oscillating ballcocks

[[email protected]]

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).

Robin

 

[Ian Stirling]

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 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).

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
unsettling.

 

[Don Pearce]

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
unsettling.

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 :-(

d

_____________________________

http://www.pearce.uk.com

 

[John Woodgate]

I read in sci.electronics.design that Ian Stirling

They understood it perfectly.

NO. They admitted on TV that they had to develop a new mathematical
analysis to explain what was happening.

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 unsettling.

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.

 

[Ian Stirling]

I read in sci.electronics.design 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).

Not quite.
The idea for soldiers breaking step is to reduce exciting oscillation
modes.

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.

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
uncomfortable.

 

[[email protected]]

Not quite.
The idea for soldiers breaking step is to reduce exciting oscillation
modes.


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
uncomfortable.

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

Robin

 

[N. Thornton]

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 happen to have some expensive pipe lagging: 3 inch OD, 1/2 inch ID.

Once it was fitted: pure silence, like magic. Those expensive ball

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

 

[Malcolm Reeves]

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

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).

Robin

--

....malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
([email protected], [email protected] or [email protected]).
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:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)

 

[Fred Abse]

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.

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