> I have just erected a new HDBX 40 in place of my 30 year old HBX 48,
sacrificing 8' of height to gain some strength and load capacity (and peace
of mind!). The stock rotator plate allows 2' of mast below the top plate
and I will be using the heavy duty Yaesu thrust bearing (along with the Yaesu
heavy duty 2800 rotator plus the absorber plate). I have added steel
angle braces to reinforce the rotator plate.
> I'm looking at using a 15' chrome/steel 2" mast (in place of my
previous 9' mast), so 13' of the mast would extend above the tower. The mast
would support a TX38 tribander just above the top plate, a 6 m beam half-way
up, and a 40m rotatable dipole near the top, 12' above the top tower plate
and the thrust bearing. The mast, of course, is very heavy, weighing around
75lbs. The tribander weighs 40lbs and with 5 ft2 surface area; the 6 m.
beam weighs 10lbs with 1.5 ft2; the dipole weighs 10lbs and is 0.5 ft2.
> Is this too much mast for this tower? (BTW - I'm aware the boom
length exceeds the 10' maximum for this tower, but the HBX 48 handled a
similar
tribander for 30 years, with occasional severe ice-loading, without any
problems.)
Well, I'd say you're pushing the limit. The capability of the tower
vis-a-vis the proposed mast depends on 2 factors. First, the whole thing -
including the top and rotator plates - are held together with aluminum
rivets. They're easy to abuse and they won't take a lot of abuse.
The second thing is that the plates are made from sheet metal. They
don't have a lot of capacity to begin with. And with only 2 feet of mast in
the tower, it's way top heavy and makes a nifty big lever arm.
And BTW, Nobles County is an 85 MPH windspeed zone. Since Rohn rates
the tower at 50 MPH, I'd say you've already exceeded the manufacturer's
specs.
So is this mast too much for this tower? I'd say so. It does have a
high potential for failure so keep your homeowner's insurance paid up.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for amateurs
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|