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

[David Gilbert]

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