Weight Saving Techniques

