How do I waterproof the drive axle

Discussion in 'Boatbuilding' started by klick, Apr 24, 2008.

  1. klick
    Joined: Apr 2008
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    klick Junior Member

    Hello all, first post here,

    I wouldn't say i'm designing a boat, but i'm toying around with a water propulsion system, a handheld one, and i'm trying to figure out how to seal the drive shaft to the outside. Obviously it hooks upto a propeller before that, but inbetween there what is the standard approach for making this rotating axle not take it water? I have some double sealed bearings from mcmaster which appear to be air-tight, the RPM is within acceptable range.

    Here was my plan, I have no picture so i'll try to desribe it, it's not really that complex. I want to use 2 double sealed ball bearings (for double protection) on the shaft. There would be a space inbetween them and the section in the middle would be completly sealed off as well. O-Rings would be places where they could be to seal the bearings to the axle. Waterproof grease would fill the interior chamber to it's utmost capacity, and of course it would be used everywhere else as usual. I hope that made sense, if not i'll try to throw together a picture later.

    See, on the surface, I think that would work, heck I think one bearing would work, but I want to have the ability to dive, and 33 feet down will create a 14.7/1 bar differential in pressure between the outside and the inside of my device. My seal needs to overcome this pressure difference and not allow anything through, as you may have guessed my drive system will be electric so I don't want it shorting out the motor/electrical system.

    Any comments/ideas are appreciated.

    Thanks,
    Ross
     
  2. rwatson
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    rwatson Senior Member

    Hi Ross

    I am very interested in you project, as I have been thnking over this type of concept for some years now.

    My thoughts are that you will need to use the same type of protection as they use for prop shafts, with the added complexity that there will be more distortion on the wheeled axles, because they are load bearing.

    There is a number of 'packless' type seals that would seem to be usefull here. The same principle is used by submarines to seal their propeller shafts, so it should work for your project. These 'packless' methods rely on a graphite pressure ring
    eg .
    http://www.correctcraftfan.com/articles/PSS.asp

    Tight fitting O rings and grease are very low tech attempts to seal out water, and a lot less effective than the newer alternatives, especillay in high pressure environments.

    Lets us all know how the project progresses
     
  3. kengrome
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    kengrome Senior Member

    The Chinese make this underwater scooter, just visit the linked page for the technical details:

    http://www.alibaba.com/product-gs/202701672/250W_Sea_Scooter_Water_Propeller_CE.html

    [​IMG]

    If it's close to what you're trying to design, you might be better off buying one.

    When I was thinking of designing one of these a decade or two ago, I considered hooking it up to a positive pressure hose from my dive gear. With a minimal amount of positive pressure inside the unit water simply could not get in. This would make sealing the unit MUCH easier.
     
  4. alan white
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    alan white Senior Member

    One way to seal a shaft from water entry is to use a double seal with a gravity drain between the seals. Obviously when going underwater, such a system is inadaquate. O rings tend to seal even better as pressure is applied, so I imagine this type of system makes more sense.
    It also makes more sense to use plain bronze bearings on the output end instead of ball bearings, because plain bearings shed moisture better than ball bearings, and also won't rust. A simple grease fitting and a spiral cut into the bearing should work down to an atmosphere. The plain bearing could have an additional seal on the forward end.
    A drain cap could be opened above (forward of) the rear bearing but aft of the forward seal to drain off any water intrusion.
    Many speacialized seals are made to accomodate pressure differences, but I like simplicity and reliability, hence the above idea.

    Alan
     
  5. alan white
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    alan white Senior Member

    Note: Ceramic seals, or bladder seals are another choice. They apply pressure on the face rather the side of two surfaces (one ceramic and the other a kind of carbon material, spring-loaded together). All commercial dishwashers use them. They are commonly made for a 5/8" shaft. About ten bucks. They last a long time. They are water-lubicated. Underwater, with pressure, they should be fine, since they are often used in pressure situations (pumps especially).


    Alan
     
  6. klick
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    klick Junior Member

    rwatson, Thanks so much for that link, that's exactly what I need. No need in designing one if I can just buy it at a reasonable price. Also this showes me what a "Stuffing box" is, but that allows drips to come through, a PSS should work much better.

    Kenneth, Yes I realize you can just buy these, obviously I want to make mine a bit different, also i'm doing it for fun as well.

    I need to read up more on this PSS and also i'm going to examine the stuffing box technique just to see how it works.

    Thanks everyone,
    Ross
     
  7. klick
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    klick Junior Member

    I know it's been a while since my last post here, but wanted to get some more advice/information on this subject.

    For my project I went ahead with the packless shaft seal, from www.shaftseal.com. Now this works but i'm hoping I can come up with something that is a little more compact then this solution.

