Re: ATM Truss tube vibrations - cloth jackets

[mjc5 <mjc5@psu.edu>]

	Waay back when, one of the original posts on this subject mentioned
putting a rope inside the tube. This is an old trick that ham radio
operators used to damp out vibrations in the beams on many of their
antennas. It is also just about the best method to use, as well as one
of the simplest. Ham radio antennas are put right into the wind path,
and the rope helps the beams last. If it didn't the vibrations would
destroy the antenna in a short order.

	Now what the rope does is rest at random points along the length of the
tube. The mass resting against these random lengths will tend to dampen
out any vibrations because of this randomness, with the rope absorbing
any energy generated where it is resting.

	The best type of rope to use is one you cannot push. Sounds like a
joke, but its true. Some of the poly ropes out there are close to
pushable, and don't work very well for this application, They are a
little light on mass as well. You want a nice compliant rope. If you
think about it, the very construction of the rope is similar to what you
are doing. The individual fibers of the rope are loose and tend to
absorb energy from one another. The rope can be made to vibrate by
putting it under great tension. Think of the tubing on the scope as a
sort of single strand "rope" that is under great tension, and will
vibrate. Add a loose rope inside, and you will be making a sort of two
strand rope that is fundamentally loose. )lets forget for a second about
all the individual strands in the inside rope for the purpose of the
argument.

	I hope this helps, and doesn't further confuse the issue. Rope is old
school, but it also works well.

	- Mike -

	

Kenneth Hunter wrote:
> 
> Anything that is "PACKED" into the tube will not dampen the vibrations completely, it will simply change the frequency. You need to have relative motion between the tube and the dampening material so that the energy is absorbed and transferred into some other form of energy... In this case heat. If the tube is packed with water, vermiculite or any other material, it will transmit the energy from one side of the tube to the other side of the tube with some losses depending on the density of the packing, This still allows the tube to vibrate.
> 
> If you were to cut and fill the tube with styrofoam "peanut" packaging material, the light pieces of styrofoam will simply go for a ride when the tube vibrates. You need enough stationary MASS to absorb and cancel the vibrational MASS.
> 
> Ughhh... Wish I could explain it better.