Kim,
Does the wheel unscrew or does it fit on a smooth shaft?
I just looked at ours and it appears it's a press fit on the shaft. In either case, I'd suggest a trip to your local auto parts store for some RED Loctite. Get some Denatured Alcohol too. Clean the inside of the wheel and the shaft with the Alcohol then "VERY CAREFULLY" put one drop of the Loctite on the inside of the wheel and slide it on with a twisting motion. You'll only get one shot at it. You DO NOT want to get any Loctite inside the arm. If you do it's gonna be glued there for eternity or till you heat it with a torch.
Well, that's what I'd do anyway.
Joe