I just installed a Shimano Hollowtech II crank and bottom bracket on my bike, and I'm wondering how they work together.
My understanding is that the bearings are located in little cups that screw into the outside of the bottom bracket shell. They are sealed but can spin freely. To install the crankset, first you insert the pedal spindle (with drive-side crankarm already attached) through the bottom bracket shell. Then you attach the other-side crankarm to the pedal spindle via a splined interface and then tighten its two bolts. That much makes sense.
But how does crankarm-spindle-othercrankarm assembly interact with the bearings? In particular, when you apply power to the crankarm (via pedaling), how does that power get transferred to the bearings? The outside of the bearing housing is flat (see first photo below) and I can't see how the inside of the crankarm would interlock with the outside of the bearing housing. The inside of the bottom bracket tube is smooth too, and recommended to be greased up. It seems like the only thing connecting the crankarm to the bearing housing is friction, and that seems like it would slip and be generally unreliable. The only thing determining how tightly the crankarm is set against the bearing housing is how much you tighten the other-side crankarm with the "bearing preload cap" (see second photo below) but the bearing preload cap is plastic, kind of flimsy, and explicitly recommended to be not very tight.
Can anyone explain how this all works together?