Engine to jet coupling

Discussion in 'Jet Drives' started by mikkijayne, Sep 7, 2016.

  1. mikkijayne
    Joined: Jul 2015
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    mikkijayne Junior Member

    Hi all,

    I'm rebuilding a Dateline jet boat, (Sidewinder clone) which has a PP65 Jet. The jet has an input flange for a UJ but I don't have the coupling for it.

    Can I couple it directly to the flywheel using this:

    [​IMG]

    (jet flange on the far right, engine would be on the left)

    or must it have a sliding coupling more like this:

    [​IMG]

    The direct coupling is much easier but obviously axial alignment has to be perfect. Are there going to be any forces at play along the crank / jet axis which could cause problems?

    The sliding coupling seems fairly common but is harder to fabricate. I'm using an Audi engine so there is nothing off the shelf to do this.

    I don't want to use the rubber donut type coupling which is common with these drives, partly because I don't have any components of those couplings, and also because they don't appear to be particularly robust at high power levels. Oh, plus they have to have the engine tilted at the same angle as the jet, whereas the UJ will allow the engine to sit horizontal.

    Any thoughts or suggestions on either option please?

    Thanks :)
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    What is on the end of the jet? Normally the input shaft to the jet is splined to allow a bit of movement

    So you would have a "short coupled" U-joint, bolted to a flywheel adaptor, which bolts to the flywheel.
    Then the jet side would be a female splined shaft to fit on the male splined jet input shaft
    The spline will give the engine and pump a little room to move as the hull flexes

    Which then brings up the issue of having an extremely rigid engine and pump mounting area.
    Ie strong stringers to hold the engine and a reinforced intake/pump mounting base.

    If you do not have an extremely rigid engine/pump area, any contact with the lake/river bottom can cause severe trauma to the pump


    With the short coupled shaft, alignment is paramount.
     
  3. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The shown parts are not suitable. You should use an elastic coupling that allows radial and axial movement of the engine, plus dampening of torsional vibrations. One of the best looks like a small "wheelbarrow tyre", having a rubber "tyre" connected to a pair of flanges.

    I think I have a picture somewhere.....I'll be back.
     
  4. mikkijayne
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    mikkijayne Junior Member

    This is the input to the jet - a keyed taper, not a spline:

    [​IMG]

    and this is the input flange:

    [​IMG]

    Designed for a UJ rather than the elastic coupling these things normally have. I know this has been covered for the Datelines on here before but mine is different to most of them and didn't come with any of the elastic coupling hardware. I read somewhere it was for a Hillman Imp UJ (can't find it again) but a Landrover joint also fits.

    I'm quite keen to avoid this:

    [​IMG]

    I'd prefer to go for an American style coupling like this since they can obviously take much more power, can cope with more radial misalignment, and are considerably easier to fabricate than the elastic couplings.

    [​IMG]

    Is the consensus that there must be some movement possible in the driveline then? If so then I can arrange it so that the spline is on the engine side like the Volvo Penta and Mercruiser sterndrives have it. I have this flywheel which has a drive plate very similar to the Volvo ones:

    [​IMG]

    I just have to find or fabricate a splined shaft which will attach to my coupling.

    Re the mounting of the drive, it is considerably stronger now compared to when I got the boat. Its got a nice solid transom and the engine bearers are 3x2 mahogany. The jet would almost flap about before - there was very little strength there, but now it is much more rigid.

    Thanks :)
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    http://www.maritimejournal.com/news101/power-and-propulsion/space-saving-jet-coupling

    This is what you should be looking at. Available in the UK

    The CV joint that you have on your bench ensures that the axes ( plural of axis) intersect BUT unless you have perfect alignment you can introduce tremendous side loads on either the jet input shaft bearing or the flywheel (rear engine) bearing

    Many jets when close coupled have an adapter where the rear of the engine sits on the jet housing itself on a rubber mount so that the engine can move slightly and relieve any side loads.

    (Kodiak, Hamilton, Berkley, AT, )

    I do not see why the spline cannot be on the flywheel side as compared to the jet side as you suggest. There will always be some misalignment as the hull will flex and the bolts and flanges that you are using to install the units will have clearances, small though as they are, that will allow some movement at installation. But when tightened can impart side loads onto the bearings

    The clearance at the spline connection will accommodate some misalignment as the tolerances cannot be tight and allow some movement axially
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Note that the splined drive disc you have shown can NOT take radial loads, the incoming shaft stub has to be radially guided by a separate bearing, such as in a gearbox or reverse gear.

    So you can't just put your double yoke unit into the splined center.

