Having built aircraft, or parts of them with lots of "blind rivets", I
feel safe is using standard "blind rivets". The hole needs to be snug,
clean, and burr free. I would prefer 4 rivets per joint in sets of 3 at
120 degrees with the second set of 2 off set from the first set by 60
degrees. I'd use good quality, blind rivets, not necessarily aircraft
"Cherry rivets" But the Cherry rivets for homebuilts are not all that
expensive.
Just make sure the tubing is clean, inside and out. I've never had them
come loose.
I purchased a used Force 12 C19XR and WARC7. After they had been up
years, the joints were still good. The rivets were drilled out to
transport the antennas. Putting them back together was a quick cleaning
with fine scotchgard pads, a thin coat of Al noalox and pop rivet back
together.
73
Roger (K8RI)
On 8/9/2015 12:57 AM, Grant Saviers wrote:
I'll offer a contrarian view - I don't use hose clamps or rivets. One
exception is a pair of hose clamps when the tip of the element is
intended to be adjusted e.g. cw to ssb. Usually a 1/2" to 3/8" tube
joint.
Riveted antennas I have acquired and rebuilt had missing and loose
rivets. I drilled the remaining ones out and used 2 bolts/machine
screws per joint. Structurally, pop rivets squeeze a joint together,
they don't fill the holes with rivet material. Bridge rivets (hot
forged) and Boeing rivets do fill the hole and prevent motion
(airplane rivets are very precise fasteners, essentially each hole is
reamed to very tight tolerances and then the rivet is cold forged
closed). A pop rivet allows the tubes to move which eventually
loosens the rivet or shears it off and it falls out. I don't think a
pop rivet can achieve the force needed for a "slip critical" joint -
https://en.wikipedia.org/wiki/Slip-critical_joint
Hose clamps are for hose, not aluminum and the thread strength is poor
(why there is this thread!). There is no secondary resistance to
loosening - no lockwasher, nylock, etc.
I subscribe to the Dave Leeson ("Physical Design of Yagi Antennas")
bolt the elements together philosophy, particularly two cross bolts
since the bolts tighten against each other as the tubing goes a bit
oval from the force. Nylocks or K-L nuts help keep them from
loosening but cross bolted, they do get tight enough to stay tight. I
have yet to see one loosen when properly tightened and then
re-tightened when the connection stress relaxes over a few days. Cross
bolts also restrain the elements in two planes, just as a pair of
orthogonal set screws are best for stuff attached to shafts. For my
2- 3" center sections I use 5/16-18 and for small 1/2 to 3/8 diameter
joints 6-32 works fine, stepping down as needed as element diameters
decrease - 1/4-20. 10-32. 8-32.
I use hex head bolts and then socket head (allen) for smaller sizes
which are preferable to phillips, although one pro builder who built
some of my antennas managed to find hex head #10 and #8 machine
screws, but those are pretty rare. With Penetrox on the threads of SS
fasteners and on the overlapped tube sections, disassembly/reassembly
with threaded fasteners is simple. Just did that for seven large
yagis that had been up 7 years. Not a single fastener was loose or
missing.
Grant KZ1W
On 8/8/2015 20:19 PM, john@kk9a.com wrote:
Hose clamps come in different widths, materials and styles. I am not
sure
what MFJ/ Cushcraft uses or what failed for K6UJ, but on my homebrew
antennas I use worm drive hose clamps with a 9/16" wide band and a 300
series stainless steel screw. For 1 1/4 or larger tubing I use a
bolt style
hose clamp.
John KK9A
To: "<towertalk@contesting.com>" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Shorty Forty Hose Clamp thread
From: Robert Harmon <k6uj@pacbell.net>
Date: Fri, 7 Aug 2015 22:02:00 -0700
Doug,
I no longer use hose clamps after having them strip as you had happen
or the
connection
loosening after flexing back and forth in the wind (I am also in the
Pacific
NW)
Now I only use rivets. I have had a number of Force 12 antennas and no
problem
at all with the connections. Their riveted conns sold me. The HF
beams I
have
fabbed have riveted elements
and no problems. I wipe on Penetrox when assembling and later when
taking
apart
the tubing is like new. Plus to change element lengths it is super
easy to
drill
out the center of the rivets, they pop right out. Now I can sleep easy
while
the winds blow, hihi.
73,
Bob
K6UJ
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73
Roger (K8RI)
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