At 10:53 AM 6/10/2005, JC Smith wrote:
>Mike,
>
>There are obviously different bearing sold to amateurs for the top of their
>towers. Although I have not disassembled it to look at the location of the
>races, my TX-472MDP's TB-2US is obviously not designed to support any axial
>weight. It's obvious because the only means of clamping the mast to the
>bearing is a single, allen/socket-head set screw. No way is anyone ever
>going to tighten that enough to support the axial load of even a moderately
>sized mast and antenna. I also have a MA-550MDP with rotor base. Now
>there's a thrust bearing that's designed to support some weight.
>Incidentally, even though the top of this tubular tower does have fairly
>heavy-duty mast clamping arrangement, my mast still slipped down inside the
>tower until the antenna (a little 3L tribander) rested on the top of the
>tower. Fortunately, no coax got pinched and since the whole tower rotates,
>no real harm was done. The mast does weigh about 70# but still a relatively
>light load considering the small antenna. The antenna was only 10' above
>the tower top. What that taught me is that when you get that mast whipping
>around a little (and we don't have much wind here) you really can't rely on
>any of these collar clamps to support an axial load over a long period of
>time. You'd have to run a hardened pin through the collar and mast to be
>guaranteed no slippage over time, and I don't think that's recommended
>either.
There's a variety of wedgelock type clamps for shafts that can take a
substantial axial load for a long time. The key is that they have the force
distributed around the entire shaft, and they're designed so that the load
tends to tighten the clamp.
One can also put a ring around the shaft with an interference fit (i.e.
heat the ring, cool the shaft).
This is an application where there's a fair amount of flexing and vibration
going on, which tends to make the shaft (which is a hollow pipe, after all)
change it's shape and size. You're going to want something that prevents
"creeping", which, to me, implies using some sort of fairly rugged
permanent attachment (epoxy? welding?)
_______________________________________________
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
and ask for Sherman, W2FLA.
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