[Towertalk] Statics 101

[K3BU@aol.com]

In a message dated 7/11/02 8:15:40 PM Eastern Daylight Time, wendell@wyly.org 
writes:

> 
>  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. 

It "do's" matter!  (According to "mechanics 1001" :-)
If you use two pulleys, one pulley at the bottom (with one end of rope tied 
to the top of tower, the other end looped through pulley at the bottom and 
over the pulley at the top) then your 100 lbs is (more-less, assuming well 
oiled pulleys) divided between two ropes going over the bottom pulley, and 
you are pulling with abt. 50 lbs force at the pulling rope (twice distance). 
The load on the gin pole is 150 lbs (assuming the top end of rope is tied to 
the gin pole. If you tie the top end to the tower, say, below gin pole and 
top pulley is tied to the gin pole, then gin pole gets abt. 100 lbs load (50 
from you pulling, 50 from half of rope going over bottom pulley). Way to 
minimize the stress on the gin pole. Of course, pulling straight down 
minimizes the bending moment on the gin pole.
Formula for pulling force F = 1/n x W, where W is the weight of antenna at 
the end of block, n is the number of ropes in the block (not counting one you 
are pulling).

I have been using two pulleys at each end (gave me F = 1/4 W) to lift my 62 
ft boom Razors by myself, with pulling rope threaded through loop at the 
hook, guiding the antenna away from the tower. Big Bertha tower with no guys 
helped. Wear hard hat, things fall from the tower.

Yuri, K3BU