[TowerTalk] More input

[Bill Coleman]

On 3/29/01 3:47 AM, Dave at xrdave@optonline.net wrote:

>Any suggestions on fixes/mods I can perform to ensure that this beast
>doesn't blow apart once in place would be greatly appreciated. I will go the
>rope route for sure, and will also beef up the rivets with sheet metal
>screws.  Any others ??

I'm a teeny bit unclear on how our F12 product unassembled itself. Did 
the elements come out whole, or were they cracked where they were 
riveted, or did the elements fail at some point other than the riveting?

If the joints are coming apart, perhaps adopting some aircraft practice 
might be prudent.

I wouldn't recommend sheet metal screws instead of / in addition to 
rivets. Properly installed rivets should hold much, much longer than 
sheet metal screws.

If you use the existing pre-drilled holes, they should be sufficiently 
deburred. However, the common practice of drilling a hole, inserting a 
rivet and pulling it leaves the rivet prone to failure. Tiny bits of 
metal can exist where the rivet is holding, and this bit or burr presents 
a stress point. Over time, vibration can cause the aluminum to crack at 
this stress point, destroying the strength of the joint.

After drilling, the parts should be separated and all burrs removed. Then 
the parts can be re-assembled and riveted. 

If only one rivet is used to hold a joint, it represents a single point 
of failure. The movement of the joint by flexing may cause the fastener 
to work loose. The solution is to use several rivets around the perimeter 
of the element. These may be offset to avoid one running into another. 
Two, three or four should be sufficent to join concentric tubing, 
depending on the diameter.

You can also double the number of rivets, offseting them by an inch or 
so. All this prevents the tubing elements from moving relative to each 
other.

Don't use the garden variety hardware store rivets. These are just a tiny 
soft aluminum tube. They are fine for holding drop ceiling tracks in 
place, but aren't structural. What you want are real structural blind 
rivets. (These are the kind Force12 uses) These are designed to break off 
a piece of the mandrel to leave a solid metal shaft in the center. 

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901