[GBC] Irreversible Screw
At first glance, this might look like any other Archimedes screw module. But because of its double walls and saw like teeth it will always transport balls forward, regardless of which direction it spins. It will keep switching direction on its own with a purely mechanical switch using standard Lego transmission parts.
This seemed like it would be a simple module to build, but it has actually taken many months. The biggest problem is size and weight. The rotating drum weighs about two kilograms. Finding a solution that would be strong enough and not have gaps or look bulky took a lot of effort. It's quite sturdy now, except for some panels that had to be rushed at the end.