Hi Kurt,
Well, I got you to go part way with me at least.
If you use a double block at the top and a single block at the load, the load
will be
supported by three strands of rope, each one supporting 1/3 of the total load.
If the
load is 200#, then each strand supports 67# and that is the tension in the
rope. The
same rope is used as a pull down rope and the tension in it must be 67#, too.
So,
therefore, on the load side of the gin pole, you have three strands of rope
pulling
down with a force of 67# each for a total of 200# and on the pull down side of
the gin
pole, you have only one strand with a tension of only 67# (same tension
throughout the
entire length of the rope).
This is how the total force pulling down on the gin pole is only 267# instead of
400#. The best thing to do here is get a double and a single pully and rig it
and
measure it.
I absolutely guarentee it will come out as I have described (neglecting
friction, of
course).
Stan w7ni@teleport.com
Kurt Andress wrote:
> Wow! A variety of ideas on this one.
>
> How about another one?
>
> 1) If the load being lifted is 200#, and the lifting line is vertical, the
> load on
> the line is 200#
>
> 2) If the tension in the line is 200# on the load side of the gin pole
> sheave, the
> line tension is also 200# on the lifting side. This neglects friction in the
> sheave assembly.
>
> 3) If we make a little sketch of the sheave on top of the gin pole we see two
> loads acting on it. On the load side is 200#, on the lift side another 200#.
> So,
> we add them up and get 400# of load on the top of the gin pole.
>
> I can't see anyway that adding a mechanical advantage to the load side or lift
> side of the system will result in a reduction in the load applied to the top
> of
> the gin pole.
> It can make the job easier for the fellows pulling on the rope, but the gin
> pole
> doesn't get to enjoy the benefit. Gotta have a net load of 200# on each side,
> no
> matter how you achieve it! The load at the top of the gin pole is determined
> by
> the line tensions exiting either side of the sheave and their angles.
>
> The reason gin poles bend over and give us heartburn is because we cannot
> practically raise the load in a position that keeps the load side parallel
> with
> the pole. We usually keep the load out away from the tower with a tag line to
> prevent it from getting fetched up in the tower. As a result, the gin pole
> rarely
> sees balanced loads. The lift side line is always running nearly vertical,
> down
> the inside of the pole, and the load side line is always running of at an
> angle.
>
> The resulting force applied to the top of the gin pole is always exactly
> halfway
> between the axes of the load side and lift side lines. So, we produce a side
> load
> on the gin pole that causes it to bend.
>
> Beyond the clamping strength of the unit, the sheave pin capability, and
> beam/column buckling properties of the pole, rating gin pole strength is
> largely a
> collection of anecdotal evidence.
>
> If we were to define the actual limits of load side line angular displacement
> from
> vertical, we could then determine a safe rating for the pole.
>
> The rating would appear something like this:
>
> This product can safely lift XXX Lbs when the load side line is XX Degrees
> from
> vertical.
> It is XXX Lbs at XX degrees.....etc.
>
> Of course, the above mentioned features would have been adjusted to suit the
> worst
> load case.
>
> Until then, keep asking others what they have had success with.
> Use good judgement and keep the load as close as possible to the tower and as
> close as possible to the tower leg that the gin pole is attached to!
>
> Happy heaving....
>
> --
> 73, Kurt
>
> K7NV "That's K7 "Nevada" (ex - NI6W)
>
> YagiStress - The Ultimate Software for Yagi Mechanical Design
> Visit http://www.freeyellow.com/members3/yagistress/
>
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