> I saw another nasty failure when a plastic-sheaved pulley (suitable for
> fibrous rope, but not wire rope) was used on wire rope.
Yikes! Thanks for the warning. I used a plastic-sheaved pulley to tram the
40-2CD on the nylon rope tram line. In fact, I got it from our esteemed
moderator's website (Steve, if you agree with Eric, you should put a warning
on the website not to use the pulley with wire rope. If you disagree, let's
hear it.) Anyway, I have a much better pulley for the job -- an aluminum
split-side pulley with a large diameter sheave, plenty wide enough for wire
rope. Rated for 5000 lbs. I got it several years ago from an interesting
website: Tools for Stagecraft (www.toolsforstagecraft.com, click on
Carpenter/Rigging/Safety). Theater stage hands need to do a lot of rigging
similar to what we do.
73, Dick WC1M
> 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
> > 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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