Rayleigh Air Bearing

Air moving at high speeds can cause some neat effects. Jet engines, for example, basically just throw a lot of air really fast in one direction, which causes the plane to move in the opposite direction, because of Newton's second law of motion. Air can also be used as a cushion, like in a hovercraft. Air bearings put that air cushion to work to reduce the friction between two spinning shafts by inducing a levitation effect. The video below shows a demonstration of an old air bearing. Yay!

Friction Welding

Friction can be your best friend or your worst enemy. If you're trying to make a efficient engine, plane, boat, car, or basically anything with moving parts, friction is not your friend. However, if you want that car to move, friction on the tires is what allows that to happen. Also, if you don't like to slip and fall, then friction is your friend. But if you do fall, friction is what gives you a cut. But if there weren't any friction you'd still get a bruise. Blah blah blah.

In the case of friction welding, friction is what gets the job done. It heats up two cylinders that are in contact and spinning very fast relative to each other. Pressure makes sure they stay in contact to form a good weld. It's most often used to weld pipes, but like it the video, it can also weld solid cylinders. I think it could do rods of a non-circular cross section, but the corners would probably get pretty funky.