Hans,
I gave Stan's posting a little more thought and he is correct on all
counts. You are correct if the gin pole rope is tied to the top of
the gin pole. In this case, the gin pole must push up with a force
equal to the weight of the load - 200 lbs in Stan's example. But if
you run the end of the rope over the gin pole pulley, you create a
2:1 mechanical advantage. The pulley axle now has to support the
downward force of the load on one side, and the opposing force of the
pull rope on the other.
Think of it this way - if I hand the pull rope to a 199 lb guy (assuming
no friction) the system will be out of balance by 1 lb. My unfortunate
friend will be lifted off the ground as the 200 lb load begins to descend
towards the ground. My friend is very quick witted though, he sees a 1
lb crescent wrench laying across the tower rungs about half way up and
grabs it. The system comes into balance again, the gin pole is now
supporting two loads - my 199 lb friend with his 1 lb crescent wrench and
the original 200 lb load, so the gin pole sees 400 lbs of load. Feeling
sorry for my friend who I know is afraid of heights, I walk over to the tower
base and grab the slack end of the rope he is holding (my friend has a
tight grip as he is very scared). I pull the rope tight and tie it off to
the base of the tower and then reassure my friend that our razor beam is
safe and that he can grab onto the tower and let go of the rope. The pulley
is still in equilibrium. Being a deaf, dumb, and blind pulley, it doesn't
know
that my friend has let go, it just knows that something is still pulling on
that end of the rope with a force of 200 lbs. The gin pole also being
somewhat deaf, dumb, and blind doesn't realize anything has changed either
and continues to push up with a force 400 lbs. Being somewhat concerned
that my flimsy gin pole is overloaded, I climb the tower with a crosley
clip in my hand. When I get to the top of the tower, a place the crosley
clip on the pull rope and then descend down the tower. Since I have been
eating lots of Little Debbie snack cakes, I am topping the scales at well
over 200 lbs and am not concerned that I will have the same uplifting
experience that my lighter friend had. I undo the knot in the pull rope and
allow the load to drop until the crosley clip jams in the gin pole pulley.
Suddenly, there is no tension on the pull rope. Now the gin pole only has
to oppose the tension in the load side of the pull rope. I reason that the
moment the pull rope went slack, the force on it went from 400 lbs to 200
lbs.
Not wanting a repeat of my friends uplifting experience the next time we
do tower work, I decide to modify my gin pole. I remove the pulley head
and install a short cross bar about 12" long on the top of the gin pole.
I place the cross bar on the gin pole symmetrically to form a "T". On one
end of the cross bar I install a large eyelet. On the other end I place
the gin pole pulley. I convince my friend to install the modified gin pole
as I am very tired and want to eat a few more snack cakes while he is on
the tower (my friend has since been to therapy and conquered his fear of
high places). I tell my friend to attach one end of the gin pole rope to the
fixed eyelet on the "T" section and to let the other end go slack to the
ground. I grab a second pulley from my tool box and head over to the 200lb
razor beam which is now sitting on the ground. I attach the pulley with a
short piece of chain to the center of gravity of the antenna. At this point,
I fumble around a bit and find the end of the pull rope that my friend has
dropped to the ground. I take the end of the rope and feed it thru the pulley
on the load (antenna) and then tie it to my climbing belt. At this point,
feeling guilty that my friend has been doing all the work, I ascend the
tower with the end of the slack rope on my belt. When I reach the gin pole,
my friend and I feed the end of the rope thru the gin pole pulley and take it
with us back down the tower (it is a long rope). We now pull the rope tight
and observe that it starts at the fixed eyelet on the gin pole, descends to
ground and thru the pulley attached to the load, ascends back up the tower
thru the gin pole pulley and back down the tower into our hands. My friend
and I pull in 10 more feet of rope and then stop to catch our breath. When
we glance over at the antenna, we notice that its only 5 feet off the ground.
