[TowerTalk] Booms

[Grant Saviers]

On 3/11/2017 8:16 AM, Jim Thomson wrote:
> Date: Fri, 10 Mar 2017 21:12:43 -0600
> From: Ed Karl <edk0kl at centurytel.net>
> To: towertalk at contesting.com
> Subject: [TowerTalk] Booms
>
> Hey Troops-
>
> I've been around for a while, always wondered about this. Looking at the
> discussion
> of square vs round booms. How come the elements are on top of the round
> boom?
> Seems like less inclination to rotate out of alignment if they were
> already on the bottom ...
>
> ed K0KL.
>
> ##  Nobody mounts els on top of the round booms, they all mount below.

 >> guess I've been doing it wrong, but it sure is easier.
> Since the els sag at their tips, I see no advantage to mounting els  below
> the round or square boom.  With eles  mounted below a boom,   you now have
> the entire weight of the ele, plus any ice loading, sitting on the mounting bolts,
> like those used on stauff clamp assy..which are only the .25 – 20 variety.
> If els mounted on top, you have zero weight on the bolts, or u bolts.
> Thats not that big a deal, except for maybe 40m eles, in
> which case you would have at least 2 x ss  u bolts on each side..or a min of 2 x
> stauff clamps per side.
I think you will have a pile of aluminum rubble long before the usual 
boom bolts fail - eg 1/4-20 for 10 or 15 for 2" booms, 3/8-16 or 1/2-13 
HDG u-bolts for 20/40 for 3" booms and 1/2-13's thru bolts for machined 
double saddles 80m 3" booms.
>
> ##  I would still like to know if I have the maths correct  as far as a 4 inch round boom
> vs  a 3 inch square boom.
Engineering Calculator agrees with your numbers below
>
> ## A 3 inch square boom has a section modulus of   1.323
> A 4 inch round boom has a section modulus of 1.45
>
> ## A 3 inch rectangular boom that is say has 40 ft long has a projected area of exactly 10 sq ft.
> A 4 inch round boom that is the same 40 ft long has a projected area of exactly  13.332 sq ft.
>
> ##  Both latest eia- tia  and also ubc-97 D specs say to multiply any flat surface  by 2.0
> Round tubes are multiplied by just 1.2
>
> ##  so the effective area of the 3 inch sq boom ends up being  10 x 2 =  20 sq ft.
> The effective area of the 4 inch round boom ends up being  13.332 x 1.2  =  15.998 sq ft
> 20  / 15.998    =  25%  more effective surface area for the 3 inch sq boom.
>
> ## Bending moment of a cylinder or tube is just section modulus X yield strength.   This is how they
> calculate  the max stress where your mast enters the top of the tower, or at any point along the length
> of a yagi ele.   Of course, with greater effective surface areas and or longer eles or a longer tower mast, the
> stress increases fast like.
> IF the same format Is used for  square  tubing, then the round boom reigns supreme..hands down.  Round boom
> has a bigger section modulus... by 9.6 %.   1.45 / 1.323 = 1.09599  = 9.6%     The sq boom has 25 %  more effective surface area.
>
> ##  mounting els  to a round or sq boom is a no brainer, several methods will work on each type.   Thats a moot point.
> If Im correct, I see no advantage to the square boom.   Its a helluva lot weaker vs a round boom, assuming the circumference
> and yield strength is identical in both cases.....and both booms are the  same length.
Although it might be easier to compress beyond yield stress a round boom 
with a u-bolt then a square boom with a plate and two bolts. Seen it 
done.  U-bolts are sloppy fits and place all the boom stress at two 
points - flat boom to element plate and crest of the u-bolt. Why DX Eng 
saddles or custom made bored split saddles are much superior ways to 
clamp to booms.

4" x .125" wall tube intuitively to me feels a bit fragile re damage 
from clamping or handling. Internally sleeving the center of a 3" boom 
seems to me a better approach than increasing the diameter. Internal 
sleeving works (ie add up the modulus) for round or square tube per 
Leeson's analysis.  One drawback for 3" and larger is the lack of 0.120" 
wall, at least I've never found it.  So the joiners for long 3" booms 
need to be machined to slip fit, eg 2.750 x .250 wall turned to 
something smaller od.  btw I have a few to spare of such joiners.

Square booms are a winner for vhf/uhf.  Drill holes thru and insulate 
elements or not with plastic bushings and a threaded hole on top or 
bottom to secure the element with a screw.

Grant KZ1W
>
> Jim  VE7RF
>
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