[TowerTalk] Mast load question

[KJ0M <kj0m@mchsi.com>]

I have a question regarding the installation of a couple of antennas 
on a US Tower TX-472 crank-up, tilt-over tower. The question involves 
lateral loads on the mast when the tower is tilted over. The tower is 
rated as I have it installed to handle quite a bit more wind loading 
than I will have on it. This includes the bending moment at the tower 
base and at the base of the mast at the thrust bearing. I'm not 
worried in this regard. It has to do with when the tower is tilted 
over.

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.

Wind loading for the 2 of them is about 17 square feet, which, as 
stated above, I've accounted for when vertical, either extended or 
retracted. I can extend the tower to 72 feet from within the shack 
and keep it nested at 21 feet unless in use. I believe and have 
calculated that I'm well within specs for the tower/antenna 
combination, both at the base of the tower and the thrust bearing.

I have a special steel jig made to support the tower when it's tilted 
over. That takes the tension off the tilting cable while I do my 
work. It makes me feel better in that I know if the cable were to 
break, the tower would stay put. I feel that when tilted over, 
there's a terrific amount of force on that cable, so I had this jig 
welded up by a local welder. I tilt the tower over, place the bracket 
under the tower to carry the weight of it and lower the tower until 
the cable gets slack.

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?

Can anyone comment on this? I'd rest easier knowing this isn't a big 
problem before I tackle it.

Thanks.

--

73,

de KJ0M
Jim
._._.
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