Let's do numbers.
If the winch and ginpole are at the pivot point of the tower, the
analysis of forces is pretty simple. It's simple mechanics (think back
to high school physics class). It's more complicated if the tower is
pivoting at a point not exactly below the top pulling force. But you can
get a ballpark estimate by looking at how high your pulling wire is at
the point it crosses the base, and how high up the attach is on the tower.
Higher is obviously better, for both.
It's VERY easy to underestimate how high you want, which is what
everyone is trying to say.
I had done some analysis for raising my HG-70HD a while back. I was
trying to answer the question "how big a ginpole, assuming I'm just
using a Fulton 1500 winch and single wire pull?" I use a guyed ginpole
with a winch at the base, pulley at top, single cable pull. (also a good
reminder on how good a cable you need!)
The key question for me was how big a ginpole, and what forces were
created where. (both compressive on the ginpole and on the back guys).
Also how much pulling force for the winch. Just using HG-70HD tower
weight here: 1200 lbs
Assuming the attach distance to the tower (from the base) is at the same
distance as the height of the ginpole, I got the following, for pulling
force at the winch (add a safety factor to this for your design). Do
your own math for your situation. (The key in the match is converting
the distributed tower load to point loads at the appropriate places).
6' - 3394 lbs
9' - 2263 lbs
12' - 1697 lbs
18' - 1131 lbs
24' -849 lbs
Remember, 45 degree angles in this analysis. If shallower angles get
created, the forces are different.
You can see why the relatively short commercial raising fixtures have to
be so beefy.
(Notice for short ginpoles, the pulling force is > the weight of the tower)
The greatest pull is required immediately off the ground, as everyone
knows. This can be reduced by using stuff like a shop crane to raise the
tower the initial 8' or so. But it's good to just use the 0' pull for
worst case. Remember that the same force exists if you ever lower. It's
easy to forget to use the shop crane on the lower! So for safety, assume
the 0' pull for analysis! ALSO: if you let the tower bounce/oscillate on
the pull, the dynamic forces can be higher! Don't do that! (sometimes
happens if the hand winch is hard to crank)
This assumes a even distribution of weight in the tower. The base will
be slightly heavier, but ignore that.
If the tower has antenna/mast/rotor while raising, then it's worse, and
all the extra weight is concentrated at the end. This analysis doesn't
include stuff like that.
Matching intuition, the higher the ginpole, the better. (the compressive
force on the ginpole is higher than this, roughly 1.5x in my setup). My
setup is about 12' ginpole (not falling derrick. guyed ginpole).
If you think of a triangle created by attach to the tower, highest
position of the pulley or pulling force, and the winch...the bigger it
is, obviously the better (for lowering forces in the overall system).
Note that as this triangle (assuming 45 degree angles at tower and top
pulley) becomes > 18', the pulling force is less than the weight of the
tower, which we like.
Let's assume any homebrew situation doesn't want forces in cables that
exceed 1500 lbs or so (for safety). I use 1/4" 7x19 wire rope, with a
WLL of 1500 lbs. Shackles, pulleys, etc in the system see more than this
(analysis not included)
Breaking it down into ballpark estimates (you should do the exact
analysis for the situation though), you can see that you want the
raising cable to be more than 12' above the tower base, and the attach
to the tower to be more than halfway up. (assuming 24' tower cranked down).
For some crazy setups, the height of the cable above the pivot point is
going to vary during the raise, like the proposed roof pull. But if the
wire is always 12' above the base for the entire pull, you can see how
it might be doable.
I know the commercial raising fixtures are smaller and use a double rope
setup, which changes the analysis.
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