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Mechanics of double helical gears

  1. Jul 21, 2013 #1
  2. jcsd
  3. Jul 24, 2013 #2


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    A herringbone gear can be made by placing two helical gears of opposite hand face to face. The two are then bolted together. When being driven, the total axial force is balanced. The equal and opposite forces from the sides either compresses the halves together (stable) or pulls them apart (unstable). If the pinion is in axial tension (unstable) then the bull gear will be in axial compression (stable). If the direction of rotation, or of energy transfer is reversed, then the opposite becomes true and the pressure is taken on the “back” of the tooth.

    Now consider a gas or steam turbine reduction gear. Since the direction of rotation and of energy transfer are fixed by the application it can be arranged that the bull gear be under (stable) compression and that a stronger solid pinion is in (unstable) tension.

    Assuming that the shafts are parallel, self alignment requires that at least one of the gears can move axially. If the shafts are not sufficiently parallel then there will be a significant asymmetry of the axial forces that will attempt to align the gears but that can never happen because the shafts remain misaligned and both gears will travel progressively away from the apex of their shaft misalignment.

    The term “disassembly” mentioned on the wiki page could mean two things.

    1. It could mean that the two halves of the bull gear separate because the forces that were holding the halves together have now reversed and so they are pulled apart, spreading the shaft bearings.

    2. Or that because of an axial misalignment, end thrust is present that moves both the poorly meshed gears along their shafts in the same direction, until they push one end bearing from the gearbox.

    The thing that you are doing wrong is assuming wikipedia has credibility.
  4. Jul 24, 2013 #3
    Analyze the force on the entire gear pair. There is a resultant force in the axial direction. The resultant force due to the other gear pair is used to nullify this axial force, so that the resultant axial force on the two pairs is zero.
  5. Sep 5, 2013 #4

    thanks, your explanation of stable and unstable make sense (as far as having two helical gears put together)

    ,hence I also understand how 1 is disassembling the gear train.

    however, can you explain 2 again ? maybe with a picture? I'm not quite sure i follow what you mean by moving both gears along their axial direction?

  6. Sep 5, 2013 #5


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    “2” can occur with any pair of gears or friction rollers.

    If the two shafts are not parallel then the slightly misaligned force of the gears pushing against each other, (due to contact angle), results in an axial component that will tend to move the gears towards the wider spaced end of the two shafts.

    The opposite can happen with misaligned belt or chain drive, when the pulleys tend to move towards the closer end of the two shafts.

    The gear does not have to move on the shaft as the shaft can be moved with the gear. It is the shaft axial pressure that can damage or push out a shaft bearing. Any contact noise between the gears will act as a hammer, with the axial component travelling along the shaft to an end bearing.
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