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oscillating ballcocks

Discussion in 'Electronic Design' started by [email protected], Oct 29, 2003.

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  1. Guest

    I was filling my newly installed cistern (in the roof void) when, just
    as the ball cock cut of the water supply, it started vibrating with a
    hellofa big NOISE.

    I tied to stop it by hanging my mediaeval plumbers wrench, cantileaver
    style, by its jaws on the feed pipe with a vague idea to push if off
    the resonant frequency but all that did was change the frequency... Ah
    Ha! it needs *damping*.

    I happen to have some expensive pipe lagging: 3 inch OD, 1/2 inch ID.
    It is extremely absorbent and exhausting to fit (you have to slit it
    longitudinally and force it open and onto the pipe inch by inch, it
    takes the energy of of your hands very well indeed) that's why I never
    got round to fitting all of it.

    Once it was fitted: pure silence, like magic. Those expensive ball
    cocks are unneccessary but no plumber will believe me. I wrote this to
    the sunday building columnist of the Telegraph but he thinks I'm round
    the bend.

    I guess ya gotta be an engineer to see it? (Mind you the engineer(s)
    who designed the Millenium bridge over the Thames did not understand
    it either).

  2. Ian Stirling

    Ian Stirling Guest

    They understood it perfectly.
    The interesting bit of that was not the bridge design, but the utterly
    new problem in bridge engineering of the people on the bridge amplifying
    the movements of the bridge unconsiously as they walk/stand on the moving
    bridge, leading to the oscillation growing from barely noticable to
  3. Don Pearce

    Don Pearce Guest

    Are you saying this was the first bridge that people walked over? What
    they failed to understand was the error in making a structure whose
    lateral resonance frequency was roughly that of the gait of a walking
    person. Even more, they believed that people's steps would remain
    essentially random and non-coherent. Of course had they thought in
    terms of forced, coupled systems they would never have made this
    mistake. The reduced Q has cured the problem (I've tried it) but
    removed all the fun :-(


  4. I read in that Ian Stirling
    NO. They admitted on TV that they had to develop a new mathematical
    analysis to explain what was happening.
    This isn't new at all; it's as old as the hills (re tales of soldiers
    breaking step when crossing bridges).

    AIUI, what WAS new was that the movements of the people excited a
    hitherto undiscovered *torsional* mode (or galaxy of modes) in that type
    of bridge structure. The fact that the mode was newly-discovered was why
    it took so long to develop a solution.
  5. Ian Stirling

    Ian Stirling Guest

    Not quite.
    The idea for soldiers breaking step is to reduce exciting oscillation
    Which would have been unimportant if the people diddn't act as
    amplifiers to excite that mode from unimportant to important.

    Normal loads from people walking over a similar dummy bridge,
    if applied to this bridge would not have caused it to oscillate nastily.

    If however you placed over a certain number of people on the bridge, even
    standing still, they excite the mode in question by their unconsious
    movements as the bridge moves under them, driving it from unnoticable to
  6. Guest

    That's people for you, little adaptive feedback systems, each with our
    own uniquely variable impulse response. Ain't it just beautiful?

  7. N. Thornton

    N. Thornton Guest

    Hi. I suppose they must have wrongly assumed you were talking
    ballcocks :) But, as you rightly demonstrate, experts are fallible,
    and here you were quite right with your solution.

    My first approach with this one is to turn down the water flow
    slightly at the service valve: that adds resistance to the oscillating
    water column, and is the easiest fix I can think of. It usually works.
    Now I suppose I'm talking ballcocks.

    Regards, NT
  8. Hi,

    Another, simpler, solution if the ballcock is the brass type is to
    bend the ball arm so the ball is off centre. Now when the ball rises
    and falls the pivot rubs against the sides. There is the friction and
    thus the damping.




    Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
    (, or ).
    Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
    electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see: or

    NEW - Desktop ToDo/Reminder program (free)
  9. Fred Abse

    Fred Abse Guest

    Until the ball rubs right through, then everything gets a damping ;-)
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