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Will a gear always mate with itself?

  1. Jan 26, 2012 #1
    I have a gearbox with a spur and pinion gear which has helical cut teeth.
    I can get the spur and pinion gears with either a LH or RH helix.
    I would like to increase the centre distance between the input and output shafts.
    Can I use a second pinion gear to act as an idler gear between the original spur and pinion?

    ie. currently LH pinon >drives> RH spur
    change to LH pinion >drives> RH pinion >drives> LH spur

    If this is possible does this mean any gear must be capable of mating with itself (helix angle reversed)?

    How do I determine the correct centre distance between the two spur gears given that I know the centre distance between the spur and pinion in the original design and the gear PCDs ?
  2. jcsd
  3. Jan 28, 2012 #2
    Yes, you can use another gear as an idler; can you get one with a bearing inside or will you add bearings to the housing?
    The gear handing that you suggest is correct.
    The number of teeth on the idler isn't critical; it won't affect the final ratio.
    Are you aware that the direction of the output will change if you add the idler (currently the gears turn opposite directions, they will turn the same direction after the idler is added in between).
    If the gear shaft axes are coplanar, then the centre distance will increase by the pitch diameter of the gear that is added between them. If the gear shaft axes are not coplanar, the answer is not so simple.
  4. Jan 28, 2012 #3


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    Welcome to PF, Cres. (I don't care whether or not you like that abbreviation of your name; it's all that you get from me.)
    Is there any particular reason for the size of your gears? I'm just thinking that the easiest way to match them up with a different axial centre would be to enlarge them to the point where they meet with the same ratio that you had to begin with. Just thinking out loud; I might be missing something.
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