stretching wire

[AlliedPianoCraft]

Re: stretching wireI'm so excited, I just can't hide it, awaiting the test results.

Al Guecia

  ----- Original Message ----- 
  From: John Delacour 
  To: Pianotech List 
  Sent: Monday, April 21, 2008 7:03 PM
  Subject: Re: stretching wire


  At 20:29 -0500 20/4/08, you wrote:


    Well I have to admit this is very compelling.
    I am tentatively sliding off the agnostic fence to join you.


  Welcome!


  Although I have massive 10ft I-beams with 30mm web supporting my upper floor, which I was planning to use for a vertical test, I have decided today to do the test horizontally on the bass string making machine.  The apparatus is as shown below.  The wire is given a normal English eye, hooked onto the machine and tensioned with a determined weight.  Near the eye end it passes through a clamp at the end of a square steel tube that lies free on the bed of the machine.  Any movement owing to the eye tightening up will therefore move the tube bodily and not affect the results.  At the far end of the tube a ball (eg. split shot) is clamped to the wire and makes contact with a lightly-sprung pointer, behind which is a dial.  The tension is applied to the wire and the tube is adjusted on the bed of the machine so that the pointer is upright and pointing to zero on the dial.  The wire is then clamped at the hook end.  This operation must be done as quickly as possible after the load is applied.


  This apparatus keeps the wire under a constant tension by means of the dead weight at the left end of the machine.  Though slight movements in temperature are hardly significant in any case, the apparatus compensates for any change by the fact that the steel tube will expand as well as the wire.  Any movement at the eye is isolated by the design from the test length.  Since only light clamping is required to secure the wire to the compensating tube, there is no distortion of the wire at either extremity of the test length.  Any movement of the pointer over a period of time can thus be attributed exclusively to an elongation of the test length.


  Does anyone see any fault in principle with the apparatus?





  ___________________________________________
  From Oscar Faber's Reinforced Concrete <http://books.google.co.uk/books?hl=en&id=_8JJewhwDnQC&dq>




  STEEL STRESSES


  Steel stresses are limited to provide a margin of safety on the steel, and also to avoid excessive stress-relaxation due to creep of the steel.  In this way, the risk of permanent deformation from overload is also reduced.


  For hard-drawn steel wire, C.P. 115 (1959) recommends the initial tensile stress should not exceed 70 per cent of the ultimate strength, or the 0.2 per cent proof stress, whichever is the less.  To reduce loss of prestress due to creep of as-drawn wires, a 10 percent overstress is sometimes held for two minutes.


  For alloy-steel bars the initial tensile stress should not exceed 70 per cent of the ultimate strength, nor 85 per cent of the 0.2 per cent proof stress, whichever is the less.
  ___________________________________________


  JD

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