[TowerTalk] Mast load question

[David Hachadorian]

----- Original Message ----- 
From: "KJ0M" <kj0m at mchsi.com>
To: <towertalk at contesting.com>
Sent: Wednesday, May 26, 2004 12:55 PM
Subject: [TowerTalk] Mast load question


>
> Currently, I have a Force-12 Model 5BA with a 32 foot boom installed
> directly above the thrust bearing. I can tilt the tower over to
> approximately a 45 degree angle to inspect or work on the antenna
and
> all the elements from the ground or using just a small step ladder
to
> reach the elements near the center of the boom. This entails tilting
> the tower up a bit, rotating the antenna to get at either end of the
> boom and tilting the tower back down. The antenna, which weighs
about
> 85# can be installed by me alone by installing the bare boom and
then
> adding all 15 elements one at a time. Again, this can be
accomplished
> from the ground or on a short step ladder.
>
> I would like to install a Force-12 240/230 4 element antenna on a 24
> foot boom above the 5BA. Due to interaction between these antennas,
I
> need to install this antenna at least 9 feet and preferably more
> above the 5BA. The plan is to install a new chromalloy mast of 20
> feet, with 4 feet inside the tower and placing the 240/230 about 12
> feet above the 5BA at the thrust bearing.That will allow room for
the
> boom truss on the 240/230. This antenna weighs around 45#. I plan on
> doing this myself by following the above model, first by installing
> the new mast, then the 4 element 30/40 meter antenna followed by the
> 5BA.

>
> My question regards when the tower is tilted over with the antennas
> both installed. With that 45# antenna about 12 feet above the thrust
> bearing and everything leaned over at about 45 degrees, it seems to
> me that there's a terrific amount of lateral force on that mast with
> then antenna 12 feet above the thrust bearing. I wonder if it's too
> much? The 5BA at the thrust bearing would have little effect, it
> seems, since it's right at the thrust bearing and it's weight would
> be supported by the thrust bearing. At least that's the way it
feels,
> intuitively, to me.
>
> Nearly all of the lateral force on the mast would come from the top
> antenna, wouldn't it? That force would be applied like a lever to
the
> rotor, which is 4 feet inside the tower, below the thrust bearing.
Is
> there any way to calculate this force? Would it shear the bolts
> holding the rotor to the rotor plate? Maybe it's nothing to worry
> about at all, considering it's only 45#. I'm sure the chromalloy
mast
> could take it. What about the rotor bolts?

In Statics, this is a "sum of the moments" problem. The clockwise
overturning moments about the thrust bearing must equal the
counterclockwise moments. You didn't specify the mast weight, but let
me assume that it weighs 5 pounds per foot.

clockwise:
(45# ant)(12')(.707) + (80# mast)(8')(.707)

counterclockwise:
(rotor force)(4') + (20# mast)(2')(.707)

Therefore:
(4')(rotor force) = 382 + 452 - 28
rotor force = 202 pounds


Dave Hachadorian, K6LL
Yuma, AZ