Dick -- Comments marked within your text. -- Eric
on 06 Dec 19 Tue 15:19 Dick Green WC1M said the following:
> Thanks for the story and the numbers.
> I've seen conflicting data on web for the safe WLL (working load limit) for
> my 7/16" 3-strand nylon rope. While the breaking strength is consistently
> reported as 4500 lbs, the WLL figures are as low as 300-400 lbs. That would
> be about 1/10 the breaking strength, rather than the 1/5 factor used for
> wire rope. Not sure about the discrepancy, but it's probably due to the fact
> that nylon rope must be derated by knots, age, condition, number of uses,
> etc. In my case, the "gotcha" is that I don't know the history of the rope.
> I bought it used from a professional tower rigger along with a bunch of
> other rigging gear. He said he used it for tramming large antennas up
> amateur towers.
> I certainly find it compelling that a break in the tram line could cause
> injury or death. The person operating the winch can't avoid being right
> under the antenna.
The winch should be relocated to a different spot off to the side, away
from the tower base. Run the pulling line off the top of the tower to
that side location.
Alternately, use an electric winch with a long cord to the control
buttons, so the operator can stand well clear of the work zone.
> Same goes for the person operating the tag line on the
> tiller. No matter where we stand, the whipping end of a broken rope could
> cause serious injury. Besides, even if we could protect ourselves, that 4-el
> SteppIR is a $2K+ antenna that I'd hate to see in pieces on the ground.
> After a lot of thought, I decided to replace the nylon rope tram line with
> 1/4" galvanized wire rope. It's has a breaking strength of about 7000 lbs
> and a safe WLL of 1400 lbs (1/5 factor.) That should provide at least a 2x
> safety factor for tramming my 4-el SteppIR. I went with galvanized wire rope
> instead of EHS because EHS is not available locally and I need to get this
> job done before the snow flies. Besides, the breaking strength of 1/4" EHS
> is about the same as regular 1/4" galvanized wire rope, though I don't know
> if the WLL is the same (anybody know?) Another factor is that wire rope is
> much more flexible and easy to work with than EHS. If anyone on the list has
> a good reason why I shouldn't use it, let me know.
> Now, a final question: should I use wire rope clips or a guy grip to form
> the end that will connect to the tower? Seems to me a guy grip would be
> best, but I want to make sure it can be used on non-EHS cable. I'll use a
> Klein grip and come-along at the bottom end.
Properly installed wire rope clips are fine, as is a properly installed
Nicropress compression fitting.
I would a wire rope clip at the bottom, rather than rely on the Klein
grip during the job. That reduces the number of potential failure points.
> Still wondering what the forces on the pull rope are. That'll still be
> 3-strand 7/16" nylon, and my impression is that nearly everyone uses some
> sort of rope for the pull line. Wouldn't want that to break, either.
Pull line has to overcome friction/inertia and do some lifting, and to
counter-act any tugging on the tag lines. But the total is less than
the tram line's force up toward the tower.
If in doubt, use a 2:1 system (i.e., run the pull line over the pulley
at the top of the tower, through another pulley at the antenna and back
toward the top of the tower where it gets tied off. Forces are divided
by two, and the amount of force that has to be exerted by the winch on
the ground is also reduced.
> 73, Dick WC1M
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