Prop shaft tube, bearings and seal design

Discussion in 'Metal Boat Building' started by pdwiley, Dec 9, 2009.

  1. pdwiley
    Joined: Jun 2008
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    pdwiley Senior Member

    Another question.

    My plans call for a straight piece of 50 NB heavy wall pipe. The prop shaft will be either 1 1/4" to 1 3/8" depending on what I can find at the right price. I don't really care as I have the tools to do the taper, threading and keyways myself. My engine is a Yanmar 3QM30H, 27HP at 2600 rpm, with a 3.5:1 gearbox if it matters.

    I seem to recall seeing a prop shaft/tube design here at one time that incorporated thrust bearings in it. Anyone have a reference? I'd prefer to keep the thrust off the g/box, not because it can't take it but because then I can put a universal/CV joint between the box and shaft, which eliminates a lot of the hassles about shaft alignment and movement due to engine torque reactions on anti-vibration mounts.

    I have a preference for plastic oilon bushings top & bottom and fill the prop tube with light grease. I know the bushings swell 11% in water and can allow for this in their machining. Is there any reason I shouldn't do either or both? Anyone want to suggest another readily available industrial plastic or similar?

    Why should I pay marine chandler prices for dripless ceramic shaft seals etc when industrial pump seals are the same thing at a fraction of the price? We're only talking about 600mm to 900mm of head, after all.

    I have a lathe more than big enough to handle the prop tube to machine seats for bushings and to true up a welded flange on the inboard end so actually making it isn't a drama, just getting the design as good as I can.

    That's probably enough for the moment. My angle grinder is telling me that it's feeling neglected.
     
  2. MikeJohns
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    MikeJohns Senior Member

    I like grease filled shafts, but use waterproof grease.
    Ultra high molecular weight polyethylene is a good marine bearing material.
    You can buy all sorts of stock sizes and turn them out yourself, or buy standard bushings.
    I think AB plastics is worth a try, There's also vesconite from Vesco plastics ( Both in Victoria ).

    I'd use a commercialy available thrust bearing, they are a good idea if you have room.

    I sent you a PM
     
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  3. Frosty

    Frosty Previous Member

    A universal joint can take thrust, its a constant velocity joint that can not.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I use stainless steel stern tubes, oil filled, integrated thrust bearings and universal joints. There are several posts with pictures and drawings on this forum. Search for DIY surface drives.
    The system has been in place for several years now and the oil bottles show the exact same level the day I filled them. No absurdly priced marine seals, just standard industrial items from SKF.
     
  5. anthony goodson
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    anthony goodson Senior Member

    Polyacetal machines well and doesn't absorb water
     
  6. pdwiley
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    pdwiley Senior Member

    That's the sort of thing I had in mind, thanks. My tube is 60.3mm OD Sched 80 black pipe. I could swap to s/steel as I haven't welded anything in place yet but to what advantage?

    The sizes aren't the same as my shaft will likely be 35mm OD but I have a bearing catalog and I see that your sleeve is 35mm OD anyway.

    Questions:

    How many hours of running time do you have on this setup?

    How did you hold concentricity between the tapered roller bearings and the radial bearing at the prop end, or didn't you worry about it? I know some bearings are self-aligning (I have a number of them).

    How did you lock your prop shaft to the thrust sleeve in your drawing? That one, for my setup, has me a bit puzzled.

    PDW
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Here are the pictures and drawings:
    http://www.boatdesign.net/forums/surface-drives/diy-tunnel-drive-20433.html#post172724

    I did not keep track of the number of running hours but looking at the date of the original post it must have been several 100's. Occasionally I lift the floor to check the oil level an decide whether or not to make a signaling system. Never noticed a change, so I forgot about it. The trick is the double seal behind the props, the chamber in between is filled with silicone oil, the best water repellent I've ever seen.

    While constructing the tubes I used a dummy shaft with discs to simulate the bearings and a threaded end to firmly clamp the flange to the tube. Because I didn't trust my ability to weld the parts together without a flaw I took the whole to an experienced welder who did it for me with a TIG welder.
    I let the dummy in place until the construction was cold, then inserted the real shaft with bearings and confirmed the concentricity.

    From the end of the prop shaft I removed appox. 2 mm to create a shoulder for the bearing carrier. That takes the forward thrust.
    The reverse thrust is taken up by the splined adapter, kept in place by the oversized clamps with 4 of the strongest Allen bolts I could find. That part also has to transmit the torque, so I probably overdid things a bit.
    If there was more space for the engine, gearbox and universal shaft, I would have probably used a more orthodox construction, but this has survived lots of abuse like bollard tests, a rope in the prop and changing gear with 3000 rpm because the electronic throttle control got stuck.
     
