[TowerTalk] Re: Gin pole loading

[Kurt Andress]

Stan Griffiths wrote:

> Hi Kurt,
>
> Well, I got you to go part way with me at least.
>

SNIP

Hi Stan,

I'd never intentionally start an arguement with you.
I was in too big a hurry and did not take time to clearly get it all out. Should not have
said anything, my apologies.

I don't disagree with what you and others have offered about how a 2:1 or 3:1 or 4:1
system actually operates. The "load" applied to the pole goes down, no arguerment.
But, that's not the whole story.


Here's my concern.
If the discussion ends with the information presented, a lot of people are going to think
that the reduction in the pole load is going to mean they can now increase the load by an
equivalent amount and be ok.
For a purely vertical lift it's ok, but if the load is off at an angle the story changes.

I haven't looked at pole buckling. Here is what I see is happening to stress with the load
out away from the tower.


Lets take a situation where we are lifting 200# with a 12 Ft long 2" dia x .125 wall
aluminum pole (6063-T832).

The top of pole is 100' high and the load is 5' away from the tower. We assume the lift
lines are straight and weightless.

We start with a 1:1 rigging, 1 sheave and 2 lines.
We look at the stress at the base of the pole.

I get 547 psi due to pure compression (11%) and 4430 psi due to bending (89%).
4978 psi combined stress, safety factor 7.0.

We go to using a 2:1 system like you mentioned, one dead end, 2 sheaves and 3 lines.
I get 410 psi pure compression (8%) and 4430 psi bending (92%).
4841 psi combined, safety factor 7.2.

The 33% reduction in "load" applied to the pole has resulted in a 2.7% reduction in
stress.

If we do it again for the same scenario but move the load out 10' from the tower, I get a
1.5% reduction in stress. Bending stress is 94% -96% of the combined stress. Safety factor
goes from 3.7 to 3.8

Take the load out to 15' from the tower and the rigging change results in a 1% stress
reduction. Safety factor goes from 2.5 to 2.6

In all cases I did not add the weight of the blocks and additional line back into the load
so my stresses are lower and reductions %'s are higher than actual.

Adding the purchase system does nothing to change the pole transverse load, it only
changes the vertical one. The rigging change affects about 1/10 of the stress problem, so
the stress reduction is pretty insignificant.

The reason the transverse load at the top never changes can be seen from either of two
perspectives.
>From the pole and tackle view, look at the resultant load vector acting on the pole top.
As we add more load sharing lines to the load side of the pole, the resultant angle
increases from the vertical axis, maintaining the transverse load.
>From the whole big picture, the load and the angle to the top is not changed by the
rigging change.

I think block and tackle mechanical advantages are really great things.
How we are using the whole system also needs to be considered.


--
73, Kurt, K7NV




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