angular contact bearings - back to back, further apart

[johnsattuk]

I guess the tube acts as an additional way of protecting from too much preload, right? without the tube, there's no way to stop the bearings from being practically destroyed by over tightening the locknut / cap-bolt?

Well not quite, if you get too enthusiastic with the wrench and collapse the tube too much, you can still destroy the bearings. If you exceed the preload on assembly the recommendations are that you use a new sleeve, which of course they always do

What it gives you is a high torque retaining nut/bolt with the ability to set a small preload and a very rigid assembly. In practice it is quite easy to feel the spindle start to take a preload and is also possible to adjust later if required, not so easy with fixed length spacers.

Differential pinions do have stringent requirements, high loads from the hypoid gear at perhaps 200hp 6000+ rpm and must run quietly.