[TowerTalk] Force12 rivets

[Grant Saviers]

If you get on an airplane, you are trusting your life to rivets, some 
are pop rivets.

Here is a manufacturer's website  which covers much of the 
engineering.   http://www.cherryaerospace.com/

There are a number of issues with screw and hose clamp fastening systems:

1. As previously posted, screw threads tend to 'worry out holes'.
2. As widely noted, hose clamps are not so great - they loosen, in small 
diameters can't be tightened much, increase wind resistance, and create 
a dissimilar metal galvanic cell.
3. Not mentioned is the difficulty of drilling holes thru round tubes - 
a big problem with screws and rivets
      a. getting them centered and straight through two tubes (three if 
sleeved for strength)
     b. getting a round hole in thin material
     c. getting a hole that is on diameter - obviously the larger the 
gap between fastener and hole the worse the wear
4. Screws work on clamping force generated friction between material 
layers as well as plain shear strength, so how does that work when the 
tubes can be compressed?
5. SS screws also create a galvanic cell.
6. Sheet metal screws have so few threads engaged in a thin material so 
are very poor for structural strength.

Aircraft rivet fastening processes use a pilot hole of a smaller than 
rivet outside diameter followed by piloted core drill which acts much 
like a reamer.  This provides a hole that is really round and very close 
to a diameter tolerance of less than a couple of 'thou.  You can't do 
that with a standard twist drill.  The rivet then expands to fill the 
hole, (minimum 'worry") brings the layers being fastened in tight 
contact (increased friction) , and provides clamping force that doesn't 
rely on compressing a tube (constant over time).  Either the rivet is 
solid and bucked from the reverse side so its full diameter provides 
shear strength or a "pop" rivet  is used that provides the body shear 
strength of the Al plus the retained pull core which can be of several 
different materials (steel, Al, SS, Monel).

Why 3 rivets? Redundancy is good. Spreading the load is good. The tubing 
has clearance so it telescopes and tightening it to ONE side is good.  
The first rivet hole, if drilled and riveted then guarantees the inner 
tube(s) won't move when drilling holes 2 & 3.

What is the downside of rivets? - care and the right drill are needed to 
get holes on size and round.  Home-Depot rivets are not a good choice.  
A tool is needed to install them properly.

So are rivets a process  "do not try this at home"?  - NOT.  While 
aircraft piloted core drills are often available on ebay, DeWalt makes a 
drill which works extremely well - pilot point drills.  (formerly Black 
and Decker Bullet Drills) They produce holes in thin materials that are 
round and very close to drill diameter using a drill press, and if care 
is used in a hand drill.  Many suitable structural grade pop rivets can 
be set using a quality hand tool.

My scratch build 40m Moxon is going together with rivets.

Grant  KZ1W

On 2/19/2011 10:36 AM, Chet wrote:
> Yesssssssss!
>
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Gene Fuller
> Sent: Saturday, February 19, 2011 12:52 PM
> To: Dick Green WC1M; towertalk at contesting.com
> Subject: Re: [TowerTalk] Force12 rivets
>
>
> Assuming one rivet is strong enough to do the job the fact that they use
> three would indicate to me that not even the manufacturer doesn't trusts
> them. Perhaps a second for backup, but three.....? I would guess that
> cheaper is the answer. Lets get on with it - past time to cut the thread.
>
> ----- Original Message -----
> From: "Dick Green WC1M"<wc1m73 at gmail.com>
> To:<towertalk at contesting.com>
> Sent: Saturday, February 19, 2011 12:07 PM
> Subject: Re: [TowerTalk] Force12 rivets
>
> snip