ARGH! How many times do we have to go through this. When you have a
2:1 advantage the ground crew only pulls down on the pole with 1/2 the
weight of the load so the load on the gin pole is reduced by 1/2 the
weight of the load. It is obvious from the vectors. Try this out for
size. Go up the pole with a rope, over the pulley, down to the load,
through another pulley, and back up to the top of the pole and tie it
off. Now lift 100# and hold it. Each rope section now has 50# of
tension in it. 2 lengths of rope from the top of the pole go to the
load offsetting its 100# weight, one length of rope goes down to the
ground crew which has to hold the 50# tension. Since all 3 are held up
by the pole the pole supports 3*50# or only 150#, not the 200# it would
have to hold without the extra pulley.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-admin@contesting.com [mailto:towertalk-
> admin@contesting.com] On Behalf Of Wendell Wyly W5FL
> Sent: Friday, July 12, 2002 00:12
> To: Chris BONDE; jljarvis
> Cc: towertalk@contesting.com
> Subject: [Towertalk] Statics 101
>
> Statics 101: An object is at rest when the forces up equal the forces
> down.
>
> 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.
>
> -----Original Message-----
> From: towertalk-admin@contesting.com
> [mailto:towertalk-admin@contesting.com]On Behalf Of Chris BONDE
> Sent: Thursday, July 11, 2002 5:20 PM
> To: jljarvis
> Cc: towertalk@contesting.com
> Subject: Re: [Towertalk] mast disaster avoidance
>
>
> 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.
>
> Chris opr VE7HCB
>
> At 02:00 PM 2002-07-11 -0400, jljarvis wrote:
>
> >Mike...
> >
> >If you had a ginpole with a 2 wheel block on top (never seen
> >one, myself. Do they exist?), you would reduce the load on each
> >run of line, proportional to the number of lines.
> >
> >But the load on the axle is still the sum of the line-loads.
> >i.e., unchanged.
> >
> >The compressive load on the ginpole itself is thus still the
> >same, and the out-of-column bending loads are the same.
> >
> >
> >Jim/N2EA
> >
> >
> >-0-
> >From: "Mike Gilmer"<n2mg@eham.net>
To: <towertalk@contesting.com>
> >Date: Thu, 11 Jul 2002 12:16:03 -0300
> >Subject: [Towertalk] Re: mast disaster avoidance
> >
> >I agree with most of N2EA's post except:
> >
> > > Solutions using added pulleys still leave the same load on
> > > the ginpole material itself
> >
> >This is not true. As has been oft-discussed on this reflector in the
> >past (and still misunderstood), judicious use of "added pulleys" can
> >reduce the total load on the ginpole.
> >
> >Mike N2MG
> >
> >
> >
> >
> >_______________________________________________
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>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
> http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and
take an
> additional 5 percent off
> any weather station price.
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>
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
> http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and
take an
> additional 5 percent off
> any weather station price.
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