Aluminium rudder stock - bearings choice

Discussion in 'Boat Design' started by Ugo, Dec 2, 2012.

  1. sean9c
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    sean9c Senior Member

    Considering the tensile strength of 6061 is around 300Mpa and the tensile of 316 is around 500Mpa either your s/s shaft is overkill or your calcs for the alum shaft are suspect. The formulas that I've seen calc in a 3/1 to 5/1 safety factor (for good reason) Have you done that?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Your figures are incorrect.

    You design using the proof stress, which for 316 is circa 220MPa, not the UTS. This is a sadly common mistake, using the incorrect material properties.
    (http://www.aalco.co.uk/datasheets/Stainless-Steel_1.4404-316L_39.ashx)

    For 6061, the proof stress is circa 240MPa. But this is unwelded. Once welded it drops to circa 115MPa. But when immersed in sea water, the "design allowable" for fatigue is circa 10-20MPa.

    It is not about static stress, but fatigue. Any member that is subjected to repeated loading, like a rudder, shall be required to take fatigue into account.
     
  3. Ugo
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    Ugo Junior Member

    Yes I did
    I designed the stock OD at bottom bearing using ISO 12215-8, then I double checked using GERR calcs and derived the safety factors.

    Stainless Steel shaft: SF=7 (oversized)
    6082 T6 alu shaft: SF = 4

    Conditions: max lift at design speed. Others suggests to design for full stall at max predictable speed (higher speed, more torque)

    rgds
    Ugo
     
  4. Ugo
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    Ugo Junior Member

    To whom may be concerned:

    it may be interesting to know that hard coating (aka "black anodizing") dramatically reduces the fatigue life of 6082 alloys.

    pls see the attachment (wholer diagram for 6082-T6 30um hard coated vs machined, rotating beam)

    source: "Costruzione di Macchine" course lectures, N. Patrone - Dep. of mechanical Engineering, University of Padova (Italy)

    rgds
    Ugo
     

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  5. sean9c
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    sean9c Senior Member

    I'd wondered about suggesting hard anodize but obviously didn't. Around these parts hard anodize is called Type-3 and is not necessarily black. It has a surface hardness of 65-70 HRC and you can get anodize thicknesses up to about 004" so it'd give you nice wear resistance at your bushings.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    It is true across the whole alloy range.
    Here is another, just another typical example...of which there are many:

    SN 6000 ally in sea water.jpg

    As I have noted before;aluminium (especially 6000 series)/sea water/welded/fatigue...is not a good combination. Thus as a designer one mitigates this, by using a different material or radically changing the design. It seems many do not heed this advice and also generally use the incorrect UTS values too...but that is their prerogative.
     
  7. Ugo
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    Ugo Junior Member

    It would be very interesting to compare salt water life of a smooth specimen to salt water life of an hard anodized one.
    Rgds
    Ugo
     
  8. Stumble
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    Stumble Senior Member

    It very much depends on the application. One of the downsides to anodizing aluminium is the aluminium oxide layer created is not as electrically active as raw aluminium. So when the part first goes into service everything is fine, but if the oxide layer is breached by scratching or damage the exposed surface is much more prone to damage from electrolysis than the rest of the part. In effect the exposed section acts like an anode for the rest of the part, and can greatly speed up galvanic damage.

    For fatigue advantages you could certainly look at shot peening aluminium parts which will result in an increase on the S/N curve, but for tubing is a pretty expensive process. Particularly if you need to to it post assembly, since most machine shops don't have the tools to do it.
     
  9. Tigawave
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    Tigawave Junior Member

    Seeing as bearing/bush material has been discussed, the fact that most people look at moisture absorption is a red herring in composites, acetals and plastics, moisture has a smaller impact than thermal expansion in most of these materials so you need to consider thermal as well as moisture.

    Most recent materials have 0.5% or less moisture absorption at 20C, however the thermal expansion can lead to far greater changes in dimensions. Many materials are also soft/melting so as the shaft works hard gets warm, has limited cooling from water flow, everything starts to get worse. The bush swells increasing friction, increasing heat causng the material to soften further and deform. Some materials have such an issue with this that you have to monitor work temperature when machining and adjust sizes as you work.

    One of the more recent materials over comes many of these problems, it's been Lloyds approved and has been around for over 15 years so it's not unproven. Maritex has a range if materials based on phenolic resins, these are hard and harden with heat. They also have low thermal expansion due to a different fibre structure. They have also done composite on composite bushes for rudders. They have shown much lower wear rates of bearing and shaft in commercial vessels, the material is hard but it has low dry friction as well as lubricants.

    Important factor is if and how much shaft flex you expect as this will effect the design of the bearing.

    If you are interested run a comparison of Thordon, Vesconite, Orkot, Maritex as a table of thermal and moisture related expansion as well as melting point.
     

  10. Ugo
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    Ugo Junior Member

    Thank you so much for your advice.
    I'll definitely check all the material one against the other.
    As regards the flexing, I'm in the range of 1.6-1.7 deg. at the exit of bottom bearing (expected if 6082 alu alloy is used for rudder stock). Any advice about bushing design criteria?

    pls find enclosed my calc sheet for further details.

    PS: I've checked the ALU rudder stock of an X-yacht X-46. the diameter at bottom bearing neck is approx 95mm, and it matches the results of my calc sheet.
     

    Attached Files:

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