Construction with gussets and bolts
 

I've been investigating some new design and construction methods that my team hasn't explored in the past, and I really want to try assembling the frame and important structures with machined gussets and bolts. I have a few questions for teams who have used this construction method. In this scenario, I'd be using two gussets, top and bottom, to create a 90 degree joint between two lengths of rectangular aluminum tubing, all 1/8" wall thickness.
1. Is it better to put a long bolt all the way through and use a nut on the other side, or to tap the gusset and tubing and use shorter bolts?
2. If a bolt all the way through is used, are large washers needed to spread the load to prevent the tube from being crushed?
3. If the parts are tapped and a short bolt is used, would it be preferable to tap both the gusset and the tubing or to drill clearance on the gusset and only tap the tubing?
4. Is locktite a necessity with the tapping method?
5. What has been your overall experience with this method of construction? Pros/Cons?
Thanks!

I should clarify, with the pretapped method, we would be using machine screws or something of the sort.

Clarification #2: We have used a lot of rivets in the past, and do like them for certain applications; however, I'm specifically interested in trying out bolts on the critical parts of the frame- primarily drivetrain- where it can see a lot of stress due to collisions or hard driving. These would be parts that we shouldn't ever have to disassemble or replace between matches (fingers crossed) so I'm not really concerned about the speed of installing/removing them.

Re: Construction with gussets and bolts
 

Are you set on using bolts? The VersaFrame line has been designed to use rivets and many teams have been very successful with it so far.

Re: Construction with gussets and bolts
 

We've used versaframe extensively, especially this last season. I'm looking into construction methods that don't rely on rivets as the primary means of fastening, and that utilizes our machining resources rather than the cash in our pockets. We have a laser-cutting sponsor, so designing gussets rather than buying them would make sense for our team.

Re: Construction with gussets and bolts
 

I'd second the idea of using rivets. Rivets have a huge weight advantage over bolts and don't vibrate loose. Most teams use 1/16" wall tubing for most things only going to 1/8" where needed. We use 3/16" rivets which come in a million different styles with different sheer strengths. McMaster Carr has most everything.

We use the wide range rivets so we don't have to stock a bunch of different lengths. The high strength rivets with steel shanks have higher strength for demanding applications but they are a pain to drill out.

Re: Construction with gussets and bolts
 

I'm going to second the rivets option. We've been using gussets and rivets for a while to build our robots and it's worked quite well for us. They're very fast to install, just as fast (if not faster) to remove, and pretty much always just as strong.

We have occasionally used bolts to hold things together during construction, or to allow assembly/dis-assembly in situations where a drill/rivet gun may not be available; when we do this we usually have the bolts go all the way through to the other side of the tube and use a nylock nut to hold it all in place. If you do want to have a bolt to allow quick dis-assembly, be sure to either buy bolts at just the right length or be ready to cut down bolts to size—it'll make assembly/dis-assembly significantly faster.

Re: Construction with gussets and bolts
 

My comment about VersaFrame was more to make the point that rivets have been used very successfully. Make your own gussets and attach with rivets. Plus, rivets are much cheaper than bolts.

Re: Construction with gussets and bolts
 

1. Whenever we have bolted a gusset/tube we have gone all the way through with a bolt. We try to keep tapped parts to standoffs and to a minimum. If its a part you may have to remove and install several times you start increasing the risk someone who is rushing won't make sure the threads are lined up and damage the threads. Our first year using tapped parts was in 2013 and by mid season a few standoffs were replaced because the threads were gone.

2. To prevent the tubing from being crushed you need to work on training teammates (and sometimes adults too) how to properly use tools. There is a difference between making sure you have a snug fit and turning the wrench to extremes to get a tight fit which typically ends in bending the tube.

A downside to using bolts extensively is they are much heavier than other alternatives such as aluminum rivets. The more parts you have in your assemblies the more weight you need to consider that is getting used holding it together.

This past build season we relied mostly on gussets from Vexpro or homemade versions held together with 3/16 aluminum rivets. We did use bolted connections in a few key areas we knew could be removed frequently and didn't want to rely on drilling out rivets over and over.

