[TowerTalk] Gin Pole Strength

[Richard L. King]

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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