Hi all, I’m curious to hear the various ways your team removes weight; from “swiss cheesing” to the various ways your team may preemptively plan your robots final weight and everything in between! I would love to hear your story or any anecdotes you may have!
I would also like to throw out a few of my own questions to other teams when it comes to strategies for weight reduction just to get the conversation going.
FEA/ FEM
Does your team do any structural analysis in CAD to see where you could possibly remove weight (i.e non-structural components) while retaining most of the structural integrity?
When designing parts you plan to mill pockets into, do you use specific designs like isogrid, orthogrid, or something in between?
What FEA/FEM software do you use or recommend? (Maybe something like SIMSCALE?) I know Autodesk Inventor has something for this (I haven’t personally played around with that quite yet)
Does your team have or take into account a certain factor of safety when building your mechanism so that you don’t go overboard on the amount of material that’s needed to complete a certain application/ task given the amount of strain/stress it might encounter?
What parts of your robot do you usually remove most of the material from?
Did your team ever dare suggest removing the cool LED lights from your robot just to save on weight?
Ever since our rookie year long ago, when we had to remove 30 lbs in three days because no one bothered to consider weight from the beginning, we plan the weight as we design the robot. Also, working hard to simplify mechanism designs, means they weigh less, cost less, and have fewer parts to break, and usually take less time to build.
We haven’t had to “lighten” robot parts any time recently, that I can remember. And we’ve been using relatively heavy custom made chassis, then the kit chassis, for a long time. I see no point in taking extra effort to lighten the chassis, when leaving it heavy both keeps the robot more stable, but also forces us to design light stuff to go “on top”.
Remove the battery. Funny story from this year, after a long day of assembly we decided to weigh the entire robot and were upset when we were way overestimate. Took about 10 minutes of academic discussion of weight saving before someone noticed the battery. Got a good laugh out of it.
But in other years, we’ve had some pretty serious robot diets. Usually we start with reviewing motors/pneumatics to make sure everything is needed. Then we looked at replacing bolts with rivets. Generally we don’t like swiss chesse, but some form of it always happens, for minimal losses.
Use thinner materials, use polycarb instead of aluminum.
We had almost no 1/8” 2x1 on this year’s robot, and use .09” max for gussets. We also use polycarbonate in many places. We avoid >1/8” materal as much as possible.
Build a short robot if the game allows for it. Anytime you’re maxing out on height you’re going to have a harder time on weight.
Use thinner walled material. You can use 1/16" thick box in a lot of places instead of 1/8", thinner plate, etc.
Plan a weight budget from the start - X pounds for drivetrain, each sub-system, don’t forget the control system. You can plan for this ahead of time. Do this and give yourself a 5-10 pound allowance because extra weight seems to sneak in places or you need to move to heavier materials, more motors, etc. Also use smaller fasteners where possible - 1/4-20 hardware adds up quick.
Weigh as you go. Keep the scale out, put all the stuff in a box and make sure you’re staying on budget.
If you do most of this stuff weight will never be an issue for you. We usually don’t do many lightening holes / patterns at all during the season except in a few spots where it just was silly not to & we already had the plate on the CNC router so why not.
FEA is very easy to do badly in FRC and can give you false confidence in a design. I think unless you have an experienced mentor knowledgeable in FEA it is best used just as an A/B comparison tool (eg to see which of two lightening patterns is more efficient).
In general, robot designs I’ve been a part of haven’t been overweight. If you are always thinking about weight and designing parts to be no heavier than necessary in the first place, and you build a simple robot, it is relatively difficult to hit the weight limit anyway.
In general, I am far more sensitive about weight above the drivetrain than I am about weight below the drivetrain. A common mistake for amateur FRC designers is to spend tons of time and effort in the off-season making the lightest drivetrain design possible, to the point of compromising strength, and then not giving due diligence to manipulator weight, resulting in top heavy robots with fragile chassis shapes. I’m not saying don’t care about weight in the drivetrain, you absolutely should, just that I would rather care about weight in the manipulator if I could only care about one thing.
We use 1/16" sheet metal for just about everything. If not that, then 1/16" wall tube
Designing with sheet metal and 1/16" wall tube is very light. It takes a few extra considerations to ensure things are strong, but I’ve seen many examples of this method working
Some things have to be stronger, like our hooks or some motor mounts. Those were made out of 1/8" material
You need a lot less material than you think you do
As for polycarbonate, if it’s nothing structural, like shielding or guides, then we get the thinnest we can buy. .02" or .03" or something like that. Electronic mounts / belly pan get slightly thicker
I’ve never used FEA. Doesn’t mean I shouldn’t or you should but with enough experience you’ll know what parts you need to strengthen and what parts can be lighter. Besides, all the failures I’ve had were from forces I didn’t expect to be applied, like a twisting force on our hooks that bent them. I only considered up/down and side/side
Weight needs to be in the thought process from the beginning.
Some (but not all) of our team rules:
1: Don’t put anything on the robot that doesn’t have a competitive function. Exception paint and an occasional sticker.
2: If you put it in your hand and you turn your hand over and it falls to the floor, it’s too heavy. This really means that you should always be thinking of how heavy thing are. Weight adds up in a hurry.
3: Use geometry for strength. For instance, a belly pan with some bends an folds doesn’t weigh more than a flat one but can serve as an integral part of the overall strength. Think airplane construction vs. tank or unibody vs. full frame.
