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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