[TowerTalk] Rope breaking strength vs working strength ???

[K7LXC@aol.com]

In a message dated 08/20/2000 9:05:41 PM Pacific Daylight Time, 
ve5sf@sk.sympatico.ca writes:

> In the midst of building a short stack on 20 (3 over 3
>  over 3 - all on 24 ft booms) and now that we are to the stage of
>  tramming antennas up the tower, the question of strength of the tram
>  line has arisen.  Refering to several internet listings and the March 92
>  NCJ have revealed that the rope ratings are given for both working and
>  breaking strength.  I presume that breaking strength is obvious, but why
>  is working strength so much less ... just a safety margin??

    JUST safety margin? Of course that's what it is. 

>  Any
>  recommendations for a tram rope to carry about 65 lbs to 145 ft with
>  tram line spread about 250 feet from the base on each side of the
>  tower.  Were thinking of using 1/2 inch polypropylene (420lb working or
>  3780lb breaking) or 3/16 dacron (700 lb working and ?? lb breaking). 
>  Plan to either backstay the mast or have a large pully at top and anchor
>  and tension both ends of the tram line carrying the load.  Problem
>  really is determining the total load on the tram rope considering the
>  need to tension the line, carry the load and the weight of the rope
>  itself.  

    Point of clarification: A tramline is a single line where the antenna is 
suspended underneath the tramline for hauling. A trolley line consists of two 
lines where the antenna is on top of the trolley lines and slid up the lines. 
Use the tramline system - there is way too much friction sliding an antenna 
up the ropes - everything gets lopsided and the antenna gets real tippy as it 
approaches the top of the tower. 

    The fixed tram line should be steel cable and not synthetic rope; use 
rope for the haul line. It's difficult to get enough tension from a rope 
whereas it's easy with a steel cable. As far as size goes, 3/16" galvanized 
or SS aircraft cable is overkill but 1/8" is okay. You'll never exceed its 
SWL (safe working load) with typical ham loads. Galvanized cable kinks so SS 
is a little easier to work with. I use a plastic extension cord reel from 
Home Depot to store it.

    If you want to use synthetic rope, use a 10 or 20% of breaking strength 
for the SWL. 

> Please pardon the ignorance.... this is my first go at tramming
>  on guyed towers, everything else has been on self supporting styles,
>  where a straight lift was used.

    No sweat. With some rare exceptions, there's no such thing as a dumb 
question. Besides, we all had the same questions at one time or other. 

Cheers & GL,   Steve    K7LXC
Tower Tech

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