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[TowerTalk] Installing a mast.

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Subject: [TowerTalk] Installing a mast.
From: (Stan Griffiths)
Date: Thu, 17 Jul 1997 23:50:30 -0700 (PDT)
>Hi All,
>I will soon be at the point of needing to put my mast down
>into the top of the tower.
>I am using a Rohn gin pole which is about 11' - 12'. The
>mast is 24'.
>How do I keep it hanging down?? Or would I be better off
>cutting about 4' off the mast? (I only need about 20' to
>handle what I'm doing).
>73, Ron,     SOWP 5545M,

Hi Ron,

Like most other guys on here, I don't like the idea of cutting 4 feet off a
perfectly good 24 foot mast.  You paid plenty for that extra four feet and
you should use it (assuming the rest of you tower system is designed to take
a mast that long).

I also own a Rohn Gin pole and I would be very uncomfortable using it as it
comes from Rohn to install that mast for two reasons.  One is what you
already see as a problem:  It isn't long enough to allow the 24 foot mast to
hang vertically and thread it into the top of the tower, unless, of course
you pull down on the short end of the mast and MAKE it hang vertically.
That idea REALLY scares me!

Consider this:  Rohn says their gin poles are for putting up 25G and 45G
sections and tops.  This means they are for at least 70 pounds which is what
a 45G section weighs.  How much does your mast weigh?  If it is steel, 2
inches in diameter and 1/4 inch wall, it will be a LOT heavier than 70
pounds.  What can you do about it?

Here is what I have done:  I bought a 16 foot aluminum mast from a friend of
mine who I talked out of using it as the regular tower mast to support his
stacked HF Yagis.  He used steel instead at my recommendation.  Anyway, the
aluminum mast he had purchased was 1/4 inch wall which is about double the
wall thickness of the Rohn gin pole mast plus it was 4 feet longer.  I
simply used the thicker wall, 16 foot aluminum mast in place of Rohn's
aluminum gin pole mast.  These two improvements raised my confidence in what
I regard as the weakest point of the Rohn gin pole, ie; I would expect gin
pole failure, if it occurred, to be the gin pole mast folding at the point
that it leaves the steel Rohn tower clamp arrangement.

Of course the new gin pole mast did not have a convenient pully mounted at
its top, but I don't like to use that pully for big loads anyway.  Here's
why.  The actual load the gin pole is expected to hold up is actually the
sum of the tensions in the two ropes that come down each side of the gin
pole.  If you have a 150 load (your 24 foot long steel mast) on one of the
ropes, you have to put 150 pounds of tension in the other rope (neglecting
friction) to just hold the mast in place, more if you are hauling it UP the
tower.  Result:  the gin pole is really supporting 300 pounds, your 150 load
and YOU as you pull down with a force of 150 pounds!

If you use a block and tackle with some mechanical advantage at the top of
the gin pole instead of a single pully, it reduces the downward tension you
must apply to raise the load such that the downward pull is now equal to the
weight of the load DIVIDED by the mechanical advantage of the block and
tackle system.  If you use a block and tackle with a 3:1 mechanical
advantage, you can haul up a 150 pound load with only a 50 pound downward
pull.  Result:  total load the gin pole must support is now 200 pounds (the
same 150 pound load but only 50 pounds of downward pull) instead of 300
pounds.  Again, all of this ignors friction and the fact that there is a LOT
more rope involved and you have to add that to the weight of the load.  Even
so, the result is much easier on the gin pole and your nerves.  You can also
use a much smaller person on the bottom to pull the rope since he only has
to pull 50 pounds instead of 150 pounds (ignoring friction and the added
weight of the rope).

A block and tackle system consisting of a double block at the top and a
single, moveable block at the bottom provides a mechanical advantage of 3:1
if it is strung correctly.  A fast rule of thumb to determine the mechanical
advantage of any block and tackle system is to count the number of ropes
actually supporting the weight of the load and that is the mechanical
advantage of the system.  You can get 4:1 out of two double blocks.

There was another neat system involving using a modified tower section with
the horizontal and diagonal members cut out of one side that was described,
I believe, by Glenn Rattman some time ago on this reflector (correct me if
I'm wrong Glenn).  Anyway, I would certainly consider sacrificing one tower
section for that purpose.  The modified tower section is used only as a
temporary "gin pole extension" and removed after the long mast is inside the
top of the tower.


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