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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