Hi Richard,
Welcome to this interesting discussion. We are in agreement except where you
say the idea is not practical because you have to stop at your lowest connecting
point which, you say, is below the top of the gin pole, on the tower. Why not
make the attachment point of the rope at the TOP of the gin pole where the gin
pole pully is? That way, you can pull the mast clear over the top of the tower
and enjoy the mechanical advantage all the way up.
Like I said in other posts, I use a double pully at the top and simply put the
double pully hook into the top of the gin pole. I do not use the pully normally
found at the top of the gin pole at all when I use my block an tackle. The end
of the rope that is tied is connected to the moveable single pully at the load.
You can pull on this system until the single pully hits the double pully and the
mast will be well over the top of the tower at that point. I also use a special
heavy wall gin pole that is about 16 feet long. I can set long and heavy masts
with this setup. It is true that this requires a lot more rope and an
investment in pullys, but I have found it to be WELL worth it. One of my ropes
is 450 feet long . . .
Stan w7ni@teleport.com
Richard L. King wrote:
> OK, I have to jump in here now.
>
> In physics all systems have to be in balance for something to stay still.
> To start something moving requires slightly more force than the force it is
> using to stay static.
>
> Consider the ginpole:
>
> We want to pull up a 200# weight with a rope. It requires 201# (+) of pull
> in the upward direction to get it moving (we will call it 200# for
> simplicity). For our convenience we install a ginpole because it has a
> pulley in it that allows us to pull from the ground instead of the top of
> the tower. The ginpole also allows us to lift something clear of the topw
> of the tower. The weight is 200#, the pull is 200# and you have 400# of
> downward force on the pulley in the ginpole.
>
> Consider the block and tackle:
>
> A block and tackle doesn't really give you mechanical advantage, but it
> seems to. What it actually does is spread the load over several connection
> points. It depends on whether the pulleys are stationairy or moveable in
> the setup. If you add all the forces up, the lifting force will always be
> 200# to lift the 200# load.
>
> If the block and tackle are on the pulling side of the ginpole, the force
> on the ginpole pully will always be 400# to lift the 200# weight. That's
> because any mechanical advantage you gain is for the pulling person and the
> forces are shared on the multiple connections points that the block and
> tackle are attached to.
>
> If you put the block and tackle between the ginpole and the load, you can
> actually decrease the forces on the ginpole pulley, but this isn't
> practical to do and here's why.
>
> Consider a 2 to 1 block and tackle between the ginpole and the weight:
>
> To get this advantage you have to attach the weight to the clip side of a
> pulley. Then a rope would be run from the lifting person, up the tower and
> through the ginpole pulley, down to the weight and through the attached
> pulley, and then back up the tower and tied to a connection point. To lift
> the load requires 200# of upward pull that is equally shared between the
> two ropes through the pulley at the weight. That would be 100# on each side
> of the rope and the total 200# of lift is shared between the ginpole
> pulley and the tie point on the tower. This subsystem is now in balance.
>
> Now that the ginpole only has 100# of downward force on it, the pull at the
> person end of the rope only needs to be 100# to get the system in balance
> and to start the lifting.
>
> The reason nobody does this is that it requires two points of attachment at
> the top of your lift. Since one point is the ginpole (which is up high) and
> the other is on the tower somewhere (which is much lower) you can only lift
> the weight to near the lower connection point. So you lose the advantage of
> using a ginpole to lift the weight up over your head to attach at the very
> top of the tower.
>
> A 3 to 1 pulley on the weight side will have the same physical problem
> limiting the range of your upward pull. But the amount of pull on the
> ginpole pulley now be only 67#. The reason for this is simple. A 3 to 1
> block and tackle requires 2 pulleys, one at the weight and the other at the
> tower connection end. The rope goes from the 200# weight, up the tower
> through the attached pulley, back to the weight and through its pulley,
> back up through the ginpole pulley, and then down to the lifting person.
>
> Now you have three ropes assisting in the lifting of the 200# weight (draw
> a picture and you will see it) and the resultant force on each rope is
> about 67#. Since two of the ropes go to the pulley attached at the tope of
> the tower, your are exerting 134# of force on that connection point. The
> other 67# of force comes from the lifting person pulling the rope that goes
> through the ginpole. So the 3 to 1 system is in equalibrium.
>
> I hope this helps some folks consider where the forces go when you use a
> block and tackle and the limitations you have due to fixed vs movable
> attachment points. For my examples I did not take the weight of the rope or
> the pulleys into consideration. That can certainly add to the pull required.
>
> I am not a physics teacher, but I play one around my house.
>
> 73, Richard
>
> k5na@texas.net
>
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