New prop shaft coupling with new shaft

Discussion in 'Inboards' started by 7228sedan, Apr 23, 2013.

  1. 7228sedan
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    7228sedan Senior Member

    Can anyone speak to the accepted machined clearance between an inboard prop shaft and a solid coupling?
    If it matters the shaft is 1.25 AQ19 alloy.
    I ordered the shaft and coupling from different locations and now need to have the two fitted locally. I wanted to get a little information prior to seeking a shop to do the work.
     
  2. Adler
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    Adler Senior Member

    Question

    I suppose they will fitted each other with cone and spline.

    Anyway should be better to make a sketch on what you asked exactly.
     
  3. jonr
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    jonr Senior Member

    My guess is as tight as possible as long as you can get it on.
     
  4. MikeJohns
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    MikeJohns Senior Member


    Before anyone can give you the options, we need to know also the shaft RPM and the power transmitted ? And the coupling is just pilot bored?
     
  5. 7228sedan
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    7228sedan Senior Member

    The shaft is 1.25 inch keyed at both the straight end for the coupling and the tapered end for the prop. Power is 340HP gasoline engine with a WOT RPM of 4600. The gear is 1.5:1 reduction. The coupling is solid steel with a straight bore and keyway. I know that this application calls for an "interference" fit if that helps. please see attached photos for example.
     

    Attached Files:

  6. MikeJohns
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    MikeJohns Senior Member

    Since it's keyed you only need whats termed a locational interference fit sometimes called a locational transitional fit, the fit determines alignment only and the torque is taken by the key. The key fit is also important since these shafts undergoe torque reversal. Also use two grub screws, a retaining dimple in the shaft and loctite rather than the wired bolt head shown in your pic.

    The coupling should ideally be 'faced' on the shaft after the fit which is best pressed on.

    Give the shaft and coupling to the machinist who will measure the shaft with a micrometer and will have a lookup table for the tolerance and fit for the interference fit.

    Preferred fits in the USA are defined in ANSI B4.2. the fit will be what is denoted H7/p6.

    I hope this helps
     
  7. Adler
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    Adler Senior Member

    Note

    According the description the shaft transmits thrust also.
    In such cases the chosen pattern for the coupling is splined conical contact like
    the propeller's one secured by nut or screwed flange.
    I wonder if there is any diameter step at the end of key channel to stop the coupling.
    If not then the fitting should be more tight than the suggested H7/p6 and will be hardly
    dismantled in maintenance service.
     

    Attached Files:

  8. 7228sedan
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    7228sedan Senior Member

    Adler, yes the shaft does transmit the trust hence why the fit is critical. I would assume that this would be a press fit however however it is not supposed to be. There is no taper, no retaining nut, and no splines. The problem I am having is locating a machine shop locally which done this type of work. I was wondering if there was an accepted clearance which I need between the coupling and shaft. As I know that the shaft is a confirmed 1.250 inches, does an interference fit or "locational transitional fit" result in a bore oversize on the coupling of 1.251, 1.255, or 1.260 etc. I wanted to more or less be able to instruct the machinist what I wanted as opposed to trusting his judgement. His best guess may not be correct as he may have not done this before.

    Thank you all for your contributions.
     
  9. jonr
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    jonr Senior Member

  10. 7228sedan
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    7228sedan Senior Member

    The rotational torque with regards to the key is I feel self explanatory. However I guess I'm having some difficulty understanding how the key will be responsible for the forward and reverse thrust on the shaft from the prop. It is my understanding that if the clearance is excessive, the shaft could pull itself out of the coupler under a hard reverse situation.
     
  11. Adler
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    Adler Senior Member

    Formula

    Dear 7228sedan,

    Please see the following address:
    http://www.engineersedge.com/manufacturing_spec/press_fit_force.htm

    According to your input data and my rough estimation the max. propeller thrust
    is 920 kgf or 2035 lbf. Of course if I had your boat hull dimensions the estimation
    could be more accurated.
    On suggested site formula you can estimate the variable [a] that is giving you
    the difference [tolerance] among the coupling's and shaft's diameters. That value
    should be in positive sign regarding to have pressure fitting. The variable [P] on that formula
    should be equal to estimated propeller thrust [920 kgf / 2035 lbf] x 1,3 [Safety factor].

    Following the above you have the tolerance value to proceed.

    When you install the coupling don't forget to preheat it into an oil [SAE30] bath
    warmed at 160°C for 10 minutes to press easier the Shaft into the Coupling.
     
  12. MikeJohns
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    MikeJohns Senior Member


    If this is taking all the axial thrust from 340HP then best practice is to use the transitional locational fit and turn the shaft down a tad to produce a small shoulder, 1/8 inch is sufficient. The end of the shaft should be turned down tapped and have a washer and nut. Forget the grub screws, usually you'll find the couplings should have enough space to accommodate a nut but they can be turned out a bit too.
     
    Last edited: Apr 25, 2013
  13. jonr
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    jonr Senior Member

    My guess would be that a split/clamp coupler would need no machining and be more likely to be removable without damage to the shaft (as compared to a rusted on interference fit).
     
  14. CDK
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    CDK retired engineer

    No matter how you look at it, the construction is doubtful.
    If the fitting is very tight, installation is a tough job; if it is less tight the coupling may leave the shaft during a hard reverse to avoid an obstacle. Tolerances are small and difficult to measure: what looks and feels like a tight fit may be no more than a small ridge at the end of the shaft.

    Engineers learn never to trust any tight fit when axial forces are involved. The shoulders as advised by MikeJohns are an absolute must, to secure the couplings against sliding backwards I would use a circlip (Seeger ring) in a groove near the end of the shafts.
     

  15. 7228sedan
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    7228sedan Senior Member

    I agree wholeheartedly that the entire set up leaves a lot to be desired. That being said, this is a highly common set up on inboard boats at least here in the U.S. I have never seen one with a shoulder machined in the shaft for forward trust, or a nut/Seeger ring for reverse retention. Everything I have researched is that a "light interference" fit is required with the 2 set screws with shaft dimples for reserve. I guess the physics of it are why I'm puzzled...
     
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