[TowerTalk] Tension on tramline

[Pete Smith]

I was talking the other day with a local antenna guy about tramming.  I 
mentioned reading various cautionary tales on towertalk about masts getting 
bent as a result of the tension on tramlines when relatively heavy antennas 
are being moved up and down.  He said he had never had this problem, but 
opined that this was because he always left sufficient slack in the 
tramline so that the tension didn't become too severe.

I can visualize that in cases of extreme slack (i.e., where the tram line 
goes almost straight down from the tower to the load) the tension on the 
tram line would be almost zero, because virtually the entire antenna weight 
would be borne by the rope that controls the antenna sliding down the tram 
line.  I have also experienced what happens when you try to pull a line 
bearing weight in the middle (like a center-fed sloper suspended from the 
top of a tower, at a 45-degree angle) really taut, in order to get the 
droop out.  The tension required seems to increase almost exponentially as 
you get closer to dead straight, and you can't ever get all the slack 
out.  The wire will break first.

Is there a mathematical formula that will tell you roughly how much tension 
is required at a given load weight and degree of slack?  Alternatively, is 
there a safe rule of thumb to tell you how much slack to leave in a tram 
line for a given load?

73, Pete N4ZR
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