How does your team reduce weight on your robot?

Don’t build to max dimensions. Make your robot as small as possible. Even shrinking a few inches off of your length and width can save a lot of weight due to the Square-Cube law.

FEA isn’t really worth it for FRC. Just build it, test, and iterate. Iterate towards good functionality first, then iterate on weight. If it doesn’t break, keep taking material away. Make it out of thinner sheets or extrusion profiles. Change steel to aluminum. Change aluminum to polycarbonate. Change polycarbonate to thinner polycarbonate. Reduce the infill on 3D printed parts. Once it horrifically breaks, back up one revision.

Brushless motors.

Think twice about pneumatics. Only use them if you need at least three separate small binary actuations. A complete pneumatics system gets heavy, fast. Don’t use any pneumatic cylinders with greater than about 14 inches³ of stroke volume (equivalent to Ø1.5" bore x 8" stroke). I know you are thinking, “but this Ø2” bore x 24" stroke cylinder seems perfect". It’s totally not. See prior point.

Use the smallest possible wheels, gears, timing belts, etc. Don’t use 8 inch wheels when you can use 3-4 inch wheels. Smaller wheels require less gear reduction for the same effective robot speed. Wheels get lighter, you need less gearing, etc.

Robots and mechanisms don’t have to be indestructible anymore. Without the bag, robots only need to survive a single competition. Assuming we’re in a post-COVID world, you can easily gut your robot and replace and/or rebuild a lot of mechanisms in between events now. This allows you to use even lighter materials or “sacrificial anode” type components / structure.

Reduce the size of all fasteners. Unless you are screwing into a tapped hole or interfacing with a specific component that expects a specific diameter bolt, replace all 1/4"-20 hardware with #10-32. Replace all #10-32 with #8-32. Replace all #8-32 with rivets. If a fastener is simply holding two or more static members together in shear, default to use rivets. These riveters are awesome.

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My kids have learned the hard way!! We used to build our robots out of this material I had at my old job. We bought so much of this 80/20 stuff. And the drop ins… and the slide ins… and the end hole bits…:frowning: So much weight! I was even surprised some of my kids could lift the robots some years haha. What we learned was to stop using heavy materials! We used to use scraps from my job where I worked as a sheet metal bender and id bring home some scraps that were big enough to be useful. That when we had our new mentor come in she told us to stop using the 80/20 because we weren’t building welding carts! We’re building robots! So we bought thin aluminum tubing rather than 80/20. And we stopped buying steal gears from Andy marks website and we bought aluminum gears. Our new mentor got some kids to make a jig to drill large holes in the gears to reduce even more weight. Only have bent one tube on the intake once and then we added a thicker aluminum tube and the robot was happy against! :slight_smile: My takeaway was if we could build the robot out of cardboard we would!!

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Who would have ever thought of such a crazy idea to use even a little bit (especially not an entire elevator) of 80/20 aluminum extrusion on their robot? Much less double down and go with 80x80 the next year?!

EDIT: Just so it’s clear, I’m poking fun at my own team. We have learned (I hope).

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This. We’ve struggled with weight for a few years, but in 2020 we came in to comps underweight. We made sure of a few design policies across the robot to get there this year:

  1. Wherever possible use aluminum rivets for fastening. Obviously you need bolts in some places, but the general attitude was to use rivets for anything that doesn’t need to be disassembled, like the superstructure and plates within a subsystem (since we made sure you can simply disassemble the entire subsystem if necessary). I’d like to believe most teams take this for granted, but i’ve met lots of people in FIRST Israel for example who apparently think 1690 is special in this department and it seems like a big portion of teams are using more bolts than they probably should and not enough rivets.

  2. People often talk about using thin wall tubes which is really important, but another thing that’s less often talked about is the actual dimensions of the tubes. American teams just use 1x2 in most of the time but with metric we have a lot more variety, which led to our team making some wacky choices in previous years with tubes like 60x30 mm, which as turns out has an even bigger effect than the thickness. Again, American teams take this for granted but i think it’s worth noting for metric teams - there’s almost never a real reason to use anything bigger than 20x40 mm and 20x20 mm, and as for the thickness 1.5mm works just fine. Alternatively you could do thick tubes with lightening but that requires better manufacturing capabilities and takes more time.

  3. Plates are a place where you can easily save weight. Our current standard is to use 2mm aluminum for gussets and 5mm aluminum with lightening for gearbox plates. Gearbox plates can be lightened pretty aggressively since there’s not supposed to be a significant force on them most of the time, we usually have 4mm thick crossbeams on our gearbox plates. This reduces (pretty consistently) around 50% of the weight of the parts, which is roughly the same as if you’d make that part out of plastic, and usually the difference is slightly favoring the aluminum version. Because of that, we don’t usually use plastic for weight reasons, but it is better to use plastic for any billet parts both for weight easier manufacturing.

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Using flawed math to do even more flawed math:

If you squint your eyes at that paper just right, you might conclude it takes about 200 electrons to flip a storage element from 0 to 1.

