I had put belts on several domestic treadles before I got my 31-15 industrial treadle. I would stretch them across my shoulders really tight to get the initial stretch out of them, then put them on.
When the guy brought my 31-15, he threaded it up and popped the belt up on the handwheel and proceeded to show me how well it worked. I sat down to try it and I was amazed at how easy it was! Soooooo smooth and almost effortless - how can that be, when it's so much bigger than any of my domestics?
Well, it's because I had the daggum belts too tight on every single one of them.
The industrial is the easiest one in the bunch to treadle - but it's so loose you wonder how it ever even catches the handwheel.