Greetings from Gloucester,
I missed this thread over the weekend. It sure has made for interesting
reading with lots of good, thought-provoking, amicable discussion.
I may have missed it, but no one mentioned that you should always use a
turning block at the base of the tower for your haul rope to keep the
haul load as close to the axis of the gin pole as possible. Some
scenarios of pulleys described might lead people to envision the haul
rope going straight from the hauler up at an angle to the pulley on the
gin pole. Don't do this! As already pointed out, you want to keep side
loads to a minimum.
Be sure that your turning block (pulley) uses a safety hook or locking
carabiner to attach it securely to a tower rung. DO NOT rely on the
tension of the haul rope to keep a simple hook engaged!
I witnessed a scary accident where the turning block unhooked itself,
releasing a section of Rohn 45 to free fall for about 30 feet. It
tumbled a little and got off to the side of the gin pole. When the slack
in the haul rope pulled tight again, the resulting shock load on the
Rohn gin pole bent it permanently. Fortunately, the tower workers and
crew were only shaken and not hurt (They got "sewing-machine legs").
Don't underestimate the energy that develops in a free fall. Shock loads
are easily several times a normal load. This is why the 'working load'
of a device is much lower than its ultimate breaking strength.
I think some of the confusion in the load issue came from where the
tackle blocks were placed. One block must be on the gin pole and the
other must be on the load in order to reduce the vertical load on the
pole.
If the blocks are only on the haul line, as some of you undoubtably
envisioned, and there is no change
to the rig between the gin pole and the load, then the vertical load on
the pole will not change and will be about twice the load weight.
Alas there is no free lunch. Aren't we always running into compromises?
The *work* that is done (force times distance) *is* still the same no
matter how you rig your tackle blocks. Friction in the sheaves actually
increases our work a little. If you have a 2:1 mechanical advantage, you
have to pull the haul line twice as far. If 3:1, then you have to slog
through three times as much rope. And, of course, you have to have a
looooooong rope. And tall glass of iced tea afterwards.
--...MARK_N1LO...--
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