By code refactoring alone, you might be able to save up to 4.01655332E-28 lbs per bit that you can flip from the “has electrons” to “doesn’t have electrons” state. That’s up to 0.00000000000000000024 lbs, if you have 70MB of code*.

and, according to the rules, a 125lbs bot is legal, but a 125.00000000000000000024 lbs bot is not. So, might as well give it a shot!

*and can refactor every bit to be the same.

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Thinking through the game strategy more thoroughly could lead to the seldom-used sub-assembly being eliminated at the concept phase of the design. This not only frees up some of the weight budget, it also makes space available for other, more useful mechanisms and makes the job of designing those more useful mechanisms easier. Of course, this also frees up time to do more iterations of the mechanisms that do get built. This may require teams to be less ambitious when choosing the game tasks AND being more ambitious when determining when a mechanism works well enough to be installed on the robot.

Working as an inspector, I have seen many robots that have major (heavy) mechanisms that are obviously not complete, the team tells me it has never worked (and may change) or it appears that the laws of physics would have to be broken for the mechanism to work as intended.

For many teams I have seen, using the KOP chassis will free up a lot of time to optimize the design of the other mechanisms, the ones that get you points.

To properly apply the suggestions made in the other posts takes time. Anything a team can do to free up time can help them produce a better design.

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I’m right in with many of the posters here. Rookie year, I was not officially a mentor, but I was there for bag night. A lot of weight was drilled and sawzalled and dremeled off of underused 80/20 style extrusion. Except that it kept us from adding ramps to the 2018 robot, weight was never again a problem. Here’s what we did in later years:

  • Never again used slotted extrusion on a competition robot.
  • Included rough weight estimates (spitballing) for mechanisms in our strategic cost/benefit analysis to select a robot architecture.
  • After the second year, to work within our capabilities; from a practical standpoint, this meant letting one game task go per year. We succeeded at this most years, failing spectacularly in 2018 when the robot was designed to do everything and did nothing until we cut stuff off and bungeed things down so we could at least drive effectively. As a result, the 2018 off-season robot (where we left two tasks undone, climbing and scale) was the best the team has made to date; only time as an alliance captain (ranked and placed #3) and first time as a first-round selection (the alliance placed #2, behind two teams who played in elims at worlds that year, 4587 and 364).
  • Tried to only slightly over-engineer everything from a strength and breakage perspective.
  • Weigh the robot and mechanisms a few times during build season so we can make adjustments earlier rather than later if needed.
  • Design robots to be big enough to do what they need to do, not as big as we can make them. I could buickle the 2017 (STEAMworks) robot in the front seat of my Saturn Vue to take it to demos, and the 2018 off-season robot was regularly carried one-handed tucked under an arm (without a battery, usually!)
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Emergency last-minute hole saw modifications.

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Is it necessary?

You’ll save more weight by not having something than by redesigning it. If your team has typically struggled to meet weight requirements, factor that into the criteria you use to make design decisions. For example, with drivetrains consider shifting gearboxes, 3 motor gearboxes, or swerve are all heavier than a single speed 2 motor WCD/KOP drive. Same with adding extra DOF to mechanisms, building to max frame perimeter/height, etc.

Is it efficiently designed?

Slotted extrusion is heavy. Don’t build 80/20 elevators.

Mecanum wheels are heavy. Don’t build mecanum drives.

Use brushless motors. Some quick math on example weight savings:
Intake: a 1 stage versaplanetary + 775pro + talon is .5 lb heavier than a falcon
Drivetrain: 2 CIMs + 2 talons + mag encoder is 4 lbs heavier than 2 falcons (8 lbs for both sides), and 3 mini CIMs/talons is 5.1 lbs heavier per side (10.2 lbs total)

Use smaller screws and thinner material. 1/4-20 bolts very rarely necessary. Unless you’re doing sheet metal, pocketing should be a last resort. The effort to weight savings ratio is very high, and often you’re not getting much structural benefits from thicker, pocketed material than thinner+solid.

If you use 3D printed parts, take advantage of the ability to print complex geometry to simplify what might otherwise need multiple parts. Design the parts smarter and not just thicker, and take advantage of print directions.

Is it effeciently integrated?

A lot of robots I’ve seen have a lack of integration at the top level, leading to extra superstructure where they could have used shared mounting points, or non-ideal mechanism locations requiring a lot of bracketing.

Did you CAD everything?

If you’re using CAD to check weight, make sure all material properties are properly set, and consider what you didn’t CAD in. The one year as a student my team didn’t easily make weight, I found several pounds of bolts and nuts that we hadn’t CADed. Fasteners, belt/chain, wires/connectors, and everything else adds weight, and you should factor in a safety margin when checking CAD.

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One golden rule which can be rephrased and sub-ruled in many different ways: “Think about the load paths”. Most of these things are obvious when you’ve enough experience and you take the time to actually think; if you get in the habit, you begin to spot room for improvement like a habit.

  • Loads don’t evaporate. Newton’s law states every action has an equal and opposite reaction. In more accessible terms: every link passes along the load; the force doesn’t vanish. Think about how every load you put on your robot will be reacted by the rest, including the drivetrain- if you do it ahead of time, you can be smart rather than scabbing tons of gussets on post-facto.

