Re: [CAD_CAM_EDM_DRO] Re: vibration damping materials

Hoyt,


When you fill or attach lead, shot or other things
it often forms something called an inertial damper.
I agree with what you say mostly, which is why
I suggested the sand, which obviously isn't stiff,
or at least doesn't seem to be.


I also damp boring bars with "things hung on them".
In fact some are made with tunable inertial dampers
built in as are softball bats, hammers, hunting bows,
airplane structures, and automobiles. However, these
devices supply forces of similar magnitude to the deflection
forces of the structure just as I said...if they didn't
they wouldn't help!

What is incorrect is the statement about similarly
stiff damping having the same resonant frequencies
and therefore not doing anything. If you put together
two structural members of the same stiffness you will
have something twice as stiff! The resonant frequencies
and Q will change.

Now the stiffness term we use in vibration control is
really a complex number- impedance. So it can describe
resistance to displacement (a spring), velocity (like fluid
viscosity) or resistance to acceleration (a mass). As
a material property we call this complex modulus.
So stiffness may mean something different to me,
but in all cases these terms describe something that
doesn't want to go anywhere when you push on it.
That's the "impede" in impedance!

And as is well known power is most efficiently
transfered from one system to another when their
impedances match.

A viscoelastic damper when deflected has to supply
very large forces to stop a heavy piece of steel. The
moving steel stores energy, and the damper must
convert that to heat.

But I think I have failed at explaining this in plain english.
It is a highly technical subject with a lot of mathematics
and not easy to explain simply. So that's about as much
as I can or will say about it.

I would like to recommend Dr. Slocums book "Precision Machine Design".
It's pretty much the bible on designing machine tools, and it has sections
on viscous and inertial damping of them. It belongs in the library
of anyone building such things. In general it has very little math, but the
sections on damping have a lot, although it is not difficult. It can explain
such things better that I can in a short post.

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html

----- Original Message -----
From: "Hoyt McKagen" <bjammin@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, January 27, 2003 8:44 AM
Subject: Re: [CAD_CAM_EDM_DRO] Re: vibration damping materials


> At 08:32 AM 1/27/03 -0500, you wrote:
> >the heck is that? Well, I will try to avoid reams of math here and simply
> >say that the forces needed to deflect the damping material need to be
> >of the same magnitude as the forces needed to similarly deflect the
> >structure.
>
> Sorry, but it's just not so. For example, boring bars can be damped by
> filling with mercury. Or they can be damped by filling with lead or steel
> shot. Or, I damp them as needed by strapping on a few tool bits with
rubber
> bands.
>
> If the damping material has the same stiffness as the structure, it will
> resonate at similar frequencies, which really adds up to no damping at
all.
>
> What does matter is getting good coupling between the structure and the
> damping material, IE overall contact or bonding between.
>
> Regards, Hoyt McKagen
>