Re: Construction with gussets and bolts
 

Rivets are the way to go. There are different choices of rivets that will give you different strengths.
Bolts are heavy and time consuming to take out during quick repair in the pit. If it is a critical joint with a very high load, use bolts. I would get it all the way through and put a spacer in between if you have access. A washer will help reduce the chance of collapsing the section but if a student wants to use a drill to tighten a bolt and not paying attention, you can still collapse it. We always use lock nuts so no locktite is needed.
To save weight, I would use 1/16" high strength aluminum alloy for the two gussets.
We usually use rivets everywhere and more rivets in area with anticipated higher loads. If any rivets failed during matches, we just replace them. If it is in an area we do not want it to fail again, we would replace with bolt and nut. If you don't have access to use nut, then use self drilling screws for repair. They work really well.

Re: Construction with gussets and bolts
 

Rivets are awesome.

Blind Rivet Supply has great prices.

http://www.blindrivetsupply.com/Butt...Alum_Alum.html

Re: Construction with gussets and bolts
 

We mainly use rivets with the Versa chassis...but in some areas we add
bolts as insurance incase of rivet failure if we have the weight and space
for them. We don't tap the Versa chassis for them. The only time we tap
any structural part for bolts is if we use 80/20 as part of the frame.
We've been able use both in the same robot. The past two years we've made
the basic frame from Versa chassis and added other structural parts using
80/20 by bolting them to the Versa chassis.

Re: Construction with gussets and bolts
 

While we mostly use rivets, we have, on occasion, used bolts as well. Rivets are extremely easy and quick to do, using a small drill bit and a pop rivet gun. Bolts, on the other hand, require larger holes, wrench and Allen wrench, and may not be as quick or easy, depending of familiarity and the exact situation.

For your specific questions, when we use bolts:

1. Most of the time we use long bolts with nuts, although we also utilize rivet nuts quite a bit - they're great, and provide a much better mount than tapping some relatively thin sheet metal.

2. We usually don't need washers- the long bolts are going through gusset plates on both sides, which act, more or less, as large washers themselves. Of course, you want to make sure you aren't over tightening things!. If we are worried about tubing being crushed, we use some plastic/nylon stock, machine it to fit inside the tube, then drill through the whole mess. It gives you support where you really need it, and is fairly light weight!,

3. We always only tap one piece and use clearance for all others. Otherwise, you create the possibility of binding issues and other difficulties putting two tapped pieces together.

4. Unlocks or locktite on everything... These robots move around and rattle so much, anything not properly secured will work it's way loose over the course of competition. And of course, fall apart at exactly the wrong moment!

5. As stated, overall we usually use pop rivets, not bolts. The exceptions being places where we want some added strength (anticipating loads the pop rivets wouldn't be able to handle, in either size or direction) or want a piece to be easily removable/swappable.

Re: Construction with gussets and bolts
 

We used 2 x 1 inch tubing with 1/8 inch walls for our chassis and lift tower and 1/16 inch wall 2 x 1 and 1 x 1 tubing for the other structures. Most of it was riveted using 3/16 inch rivets and 1/8 inch thick gusset plates that we made. Some joints, like the lift tower to chassis were done using 1/4-20 bolts to allow the lift tower to be removed for shipping. This construction was adequately strong for this years game and would probably be good enough for last year's game. Last year's robot was made from aluminum channel held together with eight 10-32 screws in the corners. There were fewer than the rivets we used this year and it held up fine with the abuse it was subjected to.

I would strongly suggest using washers under the heads of any socket cap bolts (see link below). They bearing surface area of the socket cap bolts is smaller than the bearing surface area of hex head bolts. I have seen quite a few instances where the head of the socket cap bolts have "crushed" the aluminum sheet due to the high concentration of force. Some joints have gotten loose because of the deformation of the aluminum. This is different from crushing a tube.

http://www.andymark.com/product-p/am-1207.htm


Do be careful where you use the 1/8 inch wall tubing. We ended up overweight and had to replace quite a few pieces of our structure with 1/16 inch wall tube to get the weight down.

Re: Construction with gussets and bolts
 

Tapping 1/8" aluminum for machine screws is an exercise in futility - for any thing heftier than holding an encoder in place, they're almost certain to be stripped out, or rattle loose, or both. Always end in steel, or 1/4"+ aluminum. Self-tapping or sheet metal screws in 1/8" aluminum are not too bad, but can still be stripped out more easily than I'm comfortable with for the FRC team.

