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
>
>
>
>
>_______________________________________________
>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.
>_______________________________________________
>Towertalk mailing list
>Towertalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
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.
_______________________________________________
Towertalk mailing list
Towertalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|