The pre-tension force can be checked with a Loos guage. You're correct
that it is significant.
If someone yanks 50 lb on a tag line to a balanced antenna, that's a
lot. Tag line forces are likely an order of magnitude smaller than the
Sorry for the screw-up in the early morning calculations... and glad
someone caused me to do them again.
You'll be OK with the wire rope tram -- no significant stretch at all so
it will stay nicely in place once the sag is taken up.
Be sure the tram-riding pulley has the correct sheave diameter, width
and material for wire rope. I saw another nasty failure when a
plastic-sheaved pulley (suitable for fibrous rope, but not wire rope)
was used on wire rope. While the pulley was loaded, the wire rope cut
right through the plastic. The small diameter wire rope's point load on
the plastic was high enough to just deform the plastic out of the way.
The wire rope cut down through the plastic until it was sliding over the
metal pin at the center of the sheave. The pin gouged and eventually
cut the wire rope, dropping the load.
-- Eric K3NA
on 06 Dec 19 Tue 18:14 Dick Green WC1M said the following:
>> 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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