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

[Bob K6UJ]

I remember the rope dampers Grant.  I had the same issue and resolved it 
with the poly rope.
I went out to my back yard and found element sections stuck in the 
ground like they were
shot like an arrow.

Bob
K6UJ

On 5/4/16 11:30 AM, Grant Saviers wrote:
> 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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