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    Discussion in 'The main mechanical design forum' started by greenguy, Nov 30, 2010.

    1. greenguy

      greenguy Member

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      I'm hoping to get some feedback on the best method(s) for mounting a gear to a shaft. I have a project underway that will use bevel, spur and ratchet gears. Fixing the gears to the shafts such that there is no slippage is critical. Yes, I'm a rookie.... Thanks,

      Greenguy
       
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    3. swertel

      swertel Active Member

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      There are some rules of thumb, but honestly the best approach is to contact the gear manufacturer or find the right page in their catalog that tells you what mounting positions and shaft tolerances are required for the best fit.

      You may have a gear that is press fit and needs tight tolerance and fine surface finish on the shaft.
      You may have a gear that uses a compression fitting or collet to mount it.
      You may have a gear that requires a shoulder on the shaft to index against.

      The only way to know for certain is to pick the gear you need, run the numbers to make sure your shaft strength is sufficient for cyclical loading, and then follow the manufacturer's guidelines for mounting.

      I will warn you though, if you are designing your fit and REQUIRE bearing-grade adhesive (Loctite) for it to stay on, you did a poor design. Gears, pinions, bearings, and pulleys should not require Loctite to stay affixed to the shaft. Using Loctite is OK as a secondary joint, just don't make your design depend on it.
       
    4. greenguy

      greenguy Member

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      @ swertel - Thanks much. This seems to be a very common sense approach. And you provided clarity on the topic in general, and that is that there is no "one size fits all" approach to mounting the gears. Any feedback or experience on cold welding and its usefulness in mounting gears, bearings, etc. to a shaft?
       
    5. swertel

      swertel Active Member

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      I have honestly never heard the term "cold welding" applied to a gear joint before.

      I can tell you though, that a typical press fit usually requires heating the gear or cooling the shaft -- aka shrink fit -- in order to create enough clearance between the two diameters to slide the gear into place. Once the temperature returns to equilibrium, the pre-loaded joint of the pressure fit creates enough friction that the gear does not spin or slide on the shaft.

      This is not done under a vacuum and does not plastically deform either the shaft or the gear, thus it is not cold welding. Do van der Waals forces inevitably fuze the parts together? Possibly. But nothing a little heat and a BFH (bigger fucking hammer) can't solve. :D Care must be taken during tolerancing of the shaft and hub as well as how much temperature change to apply in order to make sure the materials remain in the elastic region.

      Personally, I can't fathom why you would want to plastically deform, using the cold welding process, either the shaft or the gear. It must greatly reduce the life of the system, especially under cyclic loading. Is there a Goodman diagram that covers fatigue loading in the plastic zone of a material?
       
    6. greenguy

      greenguy Member

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      Thanks again for some very good information. Regarding cold welding, I have no experience with it but found the following article interesting: http://www.tribtech.com/app.7.htm

      While this article doesn't mention plastic deformation, I would think that it must occur. And regarding the Goodman Diagram, I'm not aware of one.
       

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