Screws vs Rivets

[SamP]

I overdrilled a rivet hole and tech support suggested I use a screw instead of a larger sized rivet. I'm trying to understand why that would be the case. Wouldn't you want to replace the rivet with a larger rivet? While the screw would support higher shear loads, the screw would only be loaded when there was displacement because of the clearance in the hole. It seems to make more sense to replace with a larger rivet, or am I missing something?

 

[gtae07]

What aircraft model? An RV? I've heard of Vans tech support suggesting that before, and I suspect it's because obtaining and/or setting the appropriate larger rivet may be difficult. As you note, a screw or bolt won't fill the hole like a rivet, but especially if it's one hole out of a larger group it shouldn't matter much. Plus, though aerospace traditionally discounts the friction between layers that's generated by the clamping force (out of conservatism), it's still physically present and contributing under load.

There's also the expectation that screws and bolts are less likely to be screwed up on installation and require drilling-out or oversizing of a hole. If ypu go the larger rivet approach and booger it up, then you have to go even larger...

 

[wsimpso1]

Screws are not rivets. Both prevent the joint from working by establishing preload. Both prevent fastener fatigue by being under tension from that preload. Sure, the rivet also fully fills the hole to further prevent joint slip, but the screw and nut more than make up for it by being larger diameter, higher preload, and higher strength than the base size rivet.

Nothing to be concerned over. Bolted joints took over from rivets on structural steel many decades ago, and are used just fine in many things in airplanes, road vehicles, marine vehicles, and heavy equipment. The nice things about rivets is that they are cheap to buy, cheap to install, and you can know the joint is solid by visual inspection. If the rivet was a tight fit in the hole, the faying surfaces drew up tight, and the both heads pass a gage check, the joint is properly set and strong and can be confirmed as such also by inspection. To to the same thing with a much more expensive screw and bolt, you need a torque wrench and either detailed record keeping or Torque Seal and record keeping.

Billski

 

[N804RV]

There's a dozen AN509 screws on the lower landing gear tower weldments on each side of the RV-8, just above where the Main landing gear legs attach. I suspect they used the screws, in combination with a whole bunch of AD4 rivets because they were much easier for the homebuilder, and much lower cost than rivnuts, yet still met the engineering requirements.

 

[TFF]

Ideas off the top of the head.

Big rivets can be hard to drive so they might be trying to help not make a mess with it.
If the spacing was close, it might break those rules, and they don’t want to set a precedent of saying OK.
Might look less like a mistake and on purpose. If not in the US, regulatory.
The company flat out don’t think it’s safe without an upgraded fastener.

It is a homebuilt airplane, so it’s going to come down to your judgment.

 

[Angusnofangus]

The Structural Repair Manuals that I am familiar with (Convair, Boeing) have fastener substitution tables. You are allowed to substitute up in strength per the tables, but never down. There is no way that I could count how many times I've substituted screws (MS or NAS), bolts, and Hi-Loks for rivets. Usually because of accessibility issues.

 

[Mad MAC]

Where in the structure it is makes a difference, as does the fastener detail design. The replys above boarder on slightly casual.

The replacement bolt should be a close tolerance bolt ideally in a reamed hole.

 

[kent Ashton]

I overdrilled a rivet hole

What is "overdrilled"? A rivet in an oversize hole will "clinch", i.e., bend over while you're trying to rivet unless you are very careful with how you hold the bucking bar. Sometime you can spread the rivet with vicegrips and it will fit better in the hole. Or use a piece of metal with a short hole as an initial bucking bar. It holds the rivet straight initially, allows it to expand in the hole, then you finish with a normal bucking bar. Experiment. They do make oversize rivets with the same size head but I don't know the nomenclature.

 

[N804RV]

What is "overdrilled"? A rivet in an oversize hole will "clinch", i.e., bend over while you're trying to rivet unless you are very careful with how you hold the bucking bar. Sometime you can spread the rivet with vicegrips and it will fit better in the hole. Or use a piece of metal with a short hole as an initial bucking bar. It holds the rivet straight initially, allows it to expand in the hole, then you finish with a normal bucking bar. Experiment. They do make oversize rivets with the same size head but I don't know the nomenclature.

