Shaft bearing for salt water

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

  1. cmaas
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    cmaas Junior Member

    I have modified an outboard to have a very long shaft (for a hydrofoil). At motor speeds in excess of about 2000 rpm I get a vibration that I believe is the driveshaft vibrating in a midshaft bearing I made from acetal. The acetal bearing is about .010" larger than the shaft as a tighter fit overheated and melted the acetal. I need a better bearing but am having trouble finding something suitable.

    The type 316 stainless ball bearings have a very low max rpm as does oilite. 440C stainless bearings can't handle salt water. The radial load is very low on this bearing but I do need it to handle up to 5000rpm. Salt water immersion is intermittent: submerged pre takeoff then dry when flying

    Any ideas?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Why the need for the mid-shaft bearing ? Did you fabricate a longer shaft, and how did you ensure the drive and shaft are properly aligned ?
     
    Last edited: Sep 26, 2020
  3. gonzo
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    gonzo Senior Member

    Is this the vertical shaft or the propeller shaft?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    vertical, I would think.
     
  5. gonzo
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    gonzo Senior Member

    Can you post a photo or diagram?
     
  6. baeckmo
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    baeckmo Hydrodynamics

    If you can arrange a small flow of water to the bearing position (a leak from the outgoing cooling water?), there is little that beats a rubber bearing ("Cutlass bearing). It is simple, has very low friction and the hysteresis in the material improves vibration damping.

    A short note on shaft resonance:
    Assuming you are talking about the vertical shaft, it is normally checked for resonance and designed to operate in the undercritical mode. A rough example: with max engine rpms=6000, the shaft resonance speed should be >8500 rpm. The resonance speed of a straight shaft is proportional to 1/L^2. You are now experiencing a resonance at 2000 rpm, ie 23,5 % of the original. If you made the new shaft with the same diameter, that indicates a length increase by a factor (1/0,235)^0,5, or about double the original. So the lengthened shaft certainly needs an extra support.

    Edit:
    And if water lubrication is not an option, there are alternatives for periodic grease lubrication. Contact SKF or Thordon Bearings for alternatives.
     
    Last edited: Sep 28, 2020
  7. cmaas
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    cmaas Junior Member

    I lengthened the existing shaft. Runout is less than .010" but I guess that's enough to start a vibration at speed.
     
  8. cmaas
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    cmaas Junior Member

    I like that idea though I have removed the water pump from the now electric motor. I did a little test yesterday and found that at 8 knots (the speed at which the bearing is no longer submerged) I can introduce water to the bearing with a little intake scoop and a tube.

    I'll look at the grease too. Though it would be a pain to take the outboard leg apart to grease it periodically. Just the sort of thing I would forget to do. Edit: Duh, I could fit a zerk fitting through the case.

    Thanks for the thoughts.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Thoughts spiraling; an old shipmate said "why don't you try lignum vitae, it survives anything". Funny, I remember seeing that material in small, local hydroturbines long ago.
     
  10. gonzo
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    gonzo Senior Member

    If you need to add grease, it is going in the water. That can end up in a fine and maybe jail; bad idea.
     
  11. cmaas
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    cmaas Junior Member

    What an intriguing idea. I can't imagine that it would withstand 5000 rpm. At least not without lubrication. But maybe?
     
  12. baeckmo
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    baeckmo Hydrodynamics

    Yes, sometimes we forget things learned in the past. Lignum vitae compares very favourably with modern polymers; it has a PV-value of 12000 (in imperial units), which is about 4 times the value of Delrin, double pressure capacity (2000 psi vs 1000) and 20 times the velocity limit (2000 sfpm vs 100). Its coefficient of friction and wear resistance compares well with Delrin, it does not melt when overloaded etc. Look for a company named "Woodex" my mate told me; if they still exist, the probably can help you out.

    So far the bearings, another solution might be to change stiffness/weight of the shaft. The resonance speed of a homogenous shaft is proportional to d/l^2, and a for a hollow shaft it is (D^2+d^2)^0.5/l^2 (D=outer diameter and d=inner diameter). This is why cardan shafts practically always are hollow. But I certainly like the use of LV. If you skip the grease and squeze a snail or two into the bearing, you won't have to worry about enprisonment for letting synthetic substances into the ocean.....! But don't tell the snail-huggers......
    Good luck!
     
  13. cmaas
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    cmaas Junior Member

    My shaft is hollow. To extend the existing shaft I cut it and welded a seamless stainless steel tube to the ends. It is still a long skinny thing though and is vibrating against my sloppy Delrin bearing.
    Wooden bearings! what an interesting find. Thanks for that. I will contact Woodex about the feasibility of using their bearings for a wet/dry situation. They seem to offer oil impregnated maple as well as lignum vitae. PEEK too.
    Wood Hydro Turbine Bearings | Woodex http://www.woodexbearing.com/product/oil-impregnated-wood-bearings-for-hydro-turbines

    Now I'm nervous about greasing the pivot bracket on my outboard. Is it worth a possible jail sentence?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I always wondered how you managed to align your electric motor to the input shaft, as a small inaccuracy is obviously going to be reflected in the shaft. How did you do that ? Are you confident they are exactly aligned ? As for the grease, If you don't tell anyone, I won't !
     

  15. Rumars
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    Rumars Senior Member

    Well, the obvious solution is to make another shaft of a bigger diameter and delete the mid shaft bearing. The problem is that by welding you introduce heat and distort the tube so after welding it needs trueing up again (not to mention if the tube was straight enough to begin with).
     
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