When we've used bolts and gussets with tubing or c-channel, we've usually gone all the way through, and used gussets on both sides to help distribute the load (minimize deformation of the tubing). Never use a single hex nut for any joint under load or vibration. Either get nylon locking nuts, or jam two nuts together to lock them in place. Sprocket-type lock nuts work when they're new, and being pushed against steel (they often mangle aluminum rather than grip). They don't work as well after they've been flattened - and how many teens will notice the difference?

Re: Construction with gussets and bolts
 

If you are sure you won't be taking it apart, look into structural adhesives. The area between a gusset plate and tube is pretty big and thin, so an epoxy or CA glue can add a lot of strength. We love Black Max (Loctite 380) because it doesn't get brittle so it can survive impacts and shock loads. Be prepared, it is extremely messy.

I would suggest self-drilling, self tapping screws to hold everything together while the glue or epoxy cures, and to add a little strength.

Re: Construction with gussets and bolts
 

I have to respectfully disagree. I've tapped 1/8 aluminum quite successfully in every thread from #4-40 up to 1/4-20 and it works just fine if you're careful about it.

Re: Construction with gussets and bolts
 

Congrats on doing some off-season research! Selecting a set construction method for your team is immensely important and I see it often overlooked resulting in some 'slapped together' style robots that tend to fail often.

Having done both gussets+bolts and gussets+rivets each for a full season, rivets are pretty awesome. We used 1/4-20 bolts in 2013, and in 2014 replaced them with 1/4" rivets. Our hole patterns and everything were treated just as if we were using bolts, but instead we dropped rivets in the holes. And if worst comes to worst and you break a rivet for whatever reason (which has not happened yet) you can drop one of the million 1/4-20's you have laying around through the hole in a pinch*. Some would argue the weight loss is a plus as well. We've used 1/8 tubes with 1/8 plate for gussets but you could go for a thinner gusset plate and probably tube wall for sure.

2013
2014

With the right kind of rivets, I guarantee you wont see them break. Here's what we've used successfully for a couple years now. They might even be overkill, but we opted to go for slightly stronger because of a bit of paranoia just to guarantee no issue. I have not used smaller rivets before, but I've also seen teams use smaller sizes (I believe 3/16"?) with a more aggressive pattern.

*Our kids also rigged up a system using the old KOP air compressor to allow us to rivet in our pit, which was pretty nice.

Re: Construction with gussets and bolts
 

We use both rivets and socket head cap scews. Hex head bolts we never use. I prefer to use rivets wherever possible, generally 1/8" or 3/16" excepting the things like the gussets on the drivebase which use 1/4".
This year we used a lot more bolts because of a want to make assemblies removable, odd loading conditions that are bad for rivets, and ascetics in some places. In any situation where you're using bolts, do a weight calculation of locknuts versus 1/4" plate; surprisingly we often find that a 1/4" tapped plate is actually lighter than nuts, and that's without any sort of lightening.
Locktite is nice, but lockwashers can get the job done just fine. Our elevator back support was held on by bolts through 1x1 to 1/4" tapped plates and it never came loose due to lockwashers.
Buttonhead screws going into tapped plates also looks very good, due to the lack of a cylinder-like head and nut. I also like the increased holding area. However, for sizes 8-32 and smaller, I like to use socket head screws because the tiny size socket of button heads make it easy to strip out a screw if it's repeatedly removed and replaced.

Re: Construction with gussets and bolts
 

None of your machining constraints really have anything to do with bolting versus riveting. You can totally build riveted frames using those same resources. To better answer your question, I'd like to know - what problems have you had with rivets?

If it's strength - stepping up to 3/16" rivets doesn't cost a ton of weight and can be a lot stronger. If it's repairability / removability - rivet nuts combined with bolts are useful for mechanisms that have to attach and detach themselves a lot.

Bolts in gussets and tube can be a bit problematic - bolts going through both sides of the tube can crush it if you're not careful, bolts have more slop leaving a less rigid frame in some cases, etc. It's still quite doable though. You can even carefully size holes to accept both #10 clearance and 3/16 rivets, or #8 clearance and 5/32 rivets.

