New prop shaft coupling with new shaft

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

  1. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    Asking around I find that it is actually common practice on leisure craft. But get a factor of safety of around 4 on the set screw shear strength ( design to fatigue at around 25% of the max thrust load ) and drill a decent receiving dimple in the shaft and make sure you assemble it properly and then use two of them, the 2nd is the backup don't presume load sharing. Be aware there's a risk of the shaft pulling out of the boat in reverse if the set screw(s) let go. Hence the retaining ring CDK was suggesting or the nut I wanted to snug the whole coupling onto a shoulder.

    I've seen a few set screws that failed from axial loads and another clamp type coupling that pulled out of a sailboat coupling recently after wrapping a rope around the shaft. If you are lucky the shaft stays in the hole but usually it drags straight out and the boat starts sinking.

    I guess we'd better state here that all this is presuming the shaft and couplings are all engineered for the job.

    [edit added] Just a thought. Another method used in low power couplings to both retain the hub axially and to provide torsional coupling...... is the 'offest bolt' . You can also put a bolt right through the coupling and shaft.
     
  2. 7228sedan
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    7228sedan Senior Member

    I thought of drilling right through the entire assembly and utilizing a cross bolt. That gives me a bit more security I believe. Not to take the place of the key, just to prevent the shaft from moving fore or aft if the interference fit failed for any reason.
     
  3. jonr
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    jonr Senior Member

    I'd use a properly torqued clamp coupling (see below) and not rely on getting a good interference fit. But I agree, if the set screw is large enough and deep enough, it will hold with no clamp. It can create pressure between the coupler and the shaft (ie, a tighter fit) which seems like an advantage. A well tightened bolt all the way through could do the same (but with two points, only in one dimension, ie, it would force the coupler into a slight oval shape, increasing clearance in areas 90 degrees from the bolt - that doesn't sound so good).

    http://www.waltergear.com/Psc7qb.jpg
     
  4. 7228sedan
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    7228sedan Senior Member

    I think I'm going to ditch the solid coupler and go with a new split coupler. I like the idea of the clamping force, as well as the ability to remove the damn thing without risking damage to the gearbox or shaft. The split coupling with set screw(s) in my application seems to be a better choice.
     
  5. 7228sedan
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    7228sedan Senior Member

    Ok, next item for discussion: Can anyone speak to what would be an acceptable torque for the 2 clamping bolts on the coupling? I have everything installed back in the boat with the coupler to shaft bolts torqued to 50 lb ft (roughly 68 nM). I had attempted to torque to 60 lb ft with the original hardware and ended up stripping the treads on one of the supplied grade 5 bolts. I replaced the supplied hardware with grade 8 and torqued to 50.
    Thoughts?
     
  6. jonr
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    jonr Senior Member

    Generally, 45 is about right for a plain/clean/dry 3/8" grade 8 bolt.
     
  7. 7228sedan
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    7228sedan Senior Member

    These are 7/16-20 so from the bolt's perspective 50 is fine then. I am just concerned with the appropriate clamping force on the shaft.
     

  8. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    my boat is like this.
    straight shaft with key to coupler
    2 bolts 180 degrees apart thread into coupling and go into dimples in shaft about 1/4 inch.
    Those square head bolts are extremely hard, like a grade 10.

    The fit between coupler and shaft is not a press fit. it is a sliding fit but a tight slide! likely about .001 when new parts are used.
    To much clearance between shaft and coupler and shaft will wobble move in coupler causing it to wear. Shaft is taking some angular forces due to engine and prop are never in perfect alignment. So that is why too loose a fit between coupler and shaft is a problem.

    I have found that the torque from the cap bolts tend to smush deform the softer shaft slightly maybe making it harder to take apart.

    Also I always grease these with marine grease. Or you could use a water displacing sealer like that white teflon paste seal for pipe threads. Rust here locks these together, so do what you can to stop the rust.
     
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