Friction coefficient

Discussion in 'Boat Design' started by YD Marine, Nov 6, 2014.

  1. YD Marine
    Joined: Nov 2014
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    YD Marine Junior Member

    Can anybody propose a document mentioning about the friction coefficient between two polished stainless steel (316L) parts.
     
  2. gonzo
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    gonzo Senior Member

    It depends on the surface condition, temperature and pressure. Friction coefficient are measured from samples. Even polished has to be quantified to what grit.
     
  3. daiquiri
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    daiquiri Engineering and Design

    The following table: http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm gives these friction coefficients between two polished mild-steel surfaces:
    - static friction coeff.: 0.74
    - dynamic frict. coeff.: 0.57
    - greased frict. coeff.: 0.09-0.19
    These numbers agree well with tables found in engineering manuals I have right in front of me.

    A 316L stainless can be assumed to behave like a freshly-polished mild steel, which seem to be confirmed by this reference too: http://www.bssa.org.uk/topics.php?article=99

    However, take any friction coefficient figure as a ballpark and give it some allowance, because it's exact value will vary with:
    - changing environmental conditions
    - surface wear and oxidation
    - sliding speed
    - duration of sliding
    - direction of sliding respect to the angle of surface machining
    - etc.

    Cheers
     
  4. YD Marine
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    YD Marine Junior Member

    Thank you very much dear.
    I also saw similar numbers on the net. But seemed high to me.
    Your comment is well appreciated.
     
  5. alan craig
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    alan craig Senior Member

  6. gonzo
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    gonzo Senior Member

    daiquiri: I can't open the first link, could you check the spelling?
     
  7. BertKu
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    BertKu Senior Member

    Gonzo, are you using opera? If so, do it under Google chrome, I was able to look at it. Bert
     
  8. Petros
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    Petros Senior Member

    it is normally considered that you will get galling (metal transfer) between two metals of near the same hardness. If you are just looking for when something will move or start to slip, this is not so much an issue, but if you are intending this to be a rubbing plate or bearing surface you will want a rockwell hardness difference of about 10 points or more to prevent galling between the parts.
     
  9. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Galling or resistance to galling does not depend upon the differences between the hardness of two materials but rather that crystal structure of the metal. If you run a steel pipe thread into an aluminum fitting, the aluminum will gall.
    Aluminum to aluminum, especially the lower strength alloys, will gall easy.
    Generally, the harder the material the more resistance to galling but not always the case
     
  10. YD Marine
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    YD Marine Junior Member

    Actually, we are producing quick release hooks for the rescue boats that we produce also. In the certificate renewal stage BV asked us to submit the calculation of max weight can be released with a force of 700 N applied on the handle. Weight of the loaded boat is 1.250 kg and can be released easily in practice. But on paper i cant proove it when the literature says that the friction coefficient is 0,74 must be less than 0,46. I guess it is, but i could not find the reference document for polished st. st.
     
  11. Barry
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    Barry Senior Member

    Coefficient of Friction values are derived from two parallel planes sliding against each other. So within your release mechanism, is this the case? If not, then you cannot apply the COF values.
    But if you must find a way to reduce the COF within your mechanism without lubrication, you could try Dynaloy. This is a flouridic based chrome plating process that is applied in thickness that are in the .0001" to .0003" ( three ten thousandths of an inch) range so no changes in the machining/casting process would need to be changed. The surface itself, but not the substrate has a hardness of around 67RC. It inhibits corrosion and enhances lubrication dispersion.
    The surface is a satin finish and applied at current density of around 4 amps per square inch which if I remember correctly is maybe 4 to 8 times hard chrome plating.
    I could not find any of my data on the COF but I suspect it might be available on the net. I do remember the sales pitch and that was that the COF was the same as Teflon on Teflon.
    It likely that other platers have a similar process.
     
  12. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    That requirement from BV is senseless if the handle is manually operated. 700 N (70 kg) is an excessive pulling force which puts the operator at risk of getting a spinal disc hernia, unless he is physically trained for that kind of loads. I have done a design of a release hook which opens with 30-40 kg force under 8t load, so it can be done. The inspection of the mechanism after the release test has revealed a moderate galling between the release cam and the hook, although both were made of high-resistance F51 duplex steel.

    As for the value of friction coefficient to use, you have another option - perform a test with the steel bars you have used for the hooks and calculate the actual coefficient of friction. Then perform the calculations for BV with this value of friction coefficient.

    Cheers
     

  13. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Found the info. COF for Dynaloy can be as low as .12
    Not to be confused with Dynalloy which is not a plating process
     
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