Which reminds me of the rope dampers in my prior TH7DX. Apparently, the
elements w/o traps were falling off due to coupling of element
mechanical resonances. HyGain/Telex came up with a clever fix by
putting a 2 ft length of polypro rope into the tips of those elements to
dampen the vibrations. A nasty property of aluminum is that it has no
fatigue limit like steel. If a certain stress level is not exceeded,
steel won't fail in fatigue. There is no such threshold in aluminum. A
small stress over many cycles and aluminum will fatigue fracture.
Grant KZ1W
On 5/4/2016 10:36 AM, David Gilbert wrote:
That's the concern I would have with some of those systems. Unless
there is mechanical loss in the coupler (damping), the energy it
momentarily decouples gets stored and returned to the system ... with
at least the theoretical possibility that it adds to forces in the
other direction. I thought I read somewhere long ago that some
rotator manufacturers stopped offering such couplers for that very
reason ... but I'm old and could be mistaken. ;)
Dave AB7E
On 5/4/2016 8:39 AM, Grant Saviers wrote:
You ask a very important question. Can these handle the static axial
load of mast and antennas?
http://www.wholesaleimportparts.com/driveshaft.php for a picture of
one with mating assemblies.
A complexity is how the shaft (mast) is supported either side of the
coupling as I don't think they are designed to handle large sideways
torques or axial thrust - i.e. each shaft is held in alignment by two
bearings which also control the axial dimension, which would not be
the case in using one above a rotator and something else at the tower
top. If the something else was a tube sleeve then it constrains the
angle the mast can attain, but not the axial dimension. If the
something else is the typical "thrust bearing" then the shaft can
move to some surprising angles, but does have axial constraint. In
neither case would a HyGain or Yaesu design rotator really be two
bearings holding its output "shaft", except when the dead (axial)
load is sufficient to keep the races tight under all circumstances.
Other rotator designs have constrained shafts with two or more bearings.
The common "Lovejoy" coupling is another version of a rubber isolated
coupling in common use in many sizes. Again, it is used where both
shafts are rigidly constrained radially and axially. A Lovejoy is
specified to handle x degrees of misalignment and y thousands of an
inch of shaft offset, at an rpm and torque value. I think those are
the primary objectives, not shock absorption. A Lovejoy is not
intended to take axial loads, so would be a bad choice without shaft
constraints.
The picture of the driveshaft components also leads me to suspect
that pins, not bolts are the shaft to coupling connection, so the
intent is no axial load on the rubber coupling.
The link recently posted
http://m4.i.pbase.com/v3/91/283791/1/50045854.P0001095.JPG shows a
rubber coupler design with what appears to have solutions to the
issues above. The tube above the rotator clearly doesn't turn and it
appears to have a bearing at the end for the mast inside. Looking
closely, it appears the end of the mast has a spline that mates with
the top attachment to the coupling. Thus, no thrust load can be
placed on the coupling.
A tower with antennas is a very complex dynamic system - many masses
and springs and few energy absorption elements. My reasoning is the
shock and vibration loads cause the destruction from high amplitude
oscillations or when hard stops are hit - rotator brakes and gears
all have backlash. Loose mast and boom clamps and rotator bolts are
another source. Peened out shear pin holes are a sure sign of problems.
Another concern with a rubber isolator is it adds another spring
(with low damping) into a system that has unknown dynamic
properties. It is an offset to the benefit of the rubber isolator
ability to reduce the peak torque values by spreading a shock pulse
energy out over time. Another potentially large force can be
created by adding a "balancing weight" at the end of a boom, so the
boom is statically balanced at the mast attachment. However, that
adds a weight on the end of a cantilever beam spring, when the other
element masses are distributed along it. I've seen it done to ease
of tramming the antenna, but adding to the rotational inertia is not
good.
One also might question what these couplings are really designed to
do. Shock transients are large amplitude low frequency content
events. Vibrations are small amplitude higher frequency and usually
continuous. Rubber isolators generally don't have much damping at
low frequencies, which are what I see when my aluminum starts waving
around in a storm.
Another idea is to adapt a rubber spring torsion axle as an
isolator. These are used on smaller trailers and can handle loads in
multiple axis. Again, with very limited damping loss.
http://www.northerntool.com/shop/tools/product_200649004_200649004
Grant KZ1W
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