[TowerTalk] [Bulk] Re: Rotator Choice for Larger Yagi

Wed May 4 11:39:02 EDT 2016

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 at gmail.com>
>>> To: Bob K6UJ <k6uj at pacbell.net>
>>> Cc: towertalk at 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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