[TowerTalk] Flex coupling for rotators

[K7LXC@aol.com]

In a message dated 7/3/01 4:45:15 AM Pacific Daylight Time, w5kp@swbell.net 
writes:

> I'm thinking of a short steel tubing sleeve with an ID that will just slip
>  over the 2" OD of the bottom of the mast. Bottom of the mast and top of the
>  sleeve would be carefully drilled for a hefty galvanized through-bolt. When
>  removing the rotator in the future, I should be able to just pull the
>  through bolt and slide the sleeve up, temporarily slipping the through bolt
>  back in through a second hole in the bottom of the sleeve to hold it up out
>  of the way, pull the rotator, etc. 

    I'm not sure what kind of antenna load you're carrying but if it's a C-3 
or similar and your windspeeds are also moderate, that'll work. Unfortunately 
I've never seen pinning a mast with a thru-bolt work for very long. With a 
moderate antenna torque load (say something with a boom 24' or longer) and 
moderate windspeeds (70 MPH), the bolt will eventually elongate the bolt 
holes and then the bolt breaks. There's just too much slop in this technique 
to be useful. 

>  The second thrust bearing and a muffler
>  clamp with a piece of angle welded to it U-bolted to a tower leg would keep
>  everything from flopping around while the rotator is out, and add some
>  safety against vertical mast slipping. 

    Why wouldn't the first top TB and muffler clamp hold the vertical weight? 
That's plenty - the TB will typically hold the weight and the muffler clamp a 
good redundant safety back-up. 

    I'm not a big fan of second TB's because it's too easy to introduce bind 
into the system if the second TB is tightened up. My suggestion would be to 
only tighten the second TB bolts when you're doing a rotator swap. Otherwise 
just let the mast float inside the second TB.

>  The Yaesu 1000 is self-centering and
>  adjustable to fit about any size sleeve in this application. 

    If you're using a G-1000, then you couldn't have too big of a system so 
thru-bolting will probably work. All bets are off if you've got anything more 
than about a 16' boom though or more than 70 MPH winds.

>  Or, as an
>  alternative, maybe some sort of easily removable heavy duty rubber coupling
>  is available that would simply fit between a short rotor stub of 2" mast 
and
>  the remainder of the mast. Don't like that as well, though, I'd fear
>  excessive flex might cause constant twisting around of the antennas in the
>  wind, unless the rubber coupling was really stiff.

    This was popularized by Dave Leeson, W6NL, ex-W6QHS, where he used BMW 
couplers (I don't remember their original function). Use of one will increase 
rotator capacity and/or reliability.

    Unfortunately no one has come up with a bolt-on, long lasting one. KLM 
used to make one but gave up after lots of failures. Yaesu shows a couple in 
their latest rotator catalog but I haven't seen one in the flesh.
>  
>  Is anybody using a setup like this, and how is it working? The only 
negative
>  possibility I see is the top of the steel sleeve might permanently deform
>  from longterm bolt pressure, and be difficult to slide up. Maybe a single
>  slot in the sleeve at 90 degrees from the bolt holes for springiness would
>  help prevent that.

    If this was my system, I'd skip the coupler and put the mast directly in 
the rotator. Using a comealong to move the mast up an inch or two is a 
slamdunk; you're only talking about doing this once a decade or so anyway. 

Cheers,   Steve    K7LXC
Tower Tech 

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