the bearing chalk full of grease until it squirts out. They do not seem to be a problem (at least in my experience).
This gets wordy, but I'll try and explain so it all makes sense.....
Grease is basically soap and oil. What lubricates a bearing is the wee bit oil that seeps/is squeezed out of the grease while the bulk of the grease is suppose to be pushed to the sidelines and is essentially a container for the oil. More than a 1/3 fill doesn't let the happen - there isn't enough room for the grease to be pushed to the side and its churned constantly. The problem comes from the fact the fluids have shear - the churning takes a bit of energy which gets translated into heat and the whole bearing heats up.
To reinforce the idea that the grease is not the lubricant, just the holder of the oil that is the lubricant, consider oil lubed bearings. They are the same bearing, but when lubbed with oil only, their speed ratings are much higher. Because there is a lot less shear and hence heat. Somewhat counter intuitively, even then less is more. One production machine I was recently working on recommended a squirt of oil every week!
Its not insignificant. SKF says, overfilling bearings accounts for over a 1/3 of premature bearing failures. Keep in mind, if over filled it may still work, but does it work for its expected life span? None of us has time or the inclination to monitor that so we rely on the bearing co's advice which is universal - they all say the same thing as will any account of how to correctly fill bearings. Here's SKF's book for example, packed with good bearing info.
https://www.skf.com/binaries/pub12/...ce-handbook---10001_1-EN(1)_tcm_12-463040.pdf lubrication starts at page 180
Why do the the trailer bearings work with the full fill? Maybe a combination of things. First off, overfilling doesn't mean it won't work, it means it reduces its service life. Unless a commercial trailer, I suspect infrequent use means reaching their expected life span might be decades so who's noticing if its cut 40%? Also, they run at a slower speed compared that what they are capable of- maybe what, 800-1000 rpm? They are also in a less tolerance sensitive situation than a lathe centre. It might also be that it is a dirty, hostile environment and overfilling has function of flushing out contaminants which are the greater evil. Might also be that they fail from contaminates long before they would from overheating so it doesn't matter. Guess work - I don't have much trailer bearing expertise and can only go by what appears to be universal bearing makers instructions on lube and fill.
Overfilling will have an adverse effect, the magnitude of which I suppose can vary - probably depended on RPM more than anything. However much of what we do (precision machines tools) is fairly sensitive and often involves expensive bearings that we want to have perform and last. A lathe centre is essentially a spindle and many use really expensive bearings, e.g. P4's (why they are expensive). Maybe a rotating centre at low RPM's with higher tolerance bearings (less precise) wouldn't know much of a difference, but if the bearings makers are all saying 'this this the right way because....' that's the approach I've taken without really second guessing how much it matters in any given situation
