I think a "second attachment" is what we are talking about.
In my scenario (2:1) the lift line runs up the tower (from the ubiquitous
snatch block, of course!) to the ginpole pulley. Then down to and through
the load's pulley then back to the ginpole where it's tied off. In this
case (with a 100lb load) the ginpole has 150lb of force on it (three ropes
with 50lb each). Ginpole gets 50lb relief; need a bit over 3XTowerHeight
worth of rope.
In Chris' (3:1) scenario (if I understand it right) the lift line runs up
the tower to the ginpole pulley. Then down to and through the load's pulley
then back to another ginpole pulley. I guess either it's a dual pulley
(never seen one on a ginpole) or another pulley attached to the ginpole up
high. Then, finally, back to the load where it's tied off. 133lb total
down force (4 ropes @ 33lb each) Ginpole gets 66lb relief; need over
4XTowerHeight worth of rope.
I don't see any reason to stop and "re-rig". One really long pull does the
job.
-Mike N2MG
W5FL wrote
Don't matter how many pulleys you use, unless you make a second attachment
to the tower you don't gain anything as the ground crew holds the rope (same
as tieing it to a ground) and the gin pole takes out twice the load since
there are two equal downward force vectors. If the forces were not equal,
the load would be going up or down really fast. The reason it looks good,
is that you must tie both the rope to the second pulley and the ground crew
to ground in which case the ground crew only has to pull half as hard (but
twice as far), but the gin pole gets no relief and you have to re-rig the
whole thing when the load is only half ways up, which is quite impossible!!!
This is one of those things that you will only try once if you don't think
about it or not at all if you do.
VE7HCB wrote:
It has been awhile since I have done any mechanical stuff with pulleys. I
think that you are on the right road but not all the way.
If you are lifting a 100lb weight with one pulley at the top, you need 100+
lbs on the other side to lift it. I would then say that there is 200+lbs
of weight on the pulley.
If you are lifting a 100lb weight with 2 pulleys at the top and one on the
load, then you have a mechanical advantage and need only 1/3 of 100+ lbs to
lift the weight (I think that that is correct, but it is a lot
less) Therefore, the total weight on the pulley is 100lbs plus the reduced
amount hence less.
I think that this is correct.
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