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Pulled rivet question

Mich48041

Well Known Member
Is a pulled rivet without the center stem just as strong as a rivet with the broken stem still in place? The reason I ask is because I punched a few of the stems out after they broke off in the wrong spot, about 3/8 of an inch outside of the rivet. You see, I made a wedge shaped tool to pull the stem at an angle because of obstructions to pulling straight. The tool angle was too big and that caused the stem to break next to the puller instead of next to the rivet's factory head. I did 6 rivets this way before making a new wedge. I did not like the looks of those stems sticking out, so I tapped them back into the rivet and then used a pin punch to drive them all of the way out the back. Now I am wondering about the strength of the rivets. Are they OK or should I drill them out and replace them?
Joe
 
In many cases, the mandrel is providing the vast majority of the shear strength of the rivet, but probably contributes to the tensile strength as well. This is almost certainly true with Cherry Max rivets that have steel mandrels.

If they break off proud of the rivet, just get your dremel out and grind it down. You should probably grind the nose of your puller down a little too to allow it to pull straight. The only place on my RV where this was necessary was with the tank z-brackets.

I'm no engineer, so take this for what it's worth, but I'd check with Van's to see if you need to drill those out and re-set them.

Good luck.
 
Joe, can you see the back side of the rivets? If they expanded and look OK then I'd leave them in. Drilling out and resetting them might leave you with a more compromised situation than just leaving them alone.
 
I could be wrong but I suspect the remaining broken off shank of a pulled rivet probably contributes little to the actual strength of the joint. Often the remaining shank falls out by itself or vibrates out during use so I doubt if the manufactures or structural design engineers consider the broken off shank when determining the loads a particular pulled rivet will support. I know some pulled rivets are designed to keep the broken off shank in place but I think that is pretty much to just seal the hole and does not contribute to the strength of the rivet. JMHO

Would be interesting to hear from an engineer regarding this.
 
Joe,

It depends on the rivet. I'm not sure which rivets are in the 12 kit, but I've built a Sonex with Cherry N rivets. Cherry N rivets do not rely on the mandrel for strength. A 1/8" stainless steel Cherry N rivet is actually stronger than a #4 rivet. A Cherry Q rivet looks similar to a Cherry N rivet, but maintains more of the mandrel to increase the strength. A CherryMax rivet has a collar that locks the full length mandrel in place and uses it to increase the strength even further. CherryMax rivets are expensive, so I'm guessing the 12 doesn't use those.

As for the mandrel sticking up. If they are the "N" rivets, I would just tap them down with a hammer until they are flush, but don't tap them all the way out. There are a bunch of rivets on my Sonex where the mandrel, at least what's left of it, is loose.

Oh, almost forgot. If you're stem breaks off that high, I would look at the back side like some others have mentioned. If it looks like it is set properly, I would grind of the stem flush with a dremel. 3/8" is way too high. The ones that I had to tap back in were only about 1/16" high.
 
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Joe,

It depends on the rivet. I'm not sure which rivets are in the 12 kit, but I've built a Sonex with Cherry N rivets.

The RV-12 mostly uses Vans part numbers LP4-3 and LP4-4 rivets.
These are actually rivets that are made by Gesipa. They are Polygrip aluminum rivets with a steel mandrel. The Gesipa part numbers are:
GSMD41-43APG and GSMD41-44APG. These rivets have a shear strength of 162 and a tensile strength of 236. I think that they do rely on the mandrel for their strength.

The nice thing about these rivets is the wide grip range. The 43 has a grip range from .039" to .197". That leads to a much smaller requirement for various rivet sizes, and simplifies kits.

Hope this helps.

Tom
 
I would be surprised if the mandrel is considered in the shear strength calculations. The steel mandrel is designed to break in a specific spot, always below the top of the head. With such a wide grip range, it would be impossible for the designer to ensure that the mandrel adequately spans the joint between the two metals, thus contributing to shear strength. This is one reason why "structural" pulled rivets have such a small grip range, and their strength calculation does include the mandrel. Other designs using similar pulled rivets do not consider the mandrel when calculating shear strength.
 
