At 07:59 AM 11/11/97 -0600, you wrote:
>Lee,
>
>I need some clarification in order to understand the comments about
>"pinning" the mast.
>
>What I had mentioned was pinning the two masts (1.00 and 1.50 telescoped
>together). It was as much for having them not untelescope while being
>lifted as anything. An alternative would be to insert shims all around
>in the space between the ID of the 1.50 and the OD of the 1.00 (top and
>bottom). My thinking is that this would make the thing stronger than if
>you just left the slop. Either way I can't understand how this would
>affect the rotator since it is dealing with the OD of the 1.50 mast via
>thrust bearings. Please help me with some clarification.
>
>It sounds like I've been lucky for a long time although it makes a big
>difference that my Cushcraft is really only about 8.5 feet above the 4
>Element 20.
>
>Thanks
>
>Stan K5GO (stans@taurus.oursc.k12.ar.us)
Stan....
You have to consider torque. Let say there is some slop in the the
telescoped sections you have pinned or bolted together. If you translate
that to the end of the boom, you have a lot of arc momentum. A little bit
of slop translates to a lot of arc at the tip of the boom. Now then, when
the wind comes up, the beams move around and places a lot of stress on that
connection. Anything below the connection won't ... but anything above the
connection will. It is like putting a cheater bar on a wrench and giving it
the old heave-ho. I have broken wrenches doing that. Well you can break
rotors too.
I had a HAM III turning a TH6DXX with a senario like yours. During a winter
storm, the momentum of the boom swinging in the wind finally fatigued the
metal in the rotor and took out the brake system Actually, it took out the
ribs in the bell housing that the brake fits into. The brake did not bust,
but the casing did....just because it had a big cheater bar hooked to it.
Note: I have also seen tripple case harded and schedule 8 bolts using to
pin the mast shear off!
Now, if I had limited the swing of the antenna to a minimum arc of no arc,
that rotor would be working today. I have gone to a complete mast over the
last 15 years and have never lost a rotor since.
Again it is not the down thrust, but the torque of the swinging boom when
the holes get wallowed out. You use a harded set of bolts and soft pipe or
mast and it will wallow out. Thus that little bit of enlargement makes a
HUGH difference at the end of the boom on your beam.
Shims would be nice to use there too, if your drive them in where the mast
wouldn't move. Welding it would be even better, but might be a problem.
Again, I went to one continous piece of mast. Hard to do, but worth it. I
don't like tower maintenance.
73
Lee
k0wa@southwind.net
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