I'm probably going to embarass myself because I can't do a little
statics analysis
but here goes: (I may be wrong, so caveat emptor. Please correct me if so)
I built a raising fixture with pipe, pulley, winch, cable, back and side
guys,
(vertical ginpole), that I used before, but now am using it to tiltup a
heavier crankup tower.
Thought I'd work out the forces in all the components, to make sure I'm
still
overdesigned for this new load. (I increased the gin pole height as a
result of this)
It varies a lot depending on where you attach to the tower, and the
height of the
gin pole. Compressive forces on the gin pole can be very high.
(no buckling analysis done on the gin pole). Also: you can see how a
2500lb winch
can be marginal. (assume no compound pulleys to reduce forces)
(I've not made allowances for the common strategy of using an alternate
lifting
device at the tower end, to get it started the first 0-8? feet, to
reduce forces. I'm
going to do that with an engine hoist. to get the tower end up 8 feet to
start)
Maybe there's a more obvious equation, but it breaks down
into two kinds of levers, since the tower weight is uniformly distributed
before and after the attach point.
First page has picture showing the cases I looked at. (although I show a
constant
distance backguy, I analyzed it as if the backguy matched the attach
distance).
Each of the lifting wires in the picture is really a different case.
Only one wire
is used.
multiply the forces by at least 2x for design margin. (not included)
bouncing or other momentary shock loads not covered.
http://home.comcast.net/~knormoyle/ginpole/ginpole_frame.htm
Second page shows the results. You can see how it's non-linear.
http://home.comcast.net/~knormoyle/ginpole_ss/ginpole_ss.htm
You can see a knee in the curve at the ginpole height = 1/2 tower length
(and attach at midtower). (around 12 feet for me)
Third page is the excel spreadsheet.
http://home.comcast.net/~knormoyle/ginpole_ss/ginpole_ss.xls
I only analyzed the worst case starting point. Forces are less after that.
winch pull, ginpole compression, wire pull, are the key forces. The ginpole
compression gives you an idea about how much ground resistance you need.
at the bottom of the ginpole. Assume side and back guys keep ginpole
vertical.
(assumed the fulcrum is strong enough because it's the tower base bolts).
You can see why all the short raising fixtures put so much strain on a
winch.
In other, lighter, scenarios, people mention using extension ladders as
vertical ginpoles
You can get a feel for how much compressive force gets generated on them.
Can't raise much that way (certainly not this tower!)
You can also see why people break ropes (if they use ropes instead of wire)
(working loads for some typical ropes are at http://www.feeneywire.com/rope
you can see why none of the reasonable diameters are good here)
-kevin
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