Rivets vs threaded fasteners

[ToddF]

This post is spurred by a conversation with a student yesterday. He suggested that we use rivets instead of #10-32 fasteners to attach a critical piece of structure. My first reaction was negative, but I found myself unable to articulate a truly good explanation why we should use fasteners over rivets. As an engineer, that's not acceptable, and is a sign that I've got some unjustifiable biases in my thinking.

Would anyone be willing to share their engineering based opinions about the pros and cons of using rivets vs threaded fasteners in FIRST robots? Are there certain applications where your team prefers one type over another? Do you have certain design rules, tools, specific rivet part numbers, etc. which you use successfully? Would you, for example, use rivets to attach a plate sprocket to a versahub? Why or why not?

We've used rivets sporadically in the past, but it always seems that by the end of competition season, many, if not all of them have been drilled out and replaced with threaded fasteners. Rivets are used quite successfully in the industrial world, on a wide variety of applications, so I suspect that we are just using them wrong.

Here are my observations based on past experience. Please correct my mistaken beliefs if your experience says I'm wrong:
- If parts need to be removed for servicing the robot, you should use threaded fasteners. Rivets should be used for installations which are expected to be permanent.
- Rivets are best in shear applications. If used in tension applications, they tend to loosen up over time, and aren't easily re-tightened except by drilling out and replacing the rivet.
- Rivets of similar strength to threaded fasteners (tension and shear) don't offer significant weight savings. Corollary: Replacing a threaded fastener with a lighter rivet results in reduced strength in that joint. (The exception may be if use of a rivet results in the elimination of a nut)
- Rivets are most useful and have the most potential for weight savings when joining very thin materials (ex 1/16" thick aluminum) which can't be tapped.
- When riveting soft materials, like lexan, the head of the rivet must be against the soft material, or a close fitting washer must be used on the "bulb" side, so the bulb doesn't pull into the soft material.

Anyone care to share their experiences? I'm hoping for more detailed responses than "We use rivets. They work great for us." How do you use them? What do you do to make them work for you?

[Craig]

I like to use them in structural applications. Idea being an overall weight savings over several mostly permanent joints. Spread the load out with more fasteners, which see less stress as a result.

I agree 100% with threaded fasteners for moving parts and modular mechanism interfaces

[Daniel_LaFleur]

Originally Posted by ToddF

Here are my observations based on past experience. Please correct my mistaken beliefs if your experience says I'm wrong:
- If parts need to be removed for servicing the robot, you should use threaded fasteners. Rivets should be used for installations which are expected to be permanent.

Threaded fasteners do make removal and replacement a bit easier. If you know you need to remove a part to service it (or another part) then use threaded fasteners.

Originally Posted by ToddF

- Rivets are best in shear applications. If used in tension applications, they tend to loosen up over time, and aren't easily re-tightened except by drilling out and replacing the rivet.

Not quite true. Screws in tension have stress risers at the threads which can cause the thread to fail. Threaded fasteners tend to loosen due to vibration. Rivets tend to loosen because they are incorrectly sized (reach is wrong or hole size is wrong). Rivets only loosen in tension if there is a shock load on them. So if you expect a lot of vibration, use rivets. If you expect heavy shock loads, use threaded fasteners (with Loctite).

Originally Posted by ToddF

- Rivets of similar strength to threaded fasteners (tension and shear) don't offer significant weight savings. Corollary: Replacing a threaded fastener with a lighter rivet results in reduced strength in that joint. (The exception may be if use of a rivet results in the elimination of a nut)

Most threaded fasteners are hardened steel. Most rivets are aluminum. For the same strength the rivet will typically weigh 2/3 the weight of screws.

Originally Posted by ToddF

- Rivets are most useful and have the most potential for weight savings when joining very thin materials (ex 1/16" thick aluminum) which can't be tapped.

Rivets tend to be useful on thin material for that reason as long as the reach of the rivet is correct, however, rivets also save weight when the strength of steel screws are not needed and nuts can be removed.

Originally Posted by ToddF

- When riveting soft materials, like lexan, the head of the rivet must be against the soft material, or a close fitting washer must be used on the "bulb" side, so the bulb doesn't pull into the soft material.

The same is true for screws. Soft material should always have washers to spread the load of the fastener (regardless of type).

Hope that helps.

[Chris Endres]

So as a general basis on my team, rivet if it is non-essential, and bolt it if it is essential. If the part your securing will see a lot of wear and tear throughout the season, bolt it. If your part will need constant replacement/isn't under tension forces, rivets work best. There are some applications where we need to use bolts, simply because we can't fit a rivet gun head where we want it. And if you do use bolts, remember to either use locknuts, or locktite on regular threaded nuts. We also bolt parts that require zero, or close to zero, lossesning throughout the season, because if the hole is not perfect for the rivet, the rivet will lossen, where ever the force.

My rule of thumb, as a "we need to rivet this", or "we need to bolt this", is "will this need to be perfectly secure the whole season?" Yes: bolt, no: rivet.

Like you said, use bolts wherever parts need to come off repeatedly.

Other than that, it's a weight and ease preference, really.