  • Use big sections: A 2x2, 1/16" wall box tube has about 5.3 times the bending stiffness, and 2.6 times the bending strength, but weighs the same as a 1x1, 1/8" wall box tube. This is why airplanes and racecars are stressed skin monocoques: moving material to the outside helps BULK strength.

  • Use thickness where you need it: bulk strength isn’t local strength. Take the above advice to the extreme: If your arm is made of .015" wall 8x8" box, sure, it’s really stiff and strong… but the wall will get dented, and maybe even pierced (.015 is aluminum foil territory).

  • Spread it out: If you can connect something with two bolts 1" apart, or two bolts 5" apart, do the latter, as this will decrease the force seen at the bolted connections by a factor of 5 during (in plane) bending loads. Also, if you’re connecting soft material to hard material, use a washer to distribute the potentially high stress. Using washers and scab plates can let you put thickness exactly where you need it (at the interfaces) rather than where you don’t (in the middle).

  • Be strong like a tree: If compliance isn’t an issue, a good way to avoid damage from impacts is to make your mechanism flex more. Energy = force times distance; if you can increase the distance your part deforms, you decrease the force your part must withstand from an impact.

  • Less things to build and break: Reduce your parts count. Bracket directly from one thing to another, or don’t even use a bracket, because fasteners and webbing for fastening to is the overwhelming majority of the weight in an optimized part. The overwhelming majority of structural failures occur not in the middle of a part… but where it meets others. And of course, the fewer links you have in a chain, the less likely it is to break. The best fastener is to not have a fastener.

  • Get to the point: If you need to connect A to B, connect A to B directly. Don’t make square frames when trapezoids will do. The more linear the load paths, the stronger your part. Parts are always weaker in bending than tension/compression.

  • Use smooth, properly-sized fasteners: Threaded fasteners will egg out over time, especially if they’re in a loose hole. If you can use pins or rivets to transmit load, these have smooth diameters which are less prone to egging out. This is why welding 6000 series aluminum, which absolutely demolishes the material strength, can still be stronger than a bolted joint: it doesn’t introduce a stress concentrator which will egg out over time. Glues and epoxies are fantastic fasteners (if not for the whole wait-til-it-cures aspect)

  • If you need to use FEM/FEA to design the part, start over: Designs should be simple. If you can’t visualize and understand the load paths (the magnitude of the loads is a different story), you’re already going down a bad path. Come up with a better design you can actually understand.

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I was surprised how much weight we were able to get out of the robot by pocketing the large aluminum gears. We were able to get between 0.25 and 0.35 lbs out of each gear.

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We haven’t ventured into 8-32 territory yet (it took years to make 10-32 the default for us!), but this is seriously missed by a lot of teams, especially ones who’ve been around a long time and got stuck in a groove before bumpers became the norm.

Also, if you can’t swing $200 on a rivet tool get this sub-$30 one. It’s been very reliable in our shop even on 3/16" rivets.

Another one I haven’t seen yet: consider your spinny bits too. If you have an axle full of wheels, could that be Versaroller or maybe a plastic pipe with broached end plugs? (We ran PVC pipe for conveyor rollers this year, and it held up fine.)

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This image re-inforces Andy’s point. The small gear was providing power to one side of a robot’s drive train, until all of its teeth stripped off. During an elimination match. Guess which alliance was eliminated.

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How big was that thing supposed to be

Before they were stripped, the small gear had 14 teeth. So its original pitch diameter was 0.7 inch and its overall diameter was about 0.8 inch.

If youre using the KOP chassis, put holes in the steel drivetrain gears. Also, design with angle bar and channel as well as tube, use 6mm aircraft plywood rather than .25" polycarb in certain areas that won’t experience significant shock loads, and DESIGN YOUR ROBOT IN CAD USING PROPER MATERIALS.

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4414 didn’t have a piece of thick wall tube on the robot.

That along with all the weight savings from using falcons (lots of single stage reductions and didn’t use all motor slots) we never had to watch our weight with what was, in my opinion, an extremely chunky robot.

Also spend the extra 2 bucks and buy the west coat product lightened gears. That stuff adds up.

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My first four days doing work for AndyMark was Championship 2016. It was peppered with heartbreaking stories of teams with aluminum gears in their drivetrains that just gave up.

There’s a reason the EVO Shifter (which came out for the 2017 season) uses weight-relieved steel gears (example) instead of aluminum.

The 14-tooth gear is particularly prone to failure (this happened all the time in 2016, due to the preference towards large wheels in that season). Aluminum gears are fine in a lot of cases, but not in that particular gearing.

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We reduce weight primarily by Swiss cheesing.

However, our real secret is to take the robot outside onto the abrasive asphalt parking lot and do drive practice for a few hours. Not only does the robot lose weight from the tire treads wearing down, but fasteners and other odds and ends fall off. In addition, this tactic provides additional benefits such as reducing the overall robot height.

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