    The donut rubber coupling is just as good as any; the fact that one has split is no reason to avoid them. If it had not split, the damage might have been far more expensive.... ! These are available in a wide range of sizes and torque settings from "Metalastik". When mounting the bolts, there must be a washer between steel insert and bolt head, AND THE BOLT HEAD SURFACE MUST BE GIVEN SOME GREASE to prevent shearing in the insert welds!
     
  7. mikkijayne
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    mikkijayne Junior Member

    Thanks Barry I will check out that NDE coupling :)

    Baeckmo - I'm taking my inspiration from Volvo Penta AQ drives which use the same sort of disc and coupling. What is the difference between the drive disc I have and this Volvo Penta one? What am I missing?

    [​IMG]

    I could actually support the splined shaft using the pilot bearing in the crank if that's necessary. Its there already so I can use it if it helps :)
     
  8. mikkijayne
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    mikkijayne Junior Member

    Would something like this be more suitable than a sprung coupling in this application?

    [​IMG]
     
  9. baeckmo
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    baeckmo Hydrodynamics

    The Volvo disc (and all similar ones) does not take radial loads; the shaft must have double bearings that keep it in line (the center guide in the flywheel can't do this!). The disc, be it the steel spring variety or the rubber type (see Mercruiser) is only able to transmit torque. The springs/rubber in the discs are there to reduce torque vibrations.

    So, either you use the rubber donuts (or the rubber tyre type) together with the naked flywheel, or you use the sprung, splined disc together with a spline shaft that runs in its own pair of bearings. If you look at a Volvo OD unit, you will see a short alu connection between flywheel cover and the transom unit. This part rests on two big rubber rings, that constitute the rear engine support, and it holds the connection shaft in its bearings.

    The shaft coupler Barry shows is flexible radially and axially, but it still needs a torsion damper of sorts, otherwise the jet impeller is at risk.
     
  10. mikkijayne
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    mikkijayne Junior Member

    Thats what I was missing thank you! I'd completely forgotten about that extra shaft in the bellhousing :rolleyes:
     
  11. Barry
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    Barry Senior Member

    Question
    Why does it need a torsion damper?
    Just about all of our jet installations were a short coupled cv joint, unsupported shaft, bolted to the flywheel , and a female splined connection onto the pump input spline

    The rear of the engines rested onto a rubber mount to deal with the misalignment problems
     
  12. baeckmo
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    baeckmo Hydrodynamics

    Since the Engine shaft is not rotating with a constant angular velocity (every power stroke generates a "torque spike"). In my world it is bad engineering practice to expose the driven shaft to such spikes. First, the impeller attachment to the shaft must tolerate the torque constantly shifting (and PP jets have a nylon drive hub to prevent galvanic corrosion). Second, the torque variations will generate increased vibration levels in the engine mounts and the jet mounting structure.

    Most (serious) jet suppliers have detailed instructions on the care to be exercised when it comes to the torsion damping. And you will not find any standard transmissions without a torsion damper either; it is there for a reason!

    BTW; Rear drive Volvo cars (f.i. 240 series) plus some BMW's have donut couplings in the cardan shaft line. They are good and come cheap as standard spare parts. If you stumble across a wrecked car, the tripod hubs come off very handy.
     
  13. Barry
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    Barry Senior Member

    The varying angular velocity or torque changes are not in question.
    But the flywheel will dampen out much of this.

    My experience with Berkley, Kodiak, Hamilton and American Turbine Jets, these serious jet manufacturers have never provided information or warnings about torsion dampening.

    Re transmissions I do not recall Velvet Drive or Vdrives making note of this being an issue.

    I would like any information that you have on this to read up on
     
  14. baeckmo
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    baeckmo Hydrodynamics

    Yes, the flywheel will reduce,, but not completely take away. There is always a risk that there will be resonance in the shaft system, when the driving and driven inertias happen to "meet" within a common resonance range.

    There are numerous examples of transmission breakdowns due to faulty spring constants in the flywheel discs. And there are even more that are still running, but the operator has to avoid certain rpm ranges, because of violent rattling from the gearbox or elsewhere.

    I'll get back to you with additional info as requested, but right now I'm heading off to see if there are any chanterells in our woods.....
     

  15. CatrigCat
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    CatrigCat Junior Member

    Some small transmissions specify reduction of rated torque if the engine has less than four cylinders.
    Example, 15% reduction for 3 cylinder and 25% for 2 cylinder engine.

    If one is using a robust enough transmission, engine torque fluctuations don't matter.


    Shaft resonance is a different issue, related more to specific engine rpm than engine torque fluctuations.
     
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