[TowerTalk] F12 C19XR Rivets

Wed Mar 9 12:45:15 EST 2016

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 at contesting.com] On Behalf Of Jim
> Thomson
> Sent: Tuesday, March 08, 2016 10:46 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] F12 C19XR Rivets
>
> Date: Tue, 8 Mar 2016 15:22:32 -0500
> From: "Bert Almemo" <balmemo at sympatico.ca>
> To: "'Roger \(K8RI\) on TT'" <K8RI-on-TowerTalk at tm.net>,
> <towertalk at 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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