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National Combo Gearbox Adjustments?


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  • National Combo Gearbox Adjustments?

    Well long story short Table gear box like coasting with good brushes in.
    I keep reading about shimming and such, but just want more details.

    Where am I shimming? The little plate opposite of output shaft?

    Besides pits and obvious wear. What to look for as being worn out,
    too much play, etc....?

    Thanks for you time...

  • #2
    You can double up the carbon brush springs to make them stiffer, that helps, but the shims would go between the outer plate and the crankcase (outer body) on the shaft opposite the output shaft- the one that has the pinion type gear and the larger gear driven by the worm. What you would be doing with the shims is moving the gear that the (motor) input worm gear drives, essentially moving it off center in relation to the worm, the more it's moved the tighter those gears will mesh, so not too much.

    Usually an "easy" gearbox is an indication that the gear teeth have become worn. A visual inspection of the gears and bearings would tell you what is going on. A good gearbox generally is tight, in other words with the gearbox on the bench if you grab the output shaft and try to turn it, it should not move. If it does, something is worn out.
    Last edited by Tablejam; 04-24-2012, 06:02 PM.


    • #3
      check the elecric breaking, with good breaks it should'nt coast.


      • #4
        Originally posted by rivmike View Post
        check the elecric breaking, with good breaks it should'nt coast.
        My exact words.
        If it cant be fixed with a hammer, use a bowling pin.


        • #5
          also if the keyway on the shaft is flogged out you will get a lot of movement


          • #6
            If at first you don't succeed, try, try, again. Then quit. There's no use being a damn fool about it." -W.C.Fields


            • #7
              I already have one of these. ^^^^

              Turns out a shim managed to work loose and was balled up inside.
              Replaced and all seems well.

              But back to the other part of my question. If I end up rebuilding a
              gearbox with new parts or checking an old one. How do I know I got
              the right number of shims in place?


              • #8
                I would say what it had before, I insides are machined alike and the shims help with the slightly different box castings.


                • #9
                  Yes, because of the slight differences in the crankcases and gears/bearings, every gearbox is slightly different eventhough they are all the same.


                  • #10
                    thanks for that link Felix, it'll come in handy


                    • #11
                      so you are saying there is no adjustment with the shims? just put it back like it was when you took it apart?
                      When you know what to do, everything is easy.


                      • #12
                        Originally posted by Grit View Post
                        so you are saying there is no adjustment with the shims? just put it back like it was when you took it apart?
                        I'm sure that may work most of the time unless you come across a gearbox someone shimmed a little
                        to get work longer before replacing worn parts, but if they didn't leave any notes! That's where I'm
                        come at this at the moment. Got an empty gearbox I would like to get up as a spare, but no shims
                        to go by for putting back together.


                        • #13
                          From what I understand the shims are there just to take up the slack from the outer bearing seat so the bearing doesn't slide back and forth in the casting. I have been going through my gearboxes(6 done 34 to go) and have come across a few that I have had to swap parts from a spare none usable gearbox to rebuild the one I was working on back up and going. Parts for these things can get expensive really quick, so good luck. Stahls Seventies can help you out with parts if you need them.


                          • #14
                            The shims are there for minute cone bearing loading
                            - just put all shims back on there after a rebuild and seal it up good with blue permatex and it should be fine


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