[TowerTalk] Falling derrick

[Richard (Rick) Karlquist]

On 3/20/2021 7:50 AM, John Simmons wrote:
> I'm planning a tower installation for the summer. It is an HDBX, 
> sections 8-4 (40 feet) with the hinged base. A 70 pound yagi and a 
> Tailtwister will be at the top with a very short mast. I want to build a 
> falling derrick system for it.
> 
> I'm searching for answers to a couple questions:
> 
>   * How long should the lever be?
>   * How much pulling force will I need to raise the tower? I realize
>     that the force will be maximum when the tower just comes off the 
> ground.
> 
> I found this reference:
> https://comtech.ru/en/production/towers/tower-accessories/ct-s3ts/ct-s3ts-falling-derrick-tilt-up-method/ 
> 
> 
> I'm neither good at physics nor math above algebra.
> 
> Comments?
> 

I've done a lot of work with falling derricks.
There is no cookbook.  Some of the calculations
are straightforward, but may be beyond your math
ability.  The trickiest issue is designing the
falling derrick itself.  First of all, you have
to determine the compression force on it.  That
is relatively straightforward if you are good
with math.  However, it is difficult to determine
how much force a given candidate for the derrick
can withstand before buckling.  I have resorted to
simply trying various things.  The first foot of lift
will tell you if the derrick is going to buckle.
If a 2 inch pipe buckles, try a 3 inch pipe, etc.
Stiffness goes as the difference between the
OD raised to the 4th power and the ID raised to
the 4th power.  The tower itself can buckle if
you don't use the right number, and placement, of the
lifting cables.

I have serious disagreements with the reference you
cited as follows:

1.  The tower has 3 way guying and depends on the
hinge to keep from falling over sideways.  It should
have 4 way guying and the hinge should be a pier
pin.  You could use temporary 4 way rope guys and
then switch over to 3 way EHS or Phillystrand guys
after erection.  I see you already have a hinged
base.  You need to make sure the side guys don't
overstress the hinge by pulling it off center.
This requires very precise layout of the side guy
anchors and just the right amount of tension.

2.  The falling derrick should have its own pier pin
and not be rigidly fastened to the tower.  It should
have side guys in addition to the lifting cables.

3.  There should be a ground mounted pulley that the
hoisting cable goes through before heading up to the
top of the derrick.  The pulley should be located
such at the top of the derrick comes down right on
top of it as the tower reaches vertical orientation.
I originated this idea around 20 years ago.  It was
obvious to me at the time that this is essential
because you have an extreme mechanical disadvantage
when the tower is near vertical.  When going down,
the tower has a "tipping" behavior that can get
out of control if the pulley is not used.

I'm sure you can find other people who have gotten
away with not following my advice.  YMMV!  My
recommendations are the result of the college of
hard knocks.  A fair amount of aluminum pipe was
sacrificed in the interest of science :-)

73
Rick N6RK