[TowerTalk] winch loads and tower raising fixtures

[Kevin Normoyle]

in an old post,
http://lists.contesting.com/archives//html/Towertalk/2005-06/msg00238.html
I calculated the forces when raising a crankup with a ginpole at various 
heights.
(there are three links there, one is an excel spreadsheet, others show what
I analyzed)

shorter ginpoles (like a raising fixture which is what, around 8' maybe) 
increase
the forces on everything a lot. I used a 14' ginpole as a result for my 
lift.
(I didn't have a raising fixture).

I calculated assuming a direct connection to the tower (no compound 
pulley like
on most raising fixtures). But you can divide by 2 in the right places 
to get the effect
of a 2:1 compound pulley setup.

adding weight to the top of the tower, when lifting, obviously causes a 
bigger effect
than linearly distributing added weight across the tower. ...i.e. 300 
lbs at the end
of the tower, is worse than a 300lb heavier tower. Because the tower is 
acting like
a class 3 lever, and the pull effort is closer to the fulcrum than the 
load is, so the 300 lbs
multiplies.

My calcs break the analysis in two. The weight past the tower attach 
point is a class 3 lever.
The weight below the tower attach is a class 2 lever. I assumed the 
attach point height
equals the ginpole height always. I assume the tower weight is linearly 
distributed, and
simplify the analysis  with an appropriate point load at the halfway 
points.
there are two, because of the two different class 3 and class 2 lever 
effects).

You can modify/do the calcs, but it's not out of the question to exceed 
a 1500 lb
pulling force with a 2:1 compound pulley,  with my example. (which had 
nothing
on top of the tower).  I would do the calcs with the antenna load at 
horizontal,
even though you would install them at a slight angle. The difference can 
be considered
design margin.

I don't know the exact height of the raising fixture in question, or the
height of the attach point to the tower, though.

Food for thought: if you think you need stronger winch, at a certain 
point you need
stronger cable (like 1/4"). I'm not sure what size cable was on your 
1500 lb winch.
The rest of the setup may be strong enough already.


-kevin
KE6RAD