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

[Grant Saviers]

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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