[TowerTalk] Al corrosion/connector seals

[force12@interserv.com]

Noticed this discussion about "pop" rivets and thought I would re-send the 
write-up I did a little while ago on the subject. If you already got it, 
just hit the "DELETE" key!

Thanks..........


Have a good day and 73,
			Tom, N6BT
			Force 12 Antennas and Systems
			(Home Page   http://www.QTH.com/force12 )

----------------------------
RIVET DISCUSSION FOLLOWS
----------------------------
Regarding aluminum "Pop" rivets:

Would like to share a little information, as maybe we have a fair amount of 
experience in this area. One of Force 12's unique product characteristics is 
that our elements are riveted (with the exception of larger diameter, 
thick-wall sections where rivets would not be appropriate). The pre-aligned 
element-to-boom brackets are also riveted to the boom (except for the large 
weldments used on 80 mtr Yagis). We have used several hundred thousand 
rivets in the last 6 years.

A quick word about the term, "Pop rivets" first...
The name "Pop" is actually a registered trademark of the POP Rivet Company. 
The generic term for a rivet that has no back washer is "blind rivet". A 
blind rivet is inserted and handled from one side only on the assembly; 
therefore, it is a different construction than a regular rivet. On the boxes 
of POP Brand Rivets, they are actually identified as "Blind Rivets." The 
term "pop" is so common, that we will probably all continue to use it (just 
like "Coke" - sorry, Pepsi!!).

Rivets of this type come in several varieties. The variables are:
	shaft diameter (i.e. 1/8". 3/16")
	head diameter and type
	rivet material (use aluminum for aluminum tubing!)
	mandrel material and type (different pulling characteristics)
	"regular" or "closed end" type construction

The most common variety of blind rivet is found at the local hardware store. 
These are a "regular" type construction. This means that the mandrel (the 
shaft that is pulled by the tool) goes through the rivet body and is 
attached to a ball at the far end of the rivet body. This is the end 
inserted into the hole. When the tool is used, the mandrel is pulled 
outward, pulling the ball along with it. This ball will mash the part of the 
rivet body that is inside the hole (inside the tubing) up against the inside 
wall of the tubing. It will continue to do so until the pulling force is 
sufficient to dislodge the mandrel from the rivet body. Different mandrel 
designs will pull the ball into the body and leave the ball filling the 
hole; another is a pull-through, leaving a clean hole; another will leave a 
portion inside the body to increase shear strength; another will flare the 
material; another will fully core and spread out a lage portion of the inner 
surface. The ones usually found are the type that separate from the ball and 
leave the ball mashed up inside the rivet body. Sometimes, the ball 
will fall out and rattle around inside an element, so be sure you tilt an 
element to clean it before putting it up! Please note that the ball material 
can be important. If it remains inside the rivet, it is most likely 
participating in the joint. For elements, if the ball falls out, or rusts 
away, the joint is probably still intact, as the material has been seated 
firmly into the hole during assembly.

Another type of blind rivet is called a "closed end" rivet. This is what 
Force 12 uses. They are designed specifically for connections where 
vibration is a primary concern.

The closed end rivet has a hole only going part way into the body. The 
mandrel is attached to the outside (top - towards the rivet head) of this 
material. After the rivet is inserted into the tubing, the mandrel is 
pulled. As the pressure increases, the material completely fills the hole, 
plus pulling tight against the inside of the tubing. When sufficient force 
has been applied, the mandrel pulls apart from the body. This leaves a solid 
"hole" and the mandrel is completely removed from the body. The connection 
has high shear strength and might even be watertight.

The mandrels we use on the closed end rivets are steel. Rivets also come 
with aluminum mandrels; however, the aluminum mandrel will separate from the 
body with less force, so the steel mandrel provides a more secure 
connection. None of the steel mandrel remains inside the rivet.

The body length of the rivet relates to it's "grip range", or the thickness 
of material it is designed to hold securely. The grip range of rivets is 
shown on the package, so be sure to get the right one. A large head is not 
desired for tubing, as the head will extend beyond the curvature of the 
tubing, so select one that is probably not the largest in the selection.

The diameter of the rivet body is important, as it also relates to the 
strength of the connection. For most elements, where shear strength is not a 
primary concern, 1/8" rivets are excellent. When we do inner liners, they 
are a combination of 1/8" and 3/16", depending on the application. The 
riveted element-to-boom brackets are secured with 3/16" rivets (with steel 
mandrels). One should use a little caution in selecting 3/16" rivets, as the 
pulling force is substantially higher than a 1/8". Unless you have very 
strong hands (using a hand blind rivet tool), stick with 1/8".

Our production lines do not use hand riveting tools, although I do all the 
time. The production tools are pneumatic and also pneumatic-hydraulic (new 
marketing term -->pneudraulic). The pneumatic/hydraulic tools are used for 
3/16" rivets. The production tools are also equipped with special nose 
pieces. We use POP Brand production tools and they are exceptional. All we 
do is keep them oiled and replace the internal springs and jaws as they wear 
out.

Hand tools have quite a range. For price, the Stanley MR-99 is my personal 
favorite to recommend to folks. The head can be changed from a right angle 
to in-line. Otherwise, the one I use is called a "Brute" by Creative 
Engineering, Inc. in Taunton, MA. There is a construction version that I 
also use, because it is grey and yellow and looks good at shows! On the 
this type riveter, there is a coil spring that is normally inserted between 
the handles to keep the tool open. However.....I remove the spring. This is 
so that I can insert a rivet into the nose and it will stay there (open the 
handles a little, insert rivet, and the handles close back together with a 
little force on the mandrel). With the spring in place, the rivet will fall 
out.

Removing an aluminum rivet is simple. The normal hole for an 1/8" rivet is 
slightly larger than 1/8", as this allows for a small amount of material to 
be pulled into the hole. To remove, use an 1/8" bit, seat it into the rivet 
head (where there is a hole already), run the drill slowly to remove the 
rivet head, then drill through the body, still running the drill fairly 
slowly. It is rare to have to run a drill very fast. How many times can you 
do this? At a convention, I once used the same hole all weekend for all the 
rivet demos. The better quesiton is how many times are you going to 
disassemble the antenna??

For DXpeditions, a single rivet per element joint is sufficient and very 
fast to disassemble - much faster than screws or clamps. If you can't take 
along a drill, then we do make elements for stainless sheet metal screws. 
These are drilled differently. The holes are first drilled through both 
pieces, just like for the rivets; then, the sections are removed and the 
outer tubing is re-drilled larger, as a clearance hole for the screw. If 
this is not done, the screw will pull material from the inner tube into the 
space between the tubes, as well as into the outer tubing hole and the 
pieces will be very difficult to separate, even being unable to separate.

Hope this was useful.

Have a good day...................73, Tom, N6BT
				Force 12 Antennas and Systems



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