[TowerTalk] Booms

[Jim Thomson]

Date: Sat, 11 Mar 2017 09:47:50 -0800
From: Grant Saviers <grants2 at pacbell.net>
To: Jim Thomson <jim.thom at telus.net>, towertalk at contesting.com
Subject: Re: [TowerTalk] Booms



On 3/11/2017 8:16 AM, Jim Thomson wrote:
> Date: Fri, 10 Mar 2017 21:12:43 -0600
> To: towertalk at contesting.com
> Subject: [TowerTalk] Booms


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.

###  Typ stauff clamps that are all the rage these days use  .25-20  SS bolts. 

Engineering Calculator agrees with your numbers below

## But  does ... bending moment =  yield strength  X section modulus....  still apply to
square tubing ??  


>
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. 

###   If its  tougher to compress a square boom in the vert plane,
it may well be also tougher to  bend square tubing in a high wind..
like a boom, broadside to the wind ?  Channel aluminum  is strong
stuff, and square tubing is sorta like 2 x  channels welded together.
Whats even worse is U bolts..and no saddles at all, fubar imo.  Typ
sheet metal saddles will rust, and are like parallel razor blades.
The solid DX eng cast al saddles are the ticket..that plus their
extra long .375 inch  SS  U bolts.   Their smaller SS U bolts are .3125
and also .25 



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.

##  m2  uses  4.5 inch OD tubing for the boom on their full sized 40m yagis.
4.5 inch OD in the center portion, then 4.0 inch OD for the ends of the booms. 
I used 4 inch  round boom  vs  3 inch square tubing  for my  maths example, since both have 
aprx the same circumference, no other reason.   4 inch x .125 has a huge section
modulus. It also has a lotta windload.   3 inch x  .120 wall is ample for most extreme cases.
Perhaps the addition of a short 2.75 x .120 wall internal sleeve, right where the ele to boom
assy resides, would provide for extra boom strength.  I see that  JK antennas does this on their
booms.   Then it has some meat to it.     Once the wall thickness gets to .240 or .250, then its
pointless to go any thicker for any boom, or mast.   At that point, its better to increase the OD. 
My chromolly 2.0 x .375 wall mast doesnt hold a candle to a 3.0 x .25 wall mast, the  section modulus
of the 3 x .25 mast is huge vs the smaller diam, but thicker mast... yet both weigh the same, and cost
the same. 

##  For joining  3 inch x .120 wall boom material, say in the exact center, where the
boom to mast plate is,  an internal sleeve of 2.75 x .120 wall, say 2- 3-6 ft long, that
straddles the center would suffice.  The same scheme could be used to splice 3 inch
x .120 wall  further out from the mast, perhaps with shorter  1-2 ft long internal 
2.75 x .120 wall sleeves. 

##  I see that Dx engineering sells a 48 ft heavy duty  3 inch od boom.  Consists  of
2 x 24 ft lengths of .120 wall.... with an internal sleeve splice consisting of a 24 ft length
of  2.715 OD  X  .132 wall.  https://www.dxengineering.com/parts/dxe-abt3-48

##  In any event, the .120 wall  6061-T8 that dx eng sells in every diam from 1.5 inch od...
up to 3.0 inch OD, in .25 inch OD increments, is the bomb imo.  Then it all just slides together,
unlike the .125 wall stuff, which requires that .005 be machined off in a lathe....pita.  At least
for the hb back yard folks like myself. 

Jim   VE7RF  

Grant KZ1W