A friend built one for my project after I became convinced the one you
looked at are unsafe. I even looked at the Rohn 45 gin pole someone had
modified to clamp clamp on the larger diameter tube. I did not like this one
either due to the offset.
The solution was quite simple, we used a 16ft section of 1/4" wall
structural aluminum tube. I can't quote the diameter as the tube is not here
right now but it was at least 2-1/2 or 2-3/4. I have to run at the moment
due to a snowstorm coming and could not locate the spreadsheet with the
buckling calculations but can look again in a few days.
Basically, we used a safety factor of 5, which is what a pro climber says
his company uses for tower erection. You reaction at the top pulley is 2x
the section weight, or 200lbs, so the buckling calculation, with no offset
is 1000 min. This does not account for any offset.
The bare pole was drilled top and bottom to accept large diameter long grade
8 bolts. On the bottom, the bolt was fitted with a large and thick flange
washer with not. This is jammed into the v-brace 4 ft below the current top
section. The tube is strapped to the tower leg just below the top, with
redundant cam straps. We used the one I use for tying in rafting equipment
for whitewater trips, and they are quite strong. Once tight a single half
hitch is added to jam the cam should it fail. Once again - multiple straps
for safety. This leaves 12 ft above the top section, where the other bolt
has a short section of strong chain attached. We had a couple inches of
offset to the chain attachment point. To this a locking carabineer and
rescue pulley is attached. Another rescue pulley is chained at the
foundation -tower leg point to redirect the haul line horizontally. That is
the basic set up. Three guys hauling and two guys on tower and everything
went right up and with confidence. It is actually easier to move the gin
pole with this method, because you are not dealing with the unyielding
clamps while holding the pole, simple remove straps and lift to next
deployment and repeat.
Here is a youtube of us using this method.
http://www.youtube.com/watch?v=UOc3cDgP0hA
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