Rivets - simple misunderstanding or misled by liars?

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rdj

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So, most of the solid rivets we use are AN426 or AN470. Here's one carried by Aircraft Spruce: AN470AD-4-2, specified as 1/8" diameter x 1/8" length which seems to match the AN decoder ring.

Now, pretty much every experimental aircraft site I visit states that the 'exposed tail' of the rivet (ie the amount sticking out the backside of two sheets of aluminum) should be 1.5 times the rivet diameter, which my calculator tells me is 0.188" for a 1/8" diameter rivet. My calculator also tells me that getting 0.188" of 'exposed tail' from a rivet only 0.125" long requires magical aluminum sheets with a thickness less than zero (-0.063" to be exact). Even more trustworthy sources than the EAA and HomebuiltHELP such as Hanson Rivet specify a 'clinch allowance' (which seems to be the same as 'exposed tail') of 0.125". That at least drops the magical sheets down to zero width sheets.

Am I misunderstanding something (probably), or is this 1.5X number just a bald-faced lie pulled out of thin air, and the truth is far more complicated? Or is Aircraft Spruce stocking rivets that no one can ever possibly use?

Bob
 

Victor Bravo

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I believe the -2 at the end of the rivet nomenclature might mean 2 8ths, not 2 16ths.

So if this is true, the rivet would be 1/4 inch long, not 1/8 inch.

edit: Further reading indicates the length is actually in sixteenths, which apparently means that hostile aliens are indeed invading the Earth and altering our sheet metal textbooks.
 
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wsimpso1

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First off, the 1.5x diameter is a "rule of thumb" that will almost always make full strength when the hole and panels and prep and technique are done to the usual standards. We also have edge margins and hole spacing, etc, that we know work without having to do full-up analyses on every rivet and joint. That can make design and fabrication easy most of the time.

Then there are situations where the standard way just won't work or full strength isn't needed. Those situations then call for unusual approaches, parts, processes, full-up analysis and testing, etc.

Apparently a short rivet was needed often enough that they are produced.

One easy reason I can come up with is this hole was put in for some other reason than attaching panels, maybe to locate some other features or to stop drill a crack or even by error. Then the hole needed to be closed to prevent further crack propagation or keep the weather out or whatever... so a rivet is set in the hole, but very little shop head is needed. Or two pieces are to be attached with very little load present and clearances are tight, so a reduced shop head is used, analyzed, and tested to make a bigger assembly work. Another possibility is the shop head is being formed in a countersinked hole and so the tail must have tiny volume to end up flush or below flush. I am positive that human endeavors can come up with more reasons...

In any event, the unusual rivet was found to have common use, and the manufacturer makes parts to fill the need. No extraterrestrials need participate.

Billski
 

Turd Ferguson

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So, most of the solid rivets we use are AN426 or AN470. Here's one carried by Aircraft Spruce: AN470AD-4-2, specified as 1/8" diameter x 1/8" length which seems to match the AN decoder ring.air, and the truth is far more complicated? Or is Aircraft Spruce stocking rivets that no one can ever possibly use?
The solution is easy, just don't buy those rivets as they will be too short.

The rivet should fit the work, instead of making the work fit the rivet.
 

BBerson

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Victor Bravo

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Then there are situations where the standard way just won't work or full strength isn't needed. Those situations then call for unusual approaches, parts, processes, full-up analysis and testing, etc.

S**T... I had just come up with a really good story about deep-cover WW2 Axis saboteur spies masquerading as Rosie the Riveter, insidiously weakening American Air Superiority through catastrophically reduced load bearing capability of our B-10 and B-18 Bolo bombers... right out of a comic book.
 

rdj

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Shhh... they are for the super secret invisible aitplanes!

That's my belief too...they're obviously for building stealth airplanes.

It seems Billski has the pulse of the rest of the experts on here (disregarding the posts of us UFO conspiracy theorists): the 1.5 'rule of thumb' is really mostly hogwash, and proper analysis and testing needs to be done. And, it seems no one really has a clue *exactly* what they're used for.

The reason this came up is I was writing some verbiage for my maintenance manual on rivets, and decided to work backwards: given the rule of thumb, what summed assembly lengths would be needed to meet that rule for all of the hard (ie 2117 'AD') solid rivets sold by Aircraft Spruce? This is what I came up with:
SolidRivetsAS.png

Many of the solid rivets can't possibly meet the 'rule of thumb' in the shorter lengths. At the other end of the length scale I can't imagine what assemblies would require 1/8" diameter rivets holding together two aluminum parts a total of 1.187" thick. Yet, you can buy them from Aircraft Spruce. Curious.

Bob
 

cvairwerks

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Bob: We used some 3-16 and longer rivets in nutplate installations for IC/IR panel installations, where engineering called for single grip length fasteners at all locations, but structure thicknesses varied. We used radius blocks and up to three shims in a stack to make the required stackups. Here's an example, for the F-16 engine bathtub cover. You can see the varying stacks under the nutplates that run across the bulkhead and the attach angles. Click on the photo and it will enlarge a bit more.

Remember that for the AN426 spec, the length included head height, whereas the AN470 spec does not.
 

wsimpso1

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Many of the solid rivets can't possibly meet the 'rule of thumb' in the shorter lengths. At the other end of the length scale I can't imagine what assemblies would require 1/8" diameter rivets holding together two aluminum parts a total of 1.187" thick. Yet, you can buy them from Aircraft Spruce.

Hmm, I can not only imagine use for long rivets in a homebuilt, I have done it. The attached photo is a dry assembly (before alodine). This is one of two aileron bellcranks with an offset between the two pushrods to allow them to cross each other. I think they are they are 3/4" thick at hub and I riveted them with solid rivets.

I actually had a more than 1.5 diameter showing. Setting the rivet:
  1. The shaft of the rivet shortens and grows in diameter until the hole is filled;
  2. Form the shop head;
  3. Compress the assembled members between the rivet heads, so that the joint ends up with the rivet in tension and members in compression. This plus fully filled holes keeps the joint from shifting under load.
With a long drilled hole, rivets with 1.5 diameters exposed barely had any material left to form a head. I was more like 2.0 diameters before I liked the shop heads I got. No the rivets were not a loose fit - most needed a tap or three with a mallet to get them home before setting them.


1652907503705.png

As for use of really short rivets, I have done an approved repair on a 0.040" thick web with holes in it that were not supposed to be there. DER gave us a doc telling us to drill specified size holes and set specific rivets in the holes. Our mechanic's service has had similar repairs, so it is more common than you think.

As to engineering of riveted or bolted joints, this is a long ways from my first rodeo. Some engineer sometimes has to get fancy with the analysis to verify squeezing enough fastener into a space will live, then testing it to prove it. No mind reading required - been there, done it, and the food was better on the Yellowstone trip.

Billski
 

agpilot24

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When I opened my maintenance shop in 1999, one of my first jobs was to replace corroded rivets on a 172 aileron balance weight. I ordered a 1/4 lb bag of an470-4-20 rivets. Even after replacing several more balance weights since then, I still have most of the bag left. The longest ones are used, but for me a quarter pounder has been a lifetime supply.
 

geraldmorrissey

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If AS is stocking a 4-2, then they must be stocking propwash by the gallon and flightline by the yard.
 
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Angusnofangus

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As mentioned earlier, about the only thing a 4-2 rivet is good for is to maybe plug a hole. It would have to be a very thin piece of material. In a long career as a rivet-smasher I doubt that I ever used even one, despite the fact that my fat fingers would have a hard time even picking one up and getting it in a hole.
 
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