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

[Sam Ferris]

Towertalkers:  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??  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.  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.

Many thanks

Sam
VE5SF

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