  8. pdwiley
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    pdwiley Senior Member

    Thanks, I've downloaded those drawings.

    If I turn the inboard end of my prop shaft down from 35mm to (say) 30mm, that gives me a shoulder to handle thrust in forward. If I put a flange on the inboard end of the shaft where the flange face butts against the other bearing cup then this will take care of thrust in reverse. The inboard seal then runs on the outer surface of the flange not the shaft, which is no big deal as long as the surface finish is within specification. The material can be carbon steel as it won't - in theory - be exposed to salt water. I can machine the flange to the SAE specification without any dramas.

    I'd do my bearing housing differently to you, as in essence a double flanged bobbin, but that reflects what we have to work with rather than anything fundamental. I'd weld it all up first, then stress relieve it in a nice wood fire, then machine it. Get one flange right, then flip it over in the lathe and machine the rest of it in one setup.
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I concur.
    My lathe is too short, otherwise I would have followed the same procedure.

    The carbon steel flange will get a bit rusty because of the environment, but that ends at the seal. A light coat of silicon oil will keep it shiny for at least a year.
    How will you secure the flange to the shaft?
     
  10. hartley
    Joined: Feb 2006
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    Location: australia

    hartley Junior Member

    prop shaft tube

    FROSTY i thought better of you .universal joints ,such as the hook type
    typified by dana spicer ,hardy spicer etc etc will not take thrust loadings , why do you think we take so much time and effort to incorporate proper thrust bearings ,if you doubt this ,take one apart ,see those little needle rollers ??? some one else made the same statement in another thread ,about
    time it was put to rest .Now frosty i like your posts so seasons greetings to you and yours ........cheers Hartley
     
  11. Frosty

    Frosty Previous Member

    It cant take as much thrust as it can take torque!!!!!

    A thrust bearing Is a different thing.

    Why am I telling you this
     
  12. Tigawave
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    Tigawave Junior Member

    Beware of grease, grease is a poor conductor of heat, so if you are planning on using a polyacetal or polymer composite bearing (which all absorb water and suffer bore closure) be very careful of thermal expansion of the bearing. I've seen many failed bearings due to too much grease working its way back from the stern gland.

    The poly bearings differ widely in their bore closure characteristics due to moisture absorption and thermal expansion. You pays your money and takes your choice.

    Phenolics exhibit some of the best bearing properties currently, and are significantly different to other poly bearing materials. There are a few suppliers each with different properties so it can be worth shopping around if you want the best. If it's low power low torque low rpm then it's not such an issue.
     
  13. Frosty

    Frosty Previous Member

    I agree with tigar . I have seen a few boats have to have the shaft cut from the boat.

    You would think that a rudder would not suffer the same, as it is minimal rotation, but that also does not seem to be the case.

    I used cutlass bearing on my rudder shaft, yes a bit expensive but 7 years down the road they are perfect and can be removed as if they were new.

    The rubber of the cutlass also absorbs vibration that high speed rudders can sometimes generate.
     
  14. pdwiley
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    pdwiley Senior Member

    Assume you mean the inner flange that's going to get connected (somehow) to the g/box via a stub shaft/universal joint setup.

    My first thought was a straight sliding fit, keyway and retaining nut as simplest and allowing adjustment of bearing preload via machined fits, spacing rings, what have you. However I see that the recommended way is a taper fit a la the reference to Gerr's book pages on Google. This has the disadvantage of making the fit to the opposed tapered rollers more difficult, assuming the taper engagement can move slightly depending on torque applied to the nut.

    So - I don't know, yet. My big lathe has a taper attachment so actually cutting tapers is relatively straightforward (once you do a dummy one or 2 in something sacrificial of course, to check the angle...).

    Ditto whether to use 316L or 2205 for the shaft. I need to check out the local prices on both before deciding.

    On universal joints, my advice is that they will NOT take end thrust, the little needle bearings will die rapidly. They also need a shaft angle of around 15 degrees so the bearings don't brinell rapidly. A CV joint deals with issue 2, but not (AFAIK) the first one.

    PDW
     

  15. CDK
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    CDK retired engineer

    That's what I meant.
    Gerr may state that a taper fit is the best way, but you'll never be able to predict the bearing play until you fasten the flange nut. Is there no slack at all you may have already damaged the bearings and if there is, you have to pull it off again which can be pretty hard. At the next fit it will slide a tiny bit further up the taper.

    If you have calculated a straight sliding fit can take the torque, I would do it that way. The only other, stronger option I can think of is splines, but you need special equipment for it. I had my SAE spline adapters made by a friend in a large machine factory and still had to spend several hours with a Dremel grinder in the support of my lathe because the splines weren't deep enough.
     
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