Re: Construction with gussets and bolts
 

We've used this construction method for the past two years on our west coast drive base and on some superstructures. Here's specific answers to your questions.
1. Mostly tapped the 1/8" tubing and used short screws. Our build standard is based on M6 socket button head screws.
2. We use bolt through or rivet nuts for mounting the superstructure to the base. We typically spread the load with washers for heavy structures like our tower this year.
3. We used tight clearance holes on the gussets and only threaded the tubing.
4. Loctite is necessary.
5. Pros / Cons: This method is slightly heavier. Not much of an issue for the drive base, but superstructures with 1/8" tubing can add up even with pocketing. We used this method because we had the tooling not necessarily for reliability. But it does hold up to competition abuse.

One disclaimer - We are buying the tooling for 3/16" rivets next year. Just never got around to before.

Let me know if you have any other questions.

David

Re: Construction with gussets and bolts
 

I have to agree with GeeToo that tapping 1/8" aluminum is not a good choice. The general rule of thumb is that for a hard material like steel the thickness of the tapped material should equal the diameter of the fastener. For soft materials with a hard bolt the thickness of the tapped material should be 2x the diameter.

So for a #10 bolt you would want the tapped material to be ~3/16" if it is steel and 3/8" if you are tapping aluminum and you want the full strength of the bolt.

If you look at standard hardware that you'll find at your local store a 1/4" nut will be 1/4" high, a 1/2" nut will be 1/2" high ect. Note that is for a standard non lock nut, nylocks, stover nuts, kep nuts ect will be slightly taller to account of the locking portion. This holds true whether the thread is coars or fine and regardless of the strength/material of the fasteners since the assumption is that you will use the same strength/material for the nut and bolt.

Now that does not mean that you are guaranteed failure is the tapped material does not follow those guidelines, just that the ultimate strength of the combination will not be realized.

In regards to the original question I prefer a bolt and nut through just the gusset and one wall of the tube. That does mean that the order of assembly must be considered and held to since if you are joining two tubes at right angles you will block access to one of the tubes with the other tube. This method will result in the ultimate strength since you'll have double the fasteners and often with less weight. Yes it is more difficult to assemble.

The other option is to use a rivnut. But for many applications I prefer a high strength pop rivet, as others have mentioned.

Re: Construction with gussets and bolts
 

We do that. More on the upper than the chassis, but we have some removable panels there as well. We specifically used bolts on our stacker this year rather than rivet because of concern for fatigue / rivets getting looser over time. Not sure I'd have that same concern with a chassis though.

Re: Construction with gussets and bolts
 

Have you considered TIG welding the tubing? If you have access to a welder, it would be a good idea to weld your drive base together (as much as possible ) because it will be very durable, especially for gussets.

Re: Construction with gussets and bolts
 

We haven't had many major issues with rivets- mostly just loosening over time in certain areas and needing replacement occasionally. I think didn't use the right rivets this last season though- we primarily used these when in the past we normally used something like this.

Re: Construction with gussets and bolts
 

We've welded in the past, and while I really like the durability and rigidity it gives us, we had a bad experience when we broke a part on our robot and were unable to replace it during the competition because we didn't have access to a welder. Since then we've moved away from welding anything except modular assemblies that can be swapped easily.

Re: Construction with gussets and bolts
 

If I understand correctly, this allows you to bolt thin sheet metal together without tapping?

Re: Construction with gussets and bolts
 

One way that you can prevent the tube from being crushed is to press wood blocks into the center of thin wall tubing. This probably is not necessary for 1/8" wall tubing, but could be useful to make robust connections in 1/16" wall or thinner. I've attached a quick cross section below.

-Nick

Re: Construction with gussets and bolts
 

Seconded. 3476 has been tapping 1/8 aluminum for a few years now. If your bolts are rattling loose, use threadlocker (given the vibrations that normal FRC machines experience, we use it on almost every bolt on our robots).

Now stripping out the threads is a different story. One of the intake pivot mounts on our 2013 machine was connected with bolts to the 1/8 tapped aluminum frame. The consistent shock loads from quickly raising/lowering the intake and normal robot/field contact was enough to strip out one of the taps. In situations like that, we found it would be better to run a bolt entirely through the frame rather than simply tap one side. (It wasn't possible on that particular mount, for reasons I won't get into ;) )

Re: Construction with gussets and bolts
 

We used some captive nuts like that to hold on the cover for our electronics panel last year. It would have gone more smoothly if we had pressed them into the chassis before installing all the other components. We also installed extras. These came into play when, over time, one or two of the captive nuts worked their way out and were lost. Most likely, someone pushed on the screw as it was being removed and pushed the captive nut out of the aluminum.