A well optimized drivetrain can save a lot weight on a robot, Particularly with brushless motors (my team uses NEOs). I’ve got our bog standard WCD chassis down to ~16lbs
I think the biggest thing you can do to reduce your robot’s final weight is to be thinking about lightness at every step of the design process. If you only discover that your robot is overweight once it’s already assembled, it’s too late to make thoughtful changes. It’s a lot better to design your parts to be light, rather than first designing the part and then lightening it. If you’re thinking about “removing weight” from your parts then you’re probably doing it wrong. There’s a saying “if you shave the ounces you won’t have to worry about the pounds”.
Using things like thin-wall tubing and lighter materials (e.g. plastics) will greatly help to keep the weight off from the get-go. Consider the weight of motors, gearboxes, pneumatics, and electronics in addition to the weight of the physical frame. It is very helpful to budget out the robot weight into individual subsystems; low-to-the-ground subsystems can weight a bit more, whereas high-up or articulated subsystems should be lower-weight. You should be leaving yourself a weight buffer from your calculated weight; I can guarantee a 125 lbs robot in CAD will not weight 125 lbs in person. I suggest leaving 10 lbs for electronics (copper wires are heavy), 10 lbs for bolts if you don’t model them (steel bolts are heavy), and another 5 lbs spare in case something ends up weighing more than expected. And make sure you keep your team’s capabilities in mind when picking a robot strategy. If you don’t have advanced manufacturing capabilities to be at the cutting edge of weight savings, maybe you need to be more selective when deciding what tasks the robot needs to do to effectively play the game.
You talk a lot about FEA in the OP, but I really want to caution against it. I talked a bit about why FEA is dangerous for FRC here: Designing custom frames. It’s a lot easier and usually more accurate to base your designs off of knowledge of what has and hasn’t worked for parts of other robot seeing similar loads, and planning for contingencies if you see that something isn’t strong enough
Back in the day we would pre-mark our lightening holes all over the frame so if needed we could start drilling. We would also use 1/16 wall box and rarely ever use material thicker than .190" plate. We optimized any parallel plate transmissions or designs using an iso-grid pattern to minimize the excess material. The main thing is to plan ahead and assume you will be overweight and can’t go back to take material out latter as you are building.
Our last resort many years was to shorten wires in ways that didn’t look as nice but got the job done to pull pounds out at the last minute. This is literally the least recommended way to reduce weight I can think of. It’s one step ahead of deciding you need to drill holes in the robot controller or motor casings to make weight. (Yes, I really saw people do this when I used to be an inspector)
In an earlier era of design we would make everything out of a sheet metal monocoque design.
That said the new motors and controllers combined with the current version of the cRIO, my first time working with them after taking 5 years off, took so much mandatory weight out of the design I never even worried about it this year. It was that significant a difference.
The best way to be underweight is based heavily off of what @Ryan_Dognaux had to say - smaller robot, thinner material.
Thinner material is the point I will stress - why are you building your ____ subsystem out of ____ thickness wall aluminum box stock? Does it need to be 1/8th? Can it be 1/10th?
You’re going to save more weight by using a thinner material than you’ll ever be able to remove with a hole saw or a good lightening pattern - and then you can still lighten the material further if needed!
For example, .125" aluminum sheet, which is 48in * 144in weighs 84.67lbs. But a .1" sheet of the same size weighs 67.74lbs , the reduction in wall thickness won’t affect performance or strength that much in most applications for FRC, and it will save you a metric crap ton of weight if you are able to consistently reduce wall thickness.
In addition, material selection. If someone ever says "oh well we have to use steel here because of " they are likely not right. Do you really need to use steel there? Or is aluminum actually strong enough? Is a failure of design constraining your material selection?
For example, as a d’oh we accidentally used too small an e-clip on a hex shaft and placed one somewhere it didn’t need to be. This created a stress riser, which caused our aluminum shaft to fail. It was suggested that we move to using steel there instead - but we could have solved the problem by changing our design to remove the excess e-clip.
Oh yes I have been waiting for this topic to pop up for a while.
As in previous years I have competing in FSAE, I have taken a lot of concepts from the very basic a=F/m, so lighter= faster and more responsive, but how to use geometry in your favor, for the most part the students I mentor, have a little of hard time grasping the difference between resistance and rigidity, I do some Basic FEA with them to show how geometry has a large influence on how “bendy” a part can be.
A good exemple I like to show is by bending paper, a flat one will bend with it own weight but if you bend it forming a series of V’s it becomes several times harder to bend
Here you can see the walls of the PC storage is heavily pocketed, why? Because they add almost no rigidity to bending, what increased the rigidity was the aluminum tube chassis mad to protect and accommodate the eletr
I highly recommend this video, and you can make the same experiment with your team,
Once you have the notion of how moment of inertia affects your design, and you start using it in your favor, it seems like magic, but it is just Engineering
Something that has not been said is to consider how you are transmitting power between two locations.
Many Gearboxes in FIRST are surprisingly heavy. Is there a lighter power transmission strategy that will serve the purpose just fine? A good example of this would be if you are running an intake with a Versa planetary on a 3 to 4:1 reduction you could potentially save a good chunk of weight by switching to a belt drive directly off the motor and use the belt to obtain the desired reduction.
Also consider motor selection. When comparing powertrain options for a mechanism, assemble the motor options to gearbox options and weigh them. It doesn’t matter if you are using a lightweight motor if you end up with a really heavy gearbox. In many cases, a motor that has a natural output speed that is close to your target speed, will take less work (and weight) to get the performance you want.
I will also say that bearings and related hardware can end up much heavier than you expect them to be.
Edit:
Also, are you building your mechanism to survive with brute strength and rigidity or by compliantly bending when something hits it? It is much harder to make a mechanism survive hits without yielding than it is to make a mechanism that can bend out of the way when it takes an impact. 1/16" poly-carbonate works in many applications 1/4" aluminum plate does not.
