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  • Stub shaft free floating on Key?

    Discussion in 'The main mechanical design forum' started by garthM, Aug 7, 2014.

    1. garthM

      garthM New Member

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      Hello. I've been asked to copy a shaft so we can make it ourselves. This is a 5" long adapter shaft between a keyed 20 HP motor shaft and a splined gearbox input. The original shaft has no set screw or any other method of securing the shaft axially. I have always seen a set screw on the key to keep everything tight. Is it common to have a shaft with no securing method? Thanks.
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      If you have an electric motor - and you are driving something like a hydraulic pump for example - then you would typically have a spider coupling to take out any lack of alignment between the two shafts - but the spider coupling would be "retained" to both the motor and the pump shafts by some system - be it a pinch bolt or grub screw.
      These spider couplings are fairly inexpensive - and I have used them to transmit 400Hp - but you can go higher than that - see data sheet below - and general picture of what they look like.

      Hope this helps.

      http://www.jbj.co.uk/spidercoupling.html


      http://www.engineeringexchange.com/photo/oep-couplings-type-sc-jaw-or-spider-coupling
       
    4. garthM

      garthM New Member

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      Thanks Lochnagar,
      But your answer doesn't apply to my question. We don't need a spider coupling because we have piloted fits and we have the shaft that I am designing as a coupling. The question I have is about retention of the coupling shaft. I've always seen shaft to shaft or shaft to anything connections retained by some sort of device to lock them together. I wonder if you have seen a shaft coupling without axial retention? This shaft will not be able to get a way because it can only move about 6mm axially before the ends hit either the gearbox or the motor shafts, but that seems strange to be floating about in there...
      Anybody else?
      Thanks.
       
    5. Lochnagar

      Lochnagar Well-Known Member

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      Hi Garth,

      The shaft you are proposing to make should be retained or clamped to one of the shafts - and not allowed to float on both shafts. However, it is worth thinking about thermal expansion and contraction (however small that maybe) - (if you have different materials in the design of the two assemblies of parts you are connecting) - since you need float - to allow for thermal expansion and contraction otherwise you may apply considerable axial loads to your bearings in the motor and gearbox - which is why I suggest you only clamp one end.

      (In the case of the spider coupling I mentioned in my first posting - both ends are clamped to their respective shafts - but then the design of the spider coupling allows axial float any way - so thermal expansion and contraction is dealt with by the design of the spider coupling).

      The only other thought I have - is if the original design of shaft (which you are copying) - was designed as an interference fit on one shaft - then that in itself would provide the necessary retention on that shaft.

      It is worth bearing in mind the "security" of the clamping system - since high rotational speed - or vibration might play a part in loosening the retaining system - then I would use Loctite to be sure the pinch bolt or grub screw do not come loose. It is also worth bearing in mind how you get access to these systems - which may mean a plug hole in the bell housing may be required.

      Hope this revised answer is of help.
       
    6. Erich

      Erich Well-Known Member

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      My personal preference would be to secure it to one shaft. BUT. You job is to reverse engineer an existing design so your company can make the part in house. That means the application all ready exists and I assume no one is complaining about any problems with the current design. If there are no issues with the current design, DON'T CHANGE IT!!!!! You may make it worse or create issues where none existed before.

      A coworker has a great saying about situations like this.

      "Solving a problem you don't have, can create a problem you don't want."

      This is a variation on the old saying "If it ain't broke, don't fix it"
       
    7. garthM

      garthM New Member

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      Thanks Guys,
      I'm in agreement with you both, especially the not changing it unnecessarily. It's uncomfortable when one encounters unusual technology and is asked to become responsible for it...
      Thanks.
       

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