Crank just spins

Thanks for reaching out to us. I can tell you right off the bat that the main issue is that the arm is completely missing; the arm on an operator like this is not designed to be removed, and if yours has come off then that means you've experienced a pretty significant failure. I would wager that the gears have stripped out beyond repair.

I'd be happy to see if we have a replacement for you, but I can't make a suggestion based on the base alone. I'll also need to see the arm (or arms) that came off when you took this out of the window. Seeing the whole thing in undamaged condition would be ideal, if you happen to have another window in the house that uses the same hardware.