Old thread but I had an problem with some LP4-3 rivets were the mandrel fell out after riveting. Van's (Ken Scott) adviced me to replace the rivets were the stem fell out.
Tomorrow I put a light in the VS and the stabilator and check were the mandrells are missing.
 
Commercial Rivets / Cherrymax.

If it is a Cherry max it only develops full strength if it is in the correct material, the right size hole, and the correct grip length.

To answer your question about the stem, it must be in place and be broken off at the correct location.

That said, the aluminum Cherry max start at 1/8th inch dia and develop 55 ksi shear when all is proper. That's more than a solid rivet and larger than the -3's that are so popular in the RV's.

Its up to you, and what you will accept, the whole RV thing is experimental, but thankfully this crowd is looking for good results!
 
I have always wondered. Why is it necessary for a steel mandrel in an aluminum rivet. I always assumed it was part of the strength issue but many of you say no.
 
More info

I have always wondered. Why is it necessary for a steel mandrel in an aluminum rivet. I always assumed it was part of the strength issue but many of you say no.

The steel in a cherry max is also necessary to pull the rivet bulb on the tail end, it takes quite a bit of force to properly and consistently form an acceptable tail:cool:
 
Avex designed to keep stem in

According to Chris Heintz (Zenith and Zodiac, et), who specifies Avex pulled rivets, the stem being retained is a significant part of the strength. The other thing that makes the Avex design better is that it expands to fill the hole unlike hardware store pop rivets which just expand on the outside of the material.
 
According to Chris Heintz (Zenith and Zodiac, et), who specifies Avex pulled rivets, the stem being retained is a significant part of the strength. The other thing that makes the Avex design better is that it expands to fill the hole unlike hardware store pop rivets which just expand on the outside of the material.

Apparently there are three types of blind rivets: 1) hollow (non-structural) 2) self-plugging/friction-lock rivets and 3) mechanical locked-stem self-plugging rivets

The hollow rivets are primarily non-structural. The friction lock rivets are prone to stem loss which "causes the shear strength of the rivet to be weakened" such that FAA AC 43.13-1B states that the diameter of a friction lock rivet must be increased one size when it replaces a solid-shank rivet. The mechanical locked rivets "in almost all cases, they can be substituted rivet (mechanical locked) for rivet (solid)". The Gesipa poly grip rivets have a mechanical locked mandrel.

"GESIPA PolyGrip rivets feature a wide grip range, enabling a single PolyGrip to replace up to three different lengths of standard blind rivets. PolyGrip rivets expand radially, filling the application hole, resulting in tighter joints and improved sealing. Add to this PolyGrip's locked mandrel core and you have a weather tight rattle resistant fastener. Improved material support is provided by the PolyGrip's larger blind side head formation" http://www.gesipausa.com/polygrip_stainless.htm

So I conclude that the stem loss is detrimental to shear strength, but due to the reliability of mechanical locks, mechanical lock rivets can often be substituted for solid rivets of equal size.

Edit: The RV12 uses the GSMD which is an aluminum rivet with a steel (not stainless steel) mandrel.

Jeff
 
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GESIPA and Avex and Heintz article

polygrip.jpg

This is the GESIPA rivet mentioned above.

It looks like an Avex to my uncritical eye. There may be differences, but the Michelin-man look is the same as is the way it retains the mandrel.

Here is a link to an article by Chris Heintz - be sure to look at the last paragraph.
http://exp-aircraft.com/library/heintz/riveted.html
 
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Pulled rivets without mandrels

This is an old thread, but since it comes up in searches I thought I should correct an error. Heintz at Zenith uses the Avdel rivets, and measured and uses their shear strength with the mandrel removed, i.e. worst case. See http://www.zenithair.com/images/kit-data/ht-87-1.html They are reliably strong, but obviously not as strong as retained-stem rivets, that's why there are 14,000 of them in the airplane...
 
This is an old thread but in conversations with a Textron Cherrymax rep I was told the mandrel in a CR3213 rivet accounts for 70% of the shear strength of the rivet. If the mandrel is not properly in place (See the Cherrymax Process Manual) the rivet strength is seriously compromised. Also, the aluminum Cherrymax rivets have a thermal limit of 250F.
 
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