We decide to tie off the rope and pounder this a little, since we are quite
confused. My friend and I reason that each leg of the loop of rope that
passes thru the pulley on the antenna must be supporting 1/2 the weight of
the antenna (100 lbs). This makes sense to us since our highly lubricated
antenna pulley isn't turning - the forces on it have to be balanced (100 lbs
on each side). We further reason that the gin pole must be pushing up with a
force of at least 200 lbs, since it supports both ends of this loop of rope.
Still confused, my friend grabs a spring scale from the garage and hooks it
to the pull rope. As we lift the antenna still higher, he notices that the
pull rope has 100 lbs of tension on it - the amount needed to balance the
load
on the other side of the gin pole pulley. We then reason that the gin pole
is pushing up with the force needed to balance the tension in the three legs
of the rope that it supports. We know that each leg of the rope has 100 lbs
of tension pulling down on it, so the gin pole must be pushing back with a
force of 300lbs. My friend and I tie off the rope and decide to have some
lunch
(more snack cakes). While I am eating, my friend decides to get me back for
his uplifting experience. He sneaks out of the kitchen a scurries up the tower
with a crosley clip in hand. Unbeknownst to me, my friend places the crosley
clip on the pull rope 2 inches below the gin pole pulley and then rejoins
me in the kitchen. When we return to finish raising the antenna, my friend
gets a wild look in his eye and says "I am tired of this hobby". He then pulls
out his Swiss Army knife and starts cutting the pull rope. I nearly faint.
As the rope severs, there is a mild jolt as the crosley clip jams into the
gin
pole pulley. The pull rope dangles slack as my friend exclaims "payback is
a b...".
At this point we return to intellectual contemplation and begin thinking about
the forces on the gin pole. It is now supporting 2 legs of the pull rope each
with 100 lbs of tension on it. We reason the gin pole must now only be pushing
up with a force of 200 lbs. As I walk over the severed end of the pull rope
and start yanking, I imagine that the gin pole must supply an incremental
opposing
force equal to the incremental force that I place on the rope until the point
that the antenna begins to move. At this point the pull rope is again under
100lbs of tension and the gin pole is resisting 300 lbs of total downward
force. My friend and I finish the job and then watch TV for a while since our
brains are tired from all this thinking.
Disclaimer - the preceeding story is a fictional account and is not based upon
any real events or individuals. Any similarity of the story or characters
to real
events or individuals is purely incidental.
Mike, W4EF.......
P.S. I am a convert on this issue. My first reaction to Stan's claim
was similary skeptical.
RE: [TowerTalk] Gin Pole Strength
Hans K0HB (k0hb@arrl.org)
Sat, 3 Apr 1999 02:59:37 +0100
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On Friday, April 02, 1999 7:37 PM, Stan Griffiths
[SMTP:w7ni@teleport.com] wrote:
> Here is what you are missing: The gin pole has to support
> TWICE the weight of the load since the load pulls down on
> one side of the gin pole and the rope you are pulling on to
> raise the load is pulling down on the other side of the gin
> pole. The gin pole sees both of these down pulls and has to
> hold them BOTH, or twice the weight of the load.
>
> By using a block and tackle, you can reduce the pull on the
> rope you are using to lift the load. If you use a 3 to 1
> block and tackle, you reduce the pulling force to 1/3 of the
> weight and reduce the total load carried by the gin pole.
> Here is an example:
>
> Supposing your big mast weights 200 pounds. Pulling it up
> with a normal gin pole with nothing but a pully at the top
> means it must support 400 pounds. If you use a block and
> tackle, you can cut the pulling force
> down to 67 pounds and the gin pole now has to support only
> 267 pounds instead of 400 pounds.
Wrong. Just plain wrong. While the block and tackle gives
you a mechanical advantage, the gin pole is carrying
*only* the weight of the antenna (200 pounds), either with
a single pulley at the top, or with the multiple pulley
arrangement of the block and tackle. In fact, the added
weight of the extra pulleys and rope places a greater
strain on the gin pole than just a single pulley at the
top.
73, Hans, K0HB
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