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

[Tom Rauch]

>     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.

Amen!

And remember if you set tension BEFORE the antenna is out near 
the center of the span the tension will skyrocket when the antenna 
hits anywhere near the midpoint.

The force multiplication can be tremendous if nothing gives!

It is important to always have enough sag in the line that the angle 
of V when the load is at the center of the line does not become too 
shallow. As that angle (when the load in the center of the line 
cause the line to V) approaches 180 degrees (a perfectly straight 
line) on the line, the tension in the line will approach infinity!  

I'd never use rope as a tram or trolly line, unless it had a working 
load a few dozen times the weight of the antenna. As a matter of 
fact, I'd never use rope. 

> 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.

I disagree with that. Never is too big a word.

I can easily break or stretch a 1/8 inch EHS line with a 50 pound 
weight, if I rig the line with too much initial tension (say two 
hundred pounds or so) and the line is between two rigid points a 
fair distance apart.

A trick I use to pull my truck or tractor out of mud is to tie a line to 
a tree, and push sideways on the line. With a hundred pounds of 
force I can easily move 4000 pounds, and even break a 1/8 inch 
line that was only tensioned by hand!!    

The key is in the angle formed by the V when the load sags the 
line, and how that V is oriented with respect to earth. The tension 
does not increase by anywhere near the weight of the antenna, it 
increases at a rate determined by the angles in the system.

I'd be mindful of the tension in the tram or trolly line at all times, 
and how the mast or tower bends! This is especially true when the 
line is long and mostly horizontal, but is still true in all cases.


73, Tom W8JI
w8ji@contesting.com

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