art wrote:
Dan Andersson wrote:

But David isn't that the object of the exercise? The object is to
dampen the occillation such that metal fatigue does not take place You
can remove or redirect the causes of occillations
but energy cannot be created or destroyed. Recognising this some
manufactures place rope inside the elements and it works even tho it
costs the manufacturer. To change the surface
of the object receiving these stress reversals only hurts what you are
trying to achieve unless what you are adding delays the effect of
stress. With foam insulation I only followed the path that is generally
used except I also wanted to isolate the inside from environmental
effects such winds from other directions as well as providing
stiffness. The bottom line is to make the elements as aerodynamic as
possible and dampen the occillation that are left. Making things less
aerodynamic only serves to worsen things. Are you sure that these
additions to chimneys is to dampen occillations as opposed to
strengthening the structure?

Looks to me like you're overconcerned about fatique problems. Given a
2M antenna the elements would have to deflect +/- some large amount, an
inch or so, probably more before the bending stresses would exceed the
endurance limit of the types of aluminum alloys normally used to build
yagis. If the stresses are below the endurance limit the element can
deflect "forever" without suffering a fatigue failure.

Further the clue in the description of the original problem indicates
that the deflections are generating oscillations with frequencies in
the audio range. If you do a freebody diagram of the "system" you'll
find it's a rather classic spring-mass with excitation vibration
problem. If you crunch the numbers and calculate the resulting time
period on the basis of the mass of a half-element, the spring constant
of the element and the observed frequency of vibration I suspect you'll
find there's no way the element can deflect +/- fast enough to generate
stresses on the element as high as the endurance limit.

Stuff rope through the element and plug the ends with some heavy goop
to add a bit of inertia out where it'll do some good and call it a day.


Cheers
Art

Brian w3rv