The main shaft in front should have a keyway on it and if the key or keyway is damaged then it causes it to move in a circular motion instead of back and forth possibly why the arm broke. Hope this helps!
The key was damaged when the arm broke and the slot was a little worn. I put a new key in there and a new arm should have I put a new sproket on tooIJ It hasn't been right since the break and I can't seem to get it to match the other side. Is there a trick to this like taking off both chainsIJ driftersifter
If the sprocket has play in it then it could throw the timing off. When the key was damaged it could have damaged the keyway on the shaft and or the sprocket so you might check that out to make sure both keyways are nice and tight. Have you got it to run since it broke the armIJ There are pieces of wood that act as guides as well at the base of the shaker pan these can be moved in and out when it broke it could have knocked one of those out allowing it to move in more of the circular motion. Your operators manual will tell you how close they are supposed to be adjusted. Good luck!