Hi Mike,
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.
BIG difference . . .
I hope this is clear. If not, please ask again. This is
important. Just
understanding this principle of pullys would have saved a
BUNCH of collapsed gin
poles in the past (and maybe some serious injuries, too).
Stan w7ni@teleport.com
Michael Tope wrote:
> Stan, I am not sure that I understand what you are getting at. As I see it,
> the gin pole
> will have to support the entire vertical load of the mast irregardless of
> any mechanical
> advantage provided by a block and tackle.
>
> What am I missing here?
>
> Thanks,
>
> Mike, W4EF.......
>
> >Use of a block and tackle with 3:1 mechanical advantage will reduce the
> load on the gin pole a
> >whole bunch.
>
> >Stan w7ni@teleport.com
>
> Michael Tope wrote:
>
> > Can anyone out there give me advice (anecdotal, analytical, or otherwise)
> on sizing a gin pole
> > for structural integrity? I have a requirement to drop 24' of 1/4" wall
> 4130 chrome moly into
> > a Rohn 25G tower (estimate wt ~120 lbs). I currently have in hand a 20'
> section of schedule 40
> > 6061-T6 water pipe (1.915" OD; 0.158" wall) which I hope to use for the
> job in conjuction with
> > WB0W gin pole fixtures (pulley and bracket). The actual working length
> (measured from the
> > bracket to the pulley) would be around 15' to allow for raising the the
> moly masts center of
> > gravity (z=+12') above the top of the tower.
> >
> > My feeling is that the aluminum pipe should be adequate to the job
> provided that we take
> > reasonable precautions to keep the load (mast) roughly collinear with the
> gin pole (e.g.
> > minimize side load). Any comments on my assumptions would be appreciated,
> especially
> > from those who have already performed similar feats.
> >
> > Thanks,
> >
> > Mike, W4EF/6
> >
> >
> >
> > --
> *********************************
> * Michael C. Tope *
> * Jet Propulsion Laboratory *
> * Section 333 *
> * M.S. 238-737 *
> * Tel (818) 354-1335 *
> * Fax (818) 354-2825 *
> * email: mtope@jpl.nasa.gov *
> *********************************
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