I'd want tri-redundant systems at a least, for everything, and zero gears to keep the rotation of the concentrical shells at the correct speed.

My washing mashine self destructed a few times. I don't want to put my children inside one for 400 years.

> I don't want to put my children inside one for 400 years.

I'm sure every parent has thought the opposite at least once.

Your washing machine probably used cheap nylon gears which self destructed. There are windmills older than 400 years old that still operate today despite using gears :)

This is false example - these windmills are actively maintained and repaired, how are you going to replace massive gears or bearings in space? Are you going to bring spare parts with you?

No, you will bring manufacturing capabilities with you. Decently sized warships carry a machine shop for a reason. A generation ship will have to be totally self sufficient, this will include not just food, air and water but also being able to manufacture pretty much anything they might need.

You mean wood gears like in https://www.alamy.com/gears-of-a-windmill-image245974580.htm... ?

I guess they were replaced a few times during the non harvesting season.

To replace the main gears of a rotating shell in space, you must "turn off" gravity for a week (or month).

My point is more narrow: if you have magical "direct fusion" technology (which is used in the main engines, as presentation says), you don't need regular toroidal nuclear fusion reactors, just use multiple redundant "direct fusion reactors". Which in turn are going to be more compact and don't need to be built as huge toruses around main cylinder.