[TowerTalk] Booms

Sat Mar 11 20:59:12 EST 2017


On 3/11/2017 15:12 PM, Jim Thomson wrote:
> 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 ??
 >> Yes (but I'm not a structural engineer).  However the modulus is 
different on the diagonal, and for the case of 3" sq x 0.125 wall is 
0.94 in^3 tip to tip vs 1.32 in^3 thru the center in X or Y.
> 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 ?

 >>  My concern is only applicable to the point load from a u-bolt on a 
round tube.  A thick flat plate clamping a square boom is distributing 
the clamping force over more tube.
>   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
 >> Standard channel webs are too thin for many applications. Either get 
the thick section version or back them up with a second plate.  Only 
works if there enough thru bolts to make them act as a single member.
>
>
>
> 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.
 >> Internal sleeving is usually a winner.  Or adding a second set of 
boom guys as on my 48' x 3" boom 3L 40m.  Built by K7ZSD, I know it 
survived some tough wind & ice on an Oregon mountain top before I bought it.
> 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.
 >> agree.
>
> ##  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

Cool.  A new product I think, available the end of March.  A bit pricey 
since 3 x  0.125 wall 6061 tube here is about $6/ft.  Then shipping 24 
footers?  Plus the crate to prevent damage?  Ouch++. Strange, I can't 
make sense of their numbers.  Why 2.715" od joiners for 3 x .120" tube?  
That is 0.055" clearance, a lot IMO.   Smaller tubing slip fits are 
predicated on 0.010" clearance  with 0.120" wall.  My joiners are 
nominally 2.736" for 2.750" id or 0.014" clearance, perhaps a bit 
snug.   However, I think DXE buys from the mill so should know the 
tolerances and doesn't want anything shipped back.
>
> ##  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.
Machining 24" long joiners is not so hard.  A bull center and big enough 
lathe dog and a 12 x 36" garage shop lathe can do it. Obviously, DXE is 
having this done from probably 2.750 x something tube, but again the 
numbers don't quite add up since 0.132 wall at 2.715 od yields an id of 
2.451.  Metric or inch that doesn't make sense to me if the joiner 
starts as either 0.1875 or 0.250" wall.
>
> Jim   VE7RF
>
> Grant KZ1W
 >>  me again  Grant KZ1W
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