NAS1097AD4-* is the "oops" rivet for AN426AD3 (3/32 flush head). You can also fatten up a 3/32 by putting a size longer rivet in the jaw of your pneumatic squeezer and adjusting it till its just exactly snug on the rivet, then adding a quarter turn, squeezing, then another quarter turn. This'll work sometimes. But, it may take a couple of tries to get it right.

 

[Angusnofangus]

NAS1097AD4-* is the "oops" rivet for AN426AD3 (3/32 flush head). You can also fatten up a 3/32 by putting a size longer rivet in the jaw of your pneumatic squeezer and adjusting it till its just exactly snug on the rivet, then adding a quarter turn, squeezing, then another quarter turn. This'll work sometimes. But, it may take a couple of tries to get it right.

NAS 1097's are a reduced head flush rivet that come in all basic sizes, They are not designed as an 'oops' rivet, but can certainly fulfill that function. There are oversize rivets, I just can't remember the number right now. They are 1/32 larger than the nominal size. Boeing uses reduced head rivets extensively for attaching skins, BACR15CE if memory serves me right.

 

[Davefl42]

NAS1241 and NAS 1242 are oversize rivits we use in out shop, use regular dash number and AN470 and AN426.

 

[Angusnofangus]

NAS1241 and NAS 1242 are oversize rivits we use in out shop, use regular dash number and AN470 and AN426.

That's the ones, thanks.

 

[Tiger Tim]

Just don’t go too far with replacing rivets with screws or else you’ll end up with the Capelis XC-12. It was designed and built by a consortium of Greek restaurants and the entire airplane was held together with nuts, bolts, and sheet metal screws. Far as I know there wasn’t a rivet to be found anywhere on it, and the screws didn’t really want to be there either. More often than not it would land missing heaps of those screws from various places all over the airframe. I guess you could always double back and follow the trail of pieces home if you were lost...

If it’s looks familiar, that’s because a major movie studio bought it after it had been grounded and used it (and a model of it) in a number of movies through the 1940s.

 

[TFF]

Dreaming of Loctite were they.

 

[Doran Jaffas]

I had an experimental airplane that was a bolt together aluminum angle construction. 6061-T52. It passed inspection as it is an acceptable construction method. Granted, it is a much different aircraft but on a small 2 place it does work WHEN done right.

 

[wktaylor]

SamP... som'ore info might help...

What was the rivet 'supposed to be' [diameter/alloy/flush/protruding]… and how much over-size [OS] is the hole before OS drilling... and how many are OS???

 

[SamP]

Thanks all for the tips

The rivet was supposed to be standard 1/8" pull type, stainless steel, protruding head for two 0.032" 6061T6 sheets. I made a hole with a #18 accidentally. There is a 3/16" hole right next to the oversized hole, and I was stepping up from the pilot hole to the final when I drilled incorrectly. Oops!

Fortunately, it was the only oversized in a long row, about 40"

 

[rv7charlie]

#18: 0.1695", right? Can you preserve edge distances by going up to 3/16 in the rivet hole? If so, just buy the same type/grade pulled rivet in 3/16".

 

[SamP]

#18: 0.1695", right? Can you preserve edge distances by going up to 3/16 in the rivet hole? If so, just buy the same type/grade pulled rivet in 3/16".

So, that was my original thought as well, and it makes more sense to me than to replace with a 3/16 bolt or screw because the rivet will take load immediately, whereas the bolt will have to have some displacement of the sheets before it takes load (there has to be some clearance, otherwise the bolt won't fit). I want to do what the tech support suggested, since they may have better understanding than I do of what loads we have to worry about.

I did read somewhere that for rows of bolts, if one is larger, then that one will actually take more load. I'm assuming the same will hold true for rivets, that the larger rivet will take more load.

 

[rv7charlie]

I did read somewhere that for rows of bolts, if one is larger, then that one will actually take more load. I'm assuming the same will hold true for rivets, that the larger rivet will take more load.

I wouldn't treat that as a universal prohibition. For instance, the RV7 & RV8 wing top and bottom spar caps are attached to the center section with 2 ea 7/16" bolts, with a 1/4" bolt on either side of the 2 larger bolts: oOOo

If that location is in a straight run of rivets, and somewhere away from the end of the run, you could almost certainly leave it out without affecting wing safety. On the 'taking more load' issue, I seriously doubt that upsizing to a stainless rivet would take more load than using the same size bolt in the same location. ;-)

Charlie