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
On 5/3/2016 23:01 PM, Jon Pearl - W4ABC wrote:
Hi Bob and Jim,
There's a whole bunch of videos on Youtube on their typical use, but
these two get to the point pretty quickly with some good close-ups:
https://www.youtube.com/watch?v=4yCxhyTlysw &
https://www.youtube.com/watch?v=v71KGSCjBrQ
Bob - as to your prior question in the earlier email, I would imagine
any decent driveshaft shop could come up with 3 bolt flanges to weld
onto whatever you could drag into their shop and do so
concentrically. In searching, I see that there are also 4 bolt models
of flanges and flex joints. I would think that the 6 bolt versions
would provide more cushion for rotator purposes as there is more
rubber between the bolts.
I have a question about the use of one of these devices, though.
Would it be possible or more likely *wise* to allow the weight of a
mast and antennas to come to rest directly on the flex joint,
compressing it (distorting it) between its two flanges? Its primary
function in automotive use is to reduce vibration through the
regularly anticipated twist of a drive shaft, not to be heavily
compressed between the transmission and pumpkin. I wouldn't mind
trying one of these flex joints, but I would hate to have to use a
bearing shelf, collar and bearing to hold the weight of the mast and
antennas off of the joint.
73,
Jon Pearl - W4ABC
www.w4abc.com
On 5/4/2016 12:21 AM, Bob K6UJ wrote:
Jim,
Thanks for the good background info on the flex discs for our ant masts.
I used to have Leesons book. Wish I still had it to check out his
BMW flex disc
configuration. I had to chuckle about M2 backing away from the
flex discs. They have
been used on automotive drive shafts for a long time, and as you said
the locktite
for the studs is standard procedure.
Bob
K6UJ
On 5/3/16 8:40 PM, Jim Thomson wrote:
Date: Tue, 3 May 2016 19:05:25 -0700
From: Jerry Gardner <jerryw6uv@gmail.com>
To: Bob K6UJ <k6uj@pacbell.net>
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] Rotator Choice for Larger Yagi
Interesting, Not being a "car guy" I've never heard of flex discs.
Do you
have any more info, perhaps a few links to websites describing them
and how
they can be used as part of an antenna/rotator system?
73,
Jerry
## The BMW flex disc is well documented in leesons book, physical
design of yagis...now out of print. It was a $25 part, available at
any
BMW dealer, looks like a giant rubber doughnut...without the hole
in the middle,
lol, and had 4 x threaded studs imbedded into the rubber.... at
each end.
IE: 4 on each end. The 4 at the front end are offset from the 4
at the back end,
## I believe BMW used the rubber flex coupler, in line with the
driveshaft on their cars.
It has just enough flex in it, when twisted in either direction, to
absorb tq.
## For rotor use, its mounted vertical of course, sandwiched
between 2 x al plates.
I think leeson then used a huge chunk of 90 deg angle AL, on both
the top and bottom
al plates , bolted. Then the long ant mas was U bolted to the
upper angle AL.
Then a real short, 1 ft long piece of pipe /tubing was U bolted to
the lower
angle al. ( which in turn is bolted to the bottom plate). Bottom
end of this 12 inch long
piece of pipe /tubing was inserted into the rotor in the normal
fashion.
## It worked superb, and provided just enough isolation, so when
yagi is at rest, and winds blowing,
the rubber flex disc absorbed the shock, vibration etc, so the teeth
in the rotor final gear don’t get trashed.
IE: the rubber flex disc absorbed a lot of the gear chatter /
backlash. The disc will also absorb the initial hit
of tq, both starting..and stopping. These days, most rotors have
ramp up and ramp down, so that part of it
is not a big issue.
## sad part of all this is... M2 in fact did make their version of
this disc assy, with heavy duty steel plates, and
the same finish as their m2 oem drive plate assy. The M2 version
used no angle al pieces at all, and the simple
sandwich was just bolted to the oem m2 drive plate. Oem m2 mast
clamp was then bolted to the upper steel plate.
## m2 stopped making the optional rubber doughnut isolation assy.
They told me, it was cuz folks...were not doing regular
maintenance on the assy. IE: the bolts that held the sandwich
assy, would work loose. Also the 8 x threaded BMW
studs would also work loose. Seems silly to me, since blue
loctite, or even red loctite would have solved that issue.
I pleaded with them at the time I bought the pair of OR-2800 rotors
to reconsider, and start making it again..but
to no avail. Their version was superb. Built like a tank.
## For automotive applications, the same 8 x threaded BMW studs
were always installed with blue loctite !
AFAIK, the BMW flex rubber couplings are still readily available.
Yaesu offered an optional rubber pad for their
big rotors, that was installed between the base of their rotor and
the mating steel triangular rotor plate in the tower.
The pad does little good, since it has its isolation at the wrong
end of the rotor.
Jim VE7RF
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