I'm going to row against the rivet tide here a bit. When I bought a
well used F12 EF180C to retrofit with a Tornado variable inductor, I
found a significant number of rivets loose or missing entirely.
Prior to that I built a Dave Leeson W6NL designed 40m Moxon "110 mph"
from scratch (there are also two other versions that are reworks of
stock antennas).
In building the Moxon I learned the advantage of cross bolting the
element joints. There are double and triple wall joints in his design.
What happens is that as one bolt is tightened the tubing flattens a bit
and increases the tension in the other bolt, and that can be measured
with a torque wrench. That slight flattening also insures total surface
contact. So, for the EF180 I cross bolted all joints. Before that I
did a Yagi-Mech analysis which showed overstress at 70mph, so I sleeved
the EF180C appropriately for about 90mph survival. (86' tip to tip).
If you consider the geometry of fasteners holding stuff on shafts, it is
well known that two at 90 degrees force concentric cylinders together so
there is no chance of movement due to the id to od clearance. Also,
that is what what Leeson recommends now for element joint fasteners. I
purchased 8 big yagis for a new tower, 7 used, one new. All have cross
bolted joints and the 7 used ones came apart, moved 150 miles, and went
back together perfectly. That's another benefit of bolts when you want
to modify or move an antenna.
I suspect that pop rivets, unless there are a lot of them, can't
generate the same force as 1/4 and 5/16 bolts. The yield stress for pop
rivet aluminum is about 30% of that of standard 18-8 grade stainless
fasteners. And a pop rivet is hollow, has less material in shear, and
easier to deform. Another concern is how they might yield/wear in
sequence over time, i.e. an end one loosens then the next etc as any
single rivet can't hold the total load, which is highly cyclic in an
element joint. Just like a zipper. In any case, if pop rivets are your
preferred fastener, put in two rows at 90 degrees apart.
73,
Grant KZ1W
On 3/8/2016 21:09 PM, Bert Almemo wrote:
Jim,
While I agree with most of you're writing I think you'll be hard pressed to
get a solid 9 square inches of surface contact in your 1 inch tube with 3
inches of overlap. Maybe if you put in a lot of SS screws or rivets at the
joint. If you're using any kind of joint compound, like Penetrox, you need a
certain pressure to make a good contact, as I'm sure you know. SS hose clamp
+ SS screws has been a good combination for me.
Bert VE3NR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Thomson
Sent: Tuesday, March 08, 2016 10:46 PM
To: towertalk@contesting.com
Subject: [TowerTalk] F12 C19XR Rivets
Date: Tue, 8 Mar 2016 15:22:32 -0500
From: "Bert Almemo" <balmemo@sympatico.ca>
To: "'Roger \(K8RI\) on TT'" <K8RI-on-TowerTalk@tm.net>,
<towertalk@contesting.com>
Subject: Re: [TowerTalk] F12 C19XR Rivets
I learned long time ago that using pop rivets, blind or not, don't make for
a good electrical connection. For all antennas, using aluminum tubing, I've
used self tapping screws for the last 30 yrs and it's served me well. It's
also much easier to take antennas apart and put them together again using
screws. As always, OMHO!
73 Bert, VE3NR
## The electrical connection is between the mating aluminum tube surfaces,
not the rivets/
self tapping screws. A 1 inch OD al tube, with say a 3 inch overlap, will
have 9 square
inches of surface contact. Unless the self tapping screw is SS, it will
rust. If it is SS, then
you have the dissimilar metal issue. Mosely etc, uses SS self tapping
screws, and they work
just fine. Heck, I used a single slot + SS hose clamps for several years
on my hb yagis...and
they worked just fine, with 100% of the RF flowing through the mating
surfaces. Hy-gain, wilson
etc, used those compression clamp assys....which I just hate with a passion.
Local fellow here had a
hy-gain 204BA up 100 ft near a cliff edge, and after a few years, some of
the tips and inner sections
were just about ready to pop right out. They had migrated outboard quite a
bit. Same fellow also
had the old KLM 4 el 40m yagi......and it self destructed. It was new at
the time. He ended up
buying a bag of SS self tapping machine screws, and drove em in everywhere.
Problem solved
with the ele migration problem. Next problem was the KLM 40 eles came right
off the boom.
The 'fix' for that was to use multiple SS hose clamps to hold the lexan
brackets to the boom.
Then drive SS self tapping screws through the slots on the SS hose clamps.
## I have never seen SS self tapping machine screws ever loosen up, they
seem to hold stuff
together quite well, just don't crank em in too tight, into al tubing,
such that they strip out.
Jim VE7RF
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