    Now, I haven't received this seal yet (I should tommorow), but does anyone think these types of seals are any good: http://www.mcmaster.com and search for: 13125K84

    They are spring loaded and will push out on the shaft and on the inside. Now honestly from the mcmaster page I can't even tell if these are meant for spinning drive shafts, or shafts that move in and out, but clearly they are meant for shafts that move. They do say 250psi for slow rotary motion, although "slow" doesn't really mean anything, wish they would of had an rpm with that. These are dramatically cheaper then a PSS and would require tons less space,and space is getting cramped.

    So i'm just wondering what everyone thinks of those, they worthless or they might work? I don't even mind if they wear out after 10 hours, or 1 hour, would be better space wish then the other thing.

    Thanks,
    Ross
     
    Last edited: Jul 14, 2008
  8. alan white
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    alan white Senior Member

    Bladder seals are used on pumps all the time. They are comprised of a ceramic face on one side and some black (carbon?) matching surface on the opposing side, which is compressed against the ceramic side by a spring within a rubber bladder. They are used in commercial dishwashers among other things.
    They last for years of everyday use. They cost under ten dollars and are available everywhere dishwasher parts are sold. They commonly are sized to 5/8" shafts. Speed is no problem. They use water for lubrication and cooling.


    Alan
     
  9. kengrome
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    kengrome Senior Member

    Ross, I studied McMaster-Carr's shaft seals for a long time, and although I haven't bought any yet I had decided to use style 1, 2 or 3 on the same page. They come as a 'cartridge' and just seem easier to install to me, not to mention more appropriate for use on a propeller shaft.

    I think the style #4 you selected is not the right type for rotary motion, I think they are more appropriate for 'wiper' type seals where the motion is linear, such as on the hydraulic spring/shocks of motorcycles for example.
     
  10. Guest625101138

    Guest625101138 Previous Member

    Ross
    One of the things to do is to pressurise the unit with air. Not high pressure but enough above atmospheric to produce tell tale bubbles when initially immersed. Almost impossible to get a submersible properly sealed the first time around. If it holds air you have a good start. I would even fit a small pressure guage until I was confident.

    Water and any electrics do not mix well. If you use the li-ion batteries then even the stabilised ones that A123 make will produce a nice explosion if there is an electrical fault in a sealed chamber. You could consider a rupture disc, weak point in the casing or relief valve that vents high pressure out of harms way.

    I only use ceramic valves for household taps. I have never needed to replace a valve through seal wear since going to them 10 years ago. One high use tap leaked because the end stop got flogged and allowed the closure to overtravel.

    I have also seen good reports of ceramic valves in tough industrial applications on pump shafts. They are the standard for many chemical pumps intended for leak-free operation.

    Rick W.
     
  11. klick
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    klick Junior Member

    I actually have a style 2 (Part # 9281K71) which I bought a long time ago and actually forgot I had eheh. I just pulled it out and now realize it was designed specifically for rotary motion. The Style 4 is a little different, and yes is designed more for linear, although may work, this style 2 probably will work better for what i'm doing.

    I guess part of my confusion is, if these things exist, why is there even a www.shaftseal.com ? the PSS they sell are 10 times more expensive, much bigger. Also stuffing boxes also seem like a pain in comparison to this. I assume they must last significantly longer (the PSS and stuffing box), but pumps that use these types (style 4) of seals run for hours a day. (or more). There must be some reason i'm not seeing.

    This is much smaller, and really it's about the same as the PSS, just doesn't come with a HUGE stainless steel thing to mount on the shaft. I think i'm just gonna go ahead and use this, the PSS was a huge waste of space in my old design and just want to get rid of it.

    Ross
     
  12. Guest625101138

    Guest625101138 Previous Member

    Ross
    There is a long discussion on stuffing boxes versus shaft lip seals in another thread. It goes on for pages.

    For your application lip seals might be OK although I think Alan's suggestion of a ceramic face seal is probably the best. If you get any vibration in the shaft or run the seal dry at high speed there is a risk that a lip seal gets damaged.

    The most advantageous feature of a stuffing box is that they do not fail catastrophically. In operation they leak a little and can be adjusted when moored to be water tight. A shaft in a bearing can completely collapse and destroy the seal. You then have a big leak in the hull. Very hard to then repair while the boat is sinking. Your situation is different. Any water could make your submersible a nice bomb.

    Rick W.
     
  13. rwatson
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    rwatson Senior Member

    They dont use stuffing boxes on submarines - they use a variation of the 'packless' flexible couplings.

    I guess thats about as critical as you can go
     
  14. Ratch
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    Ratch Junior Member

    What about using a magnetic coupling that way there is nothing to seal. A set of NIB Magnets should do the trick. These things are very strong so you should design your drive shaft in a way that it would be hard for you to place any object too close (a finger between the magnet and a knife for example would hurt).

    http://en.wikipedia.org/wiki/Neodymium_magnet
     

  15. Ratch
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    Ratch Junior Member

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