[TowerTalk] Rohn HDBX 40 - mast length

[K7LXC at aol.com]

>  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