Kurt,
Pragmatically, you are correct, since the vertical load on the gin pole
isn't really relevant to its ultimate strength. As Stan, W7NI correctly
points out, the block and tackle will reduce the vertical load on the
gin pole (I wasn't convinced of this at first), but since the strength
of any practical gin pole is so much greater in pure compression as compared
to side load, the problem becomes much more complex. The pull rope on the
typical gin pole is inside the pipe, so the forces on it will be almost
entirely parallel to the pole's axis, thus acting on it in nearly pure
compression. The load side of the rope being offset somewhat from the
axis of the pipe will, in addition to placing a compressive load on the
pipe, cause it to bend. Thus, only the force due to the load is relevant
to the possible failure of the gin pole. If the gin pole pulley was offset
by say 2' from the pipe axis, then both the weight of the load and the
reaction force of the pull rope would contribute to bending. In this case,
a block and tackle with the same average offset from the pipe axis would
probably relieve the bending stress on the pipe. You can probably imagine
a bunch of scenarios in between these to extremes where the block and tackle
might help or hinder depending on the specific geometry. Seems that the
trick to
making a robust gin pole is too minimize the load offset for a given pipe
cross section and length. Probably easier to just go out and buy a "big
mudda"
pipe and not worry about it.
Anyway, at least now I understand why no body I have talked to has been
able to say with confidence "Yes, that pipe is big enough for the job
your are doing". Every answer has been qualified with a "That SHOULD be
okay" or "That MIGHT be too small" and understandibly so. Its not a simple
problem.
Mike, W4EF.............
"Gary McDuffie, Sr." wrote:snip
>What cracks me up is the thought of all the real experts sitting on the
> side "listening" to all of this and laughing as we all spout off our own
> theories. It will be interesting when they finally speak up and we get
> the real scoop.
>
> Gary
> --
Gary,I swore I wasn't going to get back into this one! So much for good
intentions!I quickly realized that the conflicting opinions far exceeded
my available bandwidth.I chose not to be branded as a non-participant,
sitting on the side, so... Here's my simple view of the problem, not
necessarily
"the real scoop," as I'm sure we will soon see.....The gin pole usually has
two
forces acting at the top. One is a verticalforce trying to compress the pole.
The other is a horizontal load trying to bend it. The magnitudes of the
forces
are determined by the load, we are trying to lift, and the line angle from
the
pole top to that load. We can put any mechanical system between the load and
the pole or the groundand the pole to make the lifting job easier, but we
still
have the sameload, we are trying to lift, acting at the same angle. The
only way
to change the forces on the pole is to change the load or theangle, not the
mechanics of the gear between them. That's just my opinion! And, for some
reason,
I find myself unable to follow the rest of the plot. So, I gladly exit the
discussion,
having offered one perspective, and leaving it for others to finish.
Everyone have a great Easter!--73, Kurt
--
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