|Subject:||[TowerTalk] Mast load question|
|Date:||Wed, 26 May 2004 14:55:08 -0500|
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
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.
de KJ0M Jim ._._. _______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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