Construction with gussets and bolts

[Joseph Smith]

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.

[notmattlythgoe]

Originally Posted by Joseph Smith

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!

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.

[Joseph Smith]

Originally Posted by notmattlythgoe

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.

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.

[notmattlythgoe]

Originally Posted by Joseph Smith

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.

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.

[Chris is me]

Originally Posted by Joseph Smith

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.

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.

[Joseph Smith]

Originally Posted by Chris is me

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.

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.

[Chris is me]

Originally Posted by Joseph Smith

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.

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

[AdamHeard]

Originally Posted by Chris is me

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

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).

[pfreivald]

If you insist on using bolts, consider these.

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

[Dale]

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.

[Nyle]

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.

[Jon Stratis]

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.

[philso]

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.

[GeeTwo]

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?

[sanddrag]

Originally Posted by GeeTwo

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.

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.