Rivets vs Bolts / Screws in Thin Walled Structure 1

[AeroNat12]

I'd like to talk through the factors involved in using rivets vs bolts/screws in thin walled structures.

The problem I'm working with today is a piece of prototype equipment, constructed from an aluminum box frame made of .060" square extrusion (taking bending loads) with .040" skins (taking shear loads), fastened together with blind rivets. This was provided to us by a contractor. I don't have full design/structural details. I am tasked to modify it to carry additional equipment, which I planned to do by riveting fittings onto the frame (remove existing rivets, add fitting & match drill existing holes, secure fitting and structure with new rivets). Some of my colleagues are pushing to fasten the fittings with bolts instead "in case" we ever want to remove the fittings.

I'm not sure if my (brief) aviation background combined with a lack of access to the structural analysis is making me overly conservative, or if my concerns about doing this are valid. This is not safety critical, but it would be nice not to scrap the prototype. I'd like to validate some of my assumptions about fastening thin walled structures with a more general discussion anyways.

General Case:
- For an equal number and diameter of rivets vs bolts, are there any cracking, fatigue, bearing, etc. considerations? Presumably rivets are better for all of these factors.
- Should knock down factors on bearing strength for a line of bolts vs a line of rivets? I'm thinking the bolt clearances would cause loading to be less evenly distributed than for rivets.

This design case:
- By using bolts, I'll be left with a combination of bolts and rivets. With the larger clearance in the bolted connections, the remaining rivets will take more load before the bolts are loaded up.
- I could envision cracks being likely to start at one of the bolted connections with higher stress concentrations.

 

[SWComposites]

Bolts and screws are NOT the same. Bolts have a solid shank with threads on the end. Screws have threads over full length. Do not use screws where the threads will bear against the straps. Use bolts. Bolts of the same diameter will have higher strength compared to rivers.

 

[drawoh]

SWComposites,

It would be nice if there were clear definition of bolts and screws. Unfortunately, that barn ran away from the horse a long time ago.

thread1103-224665

AeroNat12,

There are all sorts of rivets, and all sorts of threaded fasteners. Metal aircraft monocoque generally are riveted together, so there are rivets out there somewhere that meet your requirements. Threaded fasteners vary in strength, fastening tool, and locking mechanism.

Can you get to the opposite face clamped by your fastener? If so, you can use solid rivets, or you can use bolts and nuts. Otherwise, you are using pop rivets of some kind, or tapped holes and screws. Tapped holes in aluminium are not good practise in aircraft, even if the wall is thicker than[ ].060". You need thread inserts of some kind. Any bolt will easily crush the .060"[ ]walled tube. You need a design that allows you to torque your bolt down hard. Bolts, nuts and screws are easily removed. Rivets must be drilled out. You need to think about there the drilled metal chips will wind up [—] in your electronics?

--
JHG

 

[klaus.]

It would be nice if there were clear definition of bolts and screws. Unfortunately, that barn ran away from the horse a long time ago.

what is the difference between pants and trousers ?
or pipe and tube.....

 

[AeroNat12]

To be fair to SWComposites, I switched terms in my original post and didn't find all instances of screw and ended up with some fairly careless initial text (since edited).

drawoh,

Thanks, all good points that I'm considering. Likely going to end up with a rivet nut or something of that nature.

I suppose the question for me comes from always seeing rivets in shear joints in sheet metal aircraft construction. I know there are a million reasons for this - lightweight, more optimal strength balance between the sheet and fastener, mitigation of loosening and inspection concerns in threaded fasteners, etc.

I've also heard the odd grey-beard talk about the benefits of hole-filling fasteners, the flexibility of aluminum rivets helping balance loads, and never mixing bolts and rivets. I've developed this bias against other fasteners in shear applications in thin sheet that needs to be challenged. (Especially since I'm currently working with non-aircraft, non-critical one-offs).

 

[Sparweb]

going to end up with a rivet nut or something of that nature.

Ah, so that means you must have determined that there are virtually no loads at all on the structure. Otherwise you wouldn't have chosen one of the poorest fasteners out there for sheet metal.

...We all develop our biases...

Just in case you remain unsure that there are or aren't substantial structural loads on this assembly...

If your examination has shown you that:
b) the structure does have substantial loads,
c) the structure does not permit access to one side of the joint when assembled

...then you want to explore blind fastener types. There are lots of blind rivets that will structurally assemble joints with access to only one side of the rivet. A common example is Cherry, but I find Huck can be installed more consistently.

If also the structure must be disassembled periodically, then there are blind bolts to choose from. A common example is the Hi-Lok, but they are costly, especially in small quantities. This is just an example to get you thinking and searching for what will work for you.

This could be a case where awareness is half the battle. Without making a specific recommendation for anyone's particular service or supply, you should explore the catalogs of Aircraft Spruce, Genuine Aircraft Hardware, and Aviall (just 3 off the top of my head) to get an idea of the options you have available. The number of options better than rivnuts are almost too numerous to count.

http://www.sparweb.ca

 

[Ron]

If your fitting needs to fit flush with the sheet surface, rivet nuts won't work because there will be a slight standoff. Agree with drawoh's commments.