Re: Construction with gussets and bolts
 

I am surprised there has not been more support on this thread for using 80/20 or Bosch Profile T-Slot framing components. Yes, we love and use rivets, but the question was more about which option to use if you are bolting gussets to extrusion. I would say neither if you can afford to go the route of T-slot extrusion which will allow you to bolt a gusset on either side of the extrusion with fewer protruding bolt heads.

The T-Slot fasteners take a LOT of tooling time away from the construction of the robot parts and you get the plus side of loosening the fasteners and adjusting the position of the components without drilling any holes most of the time. We use this for prototyping every year before we transform to a lighter design. (probably with rivets)

Check out an 80/20 catalog to see what they have, they even offer a lightweight version for robot construction and some suppliers offer discounts to FRC teams through the company. We even got about $1000 in parts and extrusion donated this last year for our pit.

Re: Construction with gussets and bolts
 

If you end up using rivets again, for critical areas like a drive chassis, where you don't expect to ever be removing the rivets, high strength aluminum rivets stay very firm over time and are just as light as regular rivets. They aren't that difficult to remove if you have to - you just need to center punch the rivet hole as the mandrel tends to break flush with the top of the rivet, rather than slightly inside the rivet.

The other benefit of these pricier high strength rivets is that they are rated to work in holes going all the way up to .201". That way, you can cut holes to .196" or so and use a tight #10 bolt clearance hole that doubles as a rivet hole.

The extra cost of these rivets isn't insignificant though - http://www.mcmaster.com/#aluminum-rivets/=x9kpy0

Re: Construction with gussets and bolts
 

We've ran regular 3/16" rives in .201 holes for years w/o issue (for anyone reading this and thinking you NEED high strength to handle the larger hole size).

Re: Construction with gussets and bolts
 

If you insist on using bolts, consider these.

http://www.mcmaster.com/#standard-rivet-nuts/=x9l7zq

Re: Construction with gussets and bolts
 

The alternative to this is just proper training on not crushing tube. It's not worth the extra weight of putting in wood blocks or standoffs when the problem can be solved through training.

Re: Construction with gussets and bolts
 

Yes and no.

By not crushing the tube (with no internal reinforcement) you limit the amount of preload and clamping force you are able to generate.

Re: Construction with gussets and bolts
 

Adam is right.

Fasteners work best when they generate clamping forces high enough that the friction between the two parts being fastened is enough to keep the joint from moving. The bolt or rivet is never in danger of shearing or loosening because there is no movement in the joint.

That is why you should either put an aluminum spacer inside the tube, or figure out how to fasten the gusset plate to the tube wall itself. This is normally achieved with blind rivets. Rivnuts or small self tapping screws will also work.

Re: Construction with gussets and bolts
 

Yes. Just press them in with an arbor press (gently) and make sure your material is stiff enough that when you clamp you don't locally deform the metal.

This is correct for bolts - the bolt loads the material it is clamping and the friction between these materials reacts shear loads. In FRC most chassis loads related to gussets or corners will be shear loads.

From my understanding of things, rivets are actually meant to be loaded mainly in shear, not tension.

Re: Construction with gussets and bolts
 

Mr. V has most of the reason - too little thickness of too-soft metal. In addition to not having enough thickness for lathed or other fully-formed formed threads, There is the loss of strength which comes from a tapped thread. In order to use a tap (or die), you must give up some of the inner or outer surface of the thread, reducing all of the forces, shear, tensile and torque locking significantly.

I concur that all of this is true. However, when our team has used gussets (actually steel flat angle braces), we have usually used gussets on both sides and through bolts for ease of assembly, at a slight loss of holding power. Since we were using C-channel in these cases, it can also be argued that we improved the torque resistance a bit more than if we'd used separate bolts on each face.

3946 has not used rivets for structural joints to date, but we plan to experiment with them this summer. The experiences I have read seem to indicate that we can do as well or better using several aluminum rivets with steel shanks where we used to use a single through hex bolt or cap screw and nylon locking nut. Still a bit of number crunching required to better define "several".

Re: Construction with gussets and bolts
 

Thank you everyone for the helpful replies. I'm planning having my team do some testing this off-season to gauge how different fastening methods hold up to vibration, cyclic forces, shock forces, etc. Your responses have given me some new insights and ideas for other construction methods that are available. Thanks!