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Re: [TowerTalk] flexible mast coupling

Subject: Re: [TowerTalk] flexible mast coupling
From: K8RI <>
Date: Thu, 05 Apr 2012 13:08:19 -0400
List-post: <">>
On 4/5/2012 12:27 PM, Bob K6UJ wrote:
> As I see my stack of beams swinging back and forth in the wind I am thinking 
> about adding a flexible shaft coupling above the rotor
> to absorb the shock,  I have read somewhere about using automotive drive 
> shaft flex couplings and adapt them to clamp on to
> our 2" masts.  I used to have one of Bill Orr, W6SAI books on antennas and if 
> I remember right he used one for a Volvo, and it
> had a rubber donut between two flanges.  Anyone make a flexible coupler for 
> their mast and would share what you used ?

A flex coupler, like a long mast adds some unknowns.  It may help and it 
may not.
Both the couple and mast can add resonance which under the right 
conditions can increase the torque applied to the rotator tremendously.  
We think of masts as being rigid, but the longer they get the more give 
they have which is exactly how a torsion bar works.  Normally it's not a 
problem, but the larger the array(s) and the longer the mast the more 
likely it is to end up resonating. With 40 feet of 2" mast (1/4" wall) 
and a large array on 
"some" windy days I'd see 10-15 degrees of twist between the bottom 
antenna and the rotator.

Pinning masts is not "normally" recommended. As has already been 
mentioned, it's a whole lot easier to realign the mast, or direction 
indicator than to replace gears.

Being blunt, I do not like rotators that use wedge brakes.  I use them 
because they are cheap and work well most of the time for small antennas 
and they are relatively easy and cheap to rebuild. The array shown is 
pretty good size, but not huge. The whole thing is not as large as a 5 
or 6L 20M monobander, but it tore the guts right out of a tail twister.

All too often these rotators are used with large antennas that are far 
beyond their capabilities.  I expect to be able to turn what ever is up 
there whether the wind is blowing or not.  On a windy day these rotators 
need to be de-rated substantially.


Roger (K8RI)


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