Hello TowerTalkers,
A couple of thoughts occurred to me after reading this latest thread on a
perennial subject. There's quite a longstanding debate on whether to pin a
mast or not.
Having a relatively smallish setup, I chose not to pin my mast. I have a
Yaesu 800SDX (circa 1998), an F12 C-3E tribander, and a 13 el 2m beam. I'm
using a 2" dia, 6061 Aluminum mast with 1/4" wall, which would easily be
wallowed out with a hard steel pin.
In one small hurricane event (not too many in SE VA), the mast slipped, and
a simple realignment was all that was necessary. The thought was that it
would be easier to realign an antenna or replace a snapped rotor loop than a
rotor, in the case that the mast slipped more than full turn or so.
The slipping also disspitates energy via friction, limiting hard forces on
the system.
This model Yaesu and other rotors come predrilled and with a steel bolt for
pinning the mast.
How about a compromise: throw away the steel pin and use an aluminum or
brass pin? This is much more easily sheared, and would probably maintain
alignment in all but the worst weather.
You can easily improvise a pin by hacking off a short length of aluminum rod
that is long enough to stick out, say, 1" through both sides of the mast
clamps. Drill the ends for simple stainless cotter pins and you're done.
Make up a couple extras for spares while you're at it.
That still doesn't address the need to provide for some shock relief to
dissipate energy and limit the forces of starting/stopping the rotation, and
wind-oscillation torquing. To some extent, the springiness of antenna
elements helps, especially if you have ropes through the centers.
The shaft couplings mentioned are an elegant way to approach this,
especially for larger arrays.
It's much better to design a mechanical system to flex in some way and
harmlessly disspiate shock energy rather than make it so rigid that
destructive forces build up.
How about an eleastomer sheet, such as, say, 1/4 or 1/2" sheet rubber, to go
between the base of the rotator and the rotator shelf?
The holes in the rotor mounting shelf would have to be drilled oversize to
allow rotational movement of the bolts and rotor together. You would use
washers under the bolt heads, before they come up and through the rotor
shelf, then torque them only enough to apply some friction between the rotor
and shelf.
You might have to replace the bolts with longer ones, and use two nuts (or
short stubs of threaded rod) under the shelf, such that you could torque the
assembly to a mid level, and lock it with the second nut.
Such an arrangement would allow the rotor to give a bit rotationally, to
limit forces on mast clamps, etc, and provide a way to disspate harmful
torque energy. It might even help relieve the age-old wedge-brake jamming
problem on rotos having this type of brake.
I have not tried this, but it seems like a relatively easy retrofit to
standard hardware to try. I'll throw it out as just a food-for-thought
thing.
Regards,
--...MARK_N1LO...--
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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