[TowerTalk] Tension on tramline

[W0UN -- John Brosnahan]

The math is not simple for a catenary with a suspended load.  You
get a lot of mental exercise doing hyperbolic sines and cosines.

You can search on catenary, tension, suspension, etc and get lots
of relevant references.   Any any decent physics book with a chapter on
mechanics should show everything you need.

Here is an interesting site for some background info.  It is on two
pages and there are some links to other interesting sites.

http://www.brantacan.co.uk/suspension.htm

73--John   W0UN




At 08:47 AM 4/4/2004, you wrote:

>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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