Chain torsion

Discussion in 'Class Societies' started by michael pierzga, Dec 20, 2013.

  1. michael pierzga
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    michael pierzga Senior Member

    Im laying two winter moorings... 14mm steel chain to 5 ton mooring blocks.

    Do i need to install a chain swivel at the chain mooring block interface to remove torsion loading on the chain ?

    Any links to the effects of torsion on chain strength?
     
    michael pierzga, Dec 20, 2013
    #1
  2. CDK
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    CDK retired engineer

    Assuming the other end of the chain can rotate freely, I see no need for a swivel.
    The zinc on a swivel is subjected to a lot of wear, making it the weakest link of the chain.
     
    Last edited: Dec 21, 2013
    CDK, Dec 20, 2013
    #2
  3. MikeJohns
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    MikeJohns Senior Member

    We attach the chain to the block and put a swivel at the upper end of the chain where it can be pulled up out of the water for inspection and replacement. Then another length of chain to the buoy.
    14mm is small chain though, not much weight in the catenary and a short life.
     
    MikeJohns, Dec 21, 2013
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  4. michael pierzga
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    michael pierzga Senior Member

    Yah 14 mm. Is a bit small...its all I can purchase locally. This is why i i was thinking that torsion might need to be considered.

    The mooring only stays set for 4 months..then its retrieved and stored

    Is there any literature on the effects of torque on chain ?

    The effects of torque on rope is well documented

    [​IMG]
    subir foto
     
    michael pierzga, Dec 21, 2013
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  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The breaking strength of 14 mm mild steel is well over 5 tons, your concrete blocks under water are approx. 3.5 tons, so why worry about torsion?
    A chain under load will straighten itself, only if both ends are immobilized the torsion can be translated into a bending force in each link. The derating factor for a single turn per meter I estimated to be 2-3 %.
     
    CDK, Dec 21, 2013
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  6. michael pierzga
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    Location: spain

    michael pierzga Senior Member

    Im not sure of the holding power for a 5 ton block on 4 to 1 scope. Substanial.

    Five ton blocks are the common small craft castings for the port.

    At any rate I skipped the swivel and laid the chain and rope tail yesterday.

    It would be interesting to see a test on chain and torsion.
     
    michael pierzga, Dec 21, 2013
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  7. jonr
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    jonr Senior Member

    Is there any advantage to threading a piece of zinc wire through the chain (for increased corrosion resistance)?
     
    jonr, Dec 21, 2013
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  9. wavesails
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    wavesails Junior Member

    Hi Michael are you by any chance the same Michael who worked as a skipper of Inspiration in Mallorca late 80's? I am Charles Ronaldo's Friend who used to run north sails in muelle viejo

    If so please drop me an e-mail I would love to catch up.

    cheers
     
    wavesails, Jan 9, 2014
    #8
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