On 3/11/2017 15:12 PM, Jim Thomson wrote:
Date: Sat, 11 Mar 2017 09:47:50 -0800
From: Grant Saviers <grants2@pacbell.net>
To: Jim Thomson <jim.thom@telus.net>, towertalk@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@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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