Shaft bearing for salt water

Discussion in 'Inboards' started by cmaas, Sep 26, 2020.

  1. cmaas
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    cmaas Junior Member

    I used a Lovejoy coupler with a urethane spider at the motor. I machined the splined crankshaft from the old engine and welded it into the coupler. Then I cut the old motor block down and used it as the base for my electric motor, which is bolted to an aluminum plate. Using the old block gave me the proper mounting holes and shaft position. It's not perfect but it's close. After welding the shaft I chucked it in the lathe and bent it to within about .010" of runout. Again not perfect but I figured the mid shaft bearing could handle that.
     
  2. cmaas
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    cmaas Junior Member

    That may be the solution, thanks. In a perfect world I would have the expertise to make a shaft with 0 runout. In reality I seem to be able to get it to within about .01o". Maybe that's close enough to be okay with a stiffer shaft. Or maybe it's worse because the shaft is not perfect and now it's heavier. I don't know. Also keep in mind I'm throwing together a quick, cheap and dirty hydrofoil using many castoff components and what I have in my shop on a remote island. "Close enough" is often the word for this project.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    In post no 12 I indicated the important dimensions for shaft resonance speed. To avoid guesswork, could you give the length and diameter of the original shaft and the length plus tubing dimensions used in the new shaft? That is necessary to make a rough check on the shafts critical ("resonance") speed, and whether it needs re-dimensioning or an intermediate bearing of sorts.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, you are no backyard tinkerer, do you notice any significant vibration ?
     
  5. cmaas
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    cmaas Junior Member

    This shaft is a hybrid of two diameters and hollow and solid. The original shaft was 540mm long, 12.9mm diameter. I cut that shaft in half and added a 640mm long tube to extend the shaft to 1m length. Tube OD is 17.15mm, ID is 12.9mm. It is seamless type 316 stainless steel.
     
  6. cmaas
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    cmaas Junior Member

    I wouldn't call it significant. The noise is annoying. And it can't be a good thing.
     
  7. baeckmo
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    baeckmo Hydrodynamics

    A rough check on critical rpms for the original shaft, with distance between supports ~510 mm (assuming splines in both ends) and semi-restrained support gives a critical speed of ~8000 to 8500 rpm.

    With the lengthened shaft you have a "soft" coupling in the motor end (an unrestrained support) and the hollow section. It is trickier to calculate correctly, but the hollow part will dominate the stiffness/mass ratio, and the combination with one semi-restrained and one unrestrained support results in a critical speed in the 2500-3000 rpm range.

    This means that the issue is critical speed, not manufacturing or balancing precision. The solution must be an increase in critical speed, either by re-design of the hollow section or introducing a support bearing. You chose the latter, which I think is a relevant path. Arranging a water lubrication to this bearing should be easier than making a new shaft. You can use a pitot pipe pick-up, either in front of the leg, or aft of the prop and just a plastic hose to the bearing, pouring a small flow onto the top of the journal. If that isn't enough, then there is the cooling water pump in the housing.
     
  8. cmaas
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    cmaas Junior Member

    FYI This is the response from Woodex, the wooden bearing mfg:

    Thank you for your inquiry. One of the attributes of maple as a bearing material is its very aggressive capillary action. That permits it to absorb a very dense load of lubricant. Unfortunately, that action is so aggressive that the grain is never completely filled, and will continue, even after oil impregnation, to absorb and release water.


    This property makes it a good choice in either wet or dry applications but, unfortunately, not in applications which cycle repeatedly between wet and dry. As the wood repeatedly expands and shrinks, it eventually surpasses the limits of its elasticity and splits. If the periods of dry operation are very brief and infrequent, it would likely work well; if those periods were prolonged and frequent, it probably wouldn’t.
     
  9. cmaas
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    cmaas Junior Member

    Thank you for taking the time to do that calculation. That is very helpful. I will try the cutlass style bearing and lubricate it with water from a small scoop and tube just forward and above the prop. I think I will test it first to see that I have adequate water at slow speeds but not a fire hose at 15 knots.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Well, that goes for maple, I have referred to lignum vitae ("Pockenholz").
     
  11. cmaas
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    cmaas Junior Member

    Maple is what they recommend for wet service ie: hydroelectric turbines.
     
  12. Blueknarr
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    Blueknarr Senior Member

    There is a history of using Log Vita in ships berings.
     
  13. sdowney717
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    sdowney717 Senior Member

    Use 'marine' grease, trailer bearing grease for boats. Standard greases actually mix with and wash away with water being soap based. Marine greases resist that. So that wont leave an oily sheen on water. Seriously people think all greases are the same, and dont wash away with water, but they can.
     
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  14. gonzo
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    gonzo Senior Member

    Water lubricated cutlass bearings are cheap and readily available. Sizes start at 3/4" and increase by 1/8" on the smaller sizes. It is a well developed technology. Submerged oil or grease contraptions will need constant maintenance and may get you into legal trouble. KISS
     
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