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How critical is gear-tooth shape?

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  • How critical is gear-tooth shape?

    I'm trying to repair an older Coats tire machine. Part of the main mechanism is a central shaft (that drives the mount/dismount arm) driven by a gear. This gear is driven by a short "partial gear" arm. This arm is driven by the air cylinder, and moves about 45 degrees- in driving the much smaller shaft gear, this gives not quite 360 degrees on the shaft.

    Anyway, the forces on the gear has been mashing the shaft to one side for years- it's worn to a third of it's diameter. We're replacing the shaft (I machined a new one from an old 12-bolt axle) and the driven gear looks pretty good (I'm assuming it's significantly harder than the drive gear-arm.)

    The problem right now is the teeth on the gear-arm are pretty badly buggered. They're good on one side, but worn at an angle to a fraction of their original thickness on the other side. (In other words, the two gears were not parallel for a considerable period of time.)

    Finding replacement parts is developing into something of a problem, so barring that, I'm looking into welding up the teeth and recutting them.

    I'm making a quick and dirty flycutter to hold a chunk of 1/2" HSS, and I've ground a pretty close approximation of the original gear tooth shape into it, but it's not perfect.

    Now, considering that the driven gear is apprently pretty hard (since it's not worn even though the drive gear is badly smoked) and the fact that the teeth always stay in a particular mesh, how critical will it be if my cutter shape isn't quite right?

    Also, the arm appears to be forged steel- what would be better, MIG or stick to weld up the teeth? Would 7018 be better/tougher?

    Doc's Machine. (Probably not what you expect.)

  • #2
    Just go for it.
    MIG the teeth up, less chance if impurities in the weld, and recut.
    Even with a tooth profile that's a bit out it's a low tech application and you can rest assured it better than the situation you have got now.
    I have built rise and fall quadrants up on big table saws this way and they have all run fine.

    Let us know how you get on.

    John S.

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


    • #3
      How fast is this thing going to turn? If "not very," then don't worry about it.

      Depending on how far you're "off" from theoretical, you'll get some amount of sliding friction, and some amount of variation in velocity (output shaft will speed up and slow down), but in practice I doubt either will be enough to matter.

      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie


      • #4
        I agree with the other fellows, go for it. I just made my first gear with the flycut method and it worked fine. Had a wormgear go bad in my milling machine and Boston Gear didn't have one so no choice but make it. I would suggest it may be better to have a loose fit between the gears than a tight fit.


        • #5
          Mig weld it. it'll still be hard to cut but not as bad a 7018 stick Carbide just about bounces off the stuff in it's as welded state.
          then just cut it and set the mesh on the loose side.
          Rule #1 be 10% smarter then what you\'re working on.
          Rule #2 see Rule #1


          • #6
            Like all things, it depends. If the speeds and required precision is low then the gear doesn't need to be that precise. If this mechanisim turns at a high speed then it will make a difference. A little more lash will offer some forgiveness. If you do need a better fit then try making two gears and running them together for a while with some lapping compound to smooth out the gear faces.



            • #7
              Wire can hve the same properties as the 7018.
              The difference is the atmosphere created by the flux as opposed to the atmosphere created by the shielding gas....Mig it up, use 7018, whichever you feel more comfortable with. Check the specs on the wire you use though.
              The welded metal will be malleable enough to "wear" into the hardended gear quick enough.....
              We use a "uni-chome" rod at work that is machinable and harder than a 7018. Welds nice and clean too. The rod is made by Arctec but I'm sure ezch manufacturer has their own version...