I’ve had a question posed to me that I’ve tried to solve in the past. Take a flywheel, add a connecting rod, piston and cylinder. Spin the flywheel and make it all move. I’ve gotten all of this working using Child-Parents and Track-To constraints, except, the piston still wants to rotate about the center point. All I need is a way to make it "slide when it’s pulled.
I’ve tried the “Lock Track” option thinking that I could lock it along an axis, but it doesn’t work. If we could get this working, we’d have a nice simulation creator.