Originally Posted by
Kontact
Precession is the description of why things don't just line up, but it isn't an explanation of the forces involved: The spindle, balls and cup form an impossible gear train because the cup is not allowed to move at the rate the bearings are trying to turn it. If the spindle, bearings and cup were cogs, the spindle would be locked up. A sun and planetary gear system wouldn't be useful if the center input gear could turn without the outer ring turning in kind, and BB cups don't turn.
The reality is that the balls are smooth and they are not rolling on either the spindle or the race, they slipping to make it possible for the spindle to turn without the cup turning in kind. Those friction forces of the balls trying to roll but being forced to slip like a car tire on ice is what pushes the cup looser or tighter.
It is also a reminder why preload is foolish - the balls are already backslipping, and preload just increases that friction more.
I don't see why you think your illustrations help clarify anything, but I don't think you get what is actually happening in a bearing system.
(bold) Respectfully, whoa-whoa-whoa. Bearing balls, when functioning properly, are rolling contact, that's actually how they are categorized as bearings. If they are not rolling but staying stationary and sliding against one or both races, that is known as bearing ball skidding, and it will flat spot the bearing ball in nothing flat, and then function is impeded from thereon. That is part of the recommendations for a slight preload on angular contact bearings, it reduces skidding. This is from online, a bearing company:
If you have a bearing without *rolling* elements, but sliding contact, that is a bushing.
Regarding precession, it's due to a radial force, that rotates direction with respect to the threads. The BB threads are set up for precession to tighten the BB cup or cartridge. If you have a fixed gear bike and they were to ride in reverse with a good amount of pedal force, the BB would back out. If you have a mid-drive bike that transmits motor power mid-spindle, and the rider is not pedaling, or just has weight on the pedals with no turning motion, the BB will not back out, because the radial force on the BB is not rotating orientation, it's all one direction from chain tension or rider weight. But if, on that same BB, the ball bearings actually seize up or stop rotating so that they are sliding on the BB, then the force on the BB cups is opposite intended direction and it will back out if the frictional force is sufficient.
Ball. Bearings. Roll. That's why they are round.
I'll have to look at your time of post, it may have been very late local to you. I've made mental errors in the wee hours.