For bringing the motion of stepper motors inside a vacuum chamber while keeping everything sealed I want to try out using a simple magnetic shaft coupling. Here is a quick test of some parts.

I used 12 pieces of 10*3mm neodymium magnets, a 608ZZ bearing and a 3D printed spacer + magnet holder/shaft for each half. 

The two halves are attracted to each other very strong and there is a lot of force required to move one half while holding the other one. The magnetic coupling can be increased by stacking more magnets on top of each other, so that rather the stepper motor skips than the magnetic shaft coupling.