Grant, please, check the "*Flex Shock Mount Clamp*";
http://www.ure.es/media/kunena/attachments/1956/3akp1012800x600.jpg
Available in many different sizes, power and even colours (aka quality);
https://www.ktr.com/en/products/power-transmission-technology/couplings/flexible-jaw-and-pin-bush-couplings/rotex/rotex-standard/
73, Maximo
> To: jonpearl@tampabay.rr.com; k6uj@pacbell.net;
towertalk@contesting.com
> From: grants2@pacbell.net
> Date: Wed, 4 May 2016 08:39:02 -0700
> Subject: Re: [TowerTalk] [Bulk] Re: Rotator Choice for Larger Yagi
>
> 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.
> >
