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Re: [TowerTalk] Shorty Forty Hose Clamp thread

To: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>, towertalk@contesting.com
Subject: Re: [TowerTalk] Shorty Forty Hose Clamp thread
From: Grant Saviers <grants2@pacbell.net>
Date: Sun, 09 Aug 2015 10:15:19 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Roger,

I think we agree that rivets need a hole "to be snug, clean, and burr free". Snug in my shop is in the sequence of tolerances of fits - (examples for a 3/16 or #10 diameter oversize of the hole, and how it is made, with pro grade tooling)

interference (dowel pins not to be removed), minus 0.0005 to 0.0015 clearance (undersize drill and reamer) line (+/- zero tolerance, pins can be pressed out, might need loctite, undersize drill, same size or slightly under reamer) snug/tight (hand fit but tight) +0.002/0.003 (oversize reamer, pilot point drill or step or core drill secondary operation) close (easy fit) +0.006 (larger drill, stub length split point or pilot point drill)
free (allowance for position errors) +0.012 (even larger drill)

Snug to me also means a round (low TIR) hole with square entry/exit. With a top grade 2 flute chisel point drill bit in a hand drill the holes I've done/seen in 0.058 wall tubing will be somewhat triangular, and the inside burr significant. The drill flutes don't engage the material before the drill breaks through. Fish tail or DeWalt pilot point (formerly Black&Decker Bullet) drills are far superior for thin material drilling but are hard to find in other than 64th sizes. These drills produce pretty close to snug/tight fits in thin materials.

I don't build aircraft parts but do have a collection of step and double margin drills used for aircraft rivet hole drilling. These are usually not in the average ham's tool box. Blind hole inside deburring tools are made, but fortunately for most antennas parts access to the interior burr is not too hard and a half round file cleans them up.

OTOH, enough 4/5/6(?) rivets you suggest may hold up in close/free or worse fit holes. I don't know how the antennas you rebuilt were drilled for rivets, the F12 EF180C (85' tip to tip) I rebuilt had missing/loose rivets and worn holes (2 or 3 rivets per joint).

Grant KZ1W


On 8/9/2015 0:59 AM, Roger (K8RI) on TT wrote:
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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