[Daniel_LaFleur]

Originally Posted by Chris Endres

My rule of thumb, as a "we need to rivet this", or "we need to bolt this", is "will this need to be perfectly secure the whole season?" Yes: bolt, no: rivet.

Considering rivets are used on aircraft, I'm surprised at this answer.

[Jared]

Our team is very much a threaded fasteners team, though we have occasionally used rivets in a few spots. Our robot has many tapped holes and countersunk screws used in applications where rivets couldn't really be used.

We often find ourselves removing, replacing, or repairing parts of our robot. It's true that we could drill out a rivet and replace it, but it's often quite difficult to fit a cordless drill and a riveter in a very tight spot. On the other side, there are many different styles of allen wrenches that can be used to fit in almost any spot. We have ball end allen keys, t handle, and a set of allen keys for 3/8" socket wrenches. If you have a universal joint for the socket wrench, you can really fit the allen wrench into tight spaces.

The second disadvantage of rivets is that they will only grab one wall of the box tubing. That's okay for light loads, or if you use a million of them. If you weld a few threaded inserts into your box tubing, and use a few 1/4-20's, you really cut down your assembly/disassembly time.

Our elevator assembly this year is a perfect example of quick assembly.
To disassemble it, you must remove 7 threaded fasteners.
You remove one 10-32 to release the chain and 6 1/4-20's to remove the 75" tall 1" x 2" uprights.
I don't see how this could be achieved with rivets.

[rich2202]

Out team started using threaded rivets this year. Drill the hole (or use the existing hole), insert the threaded rivet, and the part can be easily screwed on and off.

[staplemonx]

Rivets are lighter and reduce slop in the structural system by filling the hole. Rivets are not easily removed.
Riveting more than 4 pieces of material together with blind pop rivets is not recommended. Other permanent type fasteners like cherry max or huck bolts would be recommended for this application.
Rivets will crush material during their installation, materials that cannot be crushed or that could be delaminated with a high point compression loading should not be riveted with traditional rivets.

Bolts are heavier and need a clearance hole to be drilled.
Bolts will not reduce slop.
Bolts are easily removed.
Bolts can be tightened to a specific torque, they will only crush material if you want to crush material.

Both bolts and rivets can withstand high tensions, compression and sheer loads if both material and size are part of the joint analysis.

[MrForbes]

Since our robot this year is half made of wood, we didn't use a lot of rivets. We use a few wood screws to hold aluminum parts to wood, and some machine screws and nuts to hold other aluminum parts to wood. We have a few places where thin aluminum is attached to other thin aluminum parts, these are riveted.

We usually don't use locknuts on anything that may need to be disassembled for repair or maintenance, because it can take so long to get the screw out and back in. Instead, we depend on maintenance to keep nuts tight.

We don't use aircraft type rivets...if we did, we probably wouldn't have any trouble with riveted connections getting loose. We use mostly materials and supplies that are locally available, and that means hardware store or building supply, since there are no industrial supply places within errand distance.

When thinking of the engineering of fasteners, we usually think of the robot as a prototype, and not as a saleable commercial product.

[Brandon Holley]

I want to counter the point that rivets are not easily removable.

Rivets are VERY easily removable. Obviously care needs to be taken to ensure you are drilling out the rivet and not the hole under the rivet.

For something that will be removed/added many times during a single competition we would opt for screws. For something that needs to be added once or twice per competition, rivets are more than acceptable. The top of our lifter this year is held on by rivets that we will drill out before we leave each competition.



-Brando

[nlknauss]

Originally Posted by Brandon Holley

I want to counter the point that rivets are not easily removable.

Rivets are VERY easily removable. Obviously care needs to be taken to ensure you are drilling out the rivet and not the hole under the rivet.

For something that will be removed/added many times during a single competition we would opt for screws. For something that needs to be added once or twice per competition, rivets are more than acceptable. The top of our lifter this year is held on by rivets that we will drill out before we leave each competition.



-Brando

Agreed, we use rivets for structures we don't expect to change but often find ourselves easily drilling them to make some changes and reattaching that structure. If we find that the subsystem needs to be removed or adjusted, we use threaded hardware as a part of the design. If we're fastening permanent structure, we look to use rivets.

[AustinH]

Using quality rivets made a big difference for us. Much as I love our local hardware store, the pop rivets they stock leave much to be desired.

[EricH]

Originally Posted by rich2202

Out team started using threaded rivets this year. Drill the hole (or use the existing hole), insert the threaded rivet, and the part can be easily screwed on and off.

Threaded rivets--you mean hi-locs?

[MrBasse]

Originally Posted by EricH

Threaded rivets--you mean hi-locs?

Or Rivnuts

[Richard Wallace]

The Average Joes use a lot of rivets, and our experience is that they are much faster to remove and replace, compared with nut-and-bolt fastening in the same hole size and location. We use four rivet sizes: 1/8", 5/32", 3/16", and 1/4" to replace #6, #8, #10, and 1/4" threaded fasteners, respectively.

We were inspired by a respected mentor's claim, posted here a few years ago, that he could remove a rivet faster than any challenger could remove a nut and bolt. We tried that speed test ourselves, and when we saw he was right, we reduced our threaded fastener use considerably. We have never regretted the change.