Richard, the best simple backer system I've seen uses a chain with two sprockets mounted horizontally on each end about a foot in length. The gear motor drives one of the sprockets. It has a pulley mounted on the outside of the chain that a cable attaches to run the backer system. It's better than a rotating arm as it doesn't have as much lag time at each end of the travel as the rotating arm. The rotating arm speeds up in the middle and slows down on each end of the travel. If you have a doubled shot it will be more than likely at the end of the travel when it slows down and starts reversing. Anything over 2" per minute of travel will work. When you get too slow such as 1" per minute of travel as used to be common, horizontal groups can give you doubled shots in the backer.