> Of course, I did the algebra without the benefit of a morning
> cup of coffee... and got wrong answers. The answers for a
> 100 lb antenna should be:
> 333 lb up toward the tower
> 273 lb down toward the ground anchor
Well, that's quite a difference from 738 lbs and 602 lbs, respectively! I'd
hardly call these "enormous" forces, as some have said. Perhaps 3-strand
7/16" nylon rope would have been OK after all. Still, the history and
condition of the rope were unknowns. Also, when we raised the 40-2CD, the
nylon rope stretched a lot. I ran out of come-along line and had to parallel
another come-along to pull the line tight enough for the antenna to clear
the top guys. All in all, I'm not sorry I ran out and bought 1/4" wire rope
to tram the SteppIR, which weighs twice as much.
> As others have mentioned, additional forces at play in the
> real world include weight of the tram line, changing angles
> as the antenna moves up the tram, and transient forces used
> to overcome friction, start the antenna moving, swaying, etc.
> So 333 lb is just a starting point...
My sense is that these are not "enormous" forces either. Neither nylon rope
nor 1/4" wire rope weighs all that much -- I can easily carry 250' of either
in one hand (EHS is a lot heavier, though.) If the antenna is badly
balanced, such that it spins and/or sways a lot, perhaps there are some
additional forces, but how large could they be? Certainly pulling on tag
lines doesn't put all that much force on the tram line. Besides, if the
antenna is moving around that much, it's probably not rigged properly and
should be brought down.
Seems to me the force that you have not included, which was mentioned by
K8RI, is the pretension on the tram line. I'll bet it takes several hundred
lbs of force to get the tram line high enough and straight enough for the
antenna to clear the top guys. As I was cranking the come-along(s) to raise
the tram line, that was the force that most concerned me.
73, Dick WC1M
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