Eric,
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. 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.
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.
73, Dick WC1M
> -----Original Message-----
> From: Eric Scace K3NA [mailto:eric@k3na.org]
> Sent: Tuesday, December 19, 2006 12:32 AM
> To: wc1m@msn.com; Towertalk
> Subject: Re: [TowerTalk] Tram rope
>
> Yes, I saw a tram line part and the results were not pretty.
> The person operating the winch narrowly escaped injury by the
> antenna and by the parted line. This occurred early in my
> ham antenna career as was a real wake-up call to the
> forethought required for safe installation.
>
> A tram line's forces act against the dead weight and wind
> load of the antenna being installed. Because the tram lines
> come off at an angle, those forces are much larger. (Draw
> out a force diagram using high school physics.) Assume a
> tram line leaves the pulley holding up the antenna at a 45
> degree angle up and toward the tower, and on the other side
> continues on at a 30 degree angle below horizontal in the
> opposite direction toward the ground. For a 100 lb antenna,
> the force on the line to the tower is 738 lbs and on the line
> to the ground is 602 lb.
>
> That is the static case. Loads will vary when one attempts
> to start the antenna moving from that point.
>
> -- Eric K3NA
>
> on 06 Dec 18 Mon 13:56 Dick Green WC1M said the following:
> > I don't want to be contrarian, but has anyone seen a tram
> line made of
> > reasonably strong rope break? Has anyone calculated the
> actual forces
> > on the tram line? For example, the breaking strength of 7/16" nylon
> > rope is about 4,500 lbs, with a safe working load between
> 450 lbs and
> > 900 lbs (some references say no more that 1/5 breaking strength,
> > others say no more than 1/10 breaking strength.) Would the forces
> > exerted by a 100 lb antenna on a tensioned tram line exceed that?
> > Also, the haul rope supports some of the weight of the
> antenna. Doesn't that figure into the equation as well?
> >
> > I ask this question because yesterday I successfully
> trammed a 44 lb
> > Cushcraft 40-2CD to 110 feet using 7/16" 3-strand nylon
> rope. The next
> > antenna to be trammed is an 81 lb SteppIR. Is there really a chance
> > that my tram line will break?
> >
> > 73, Dick WC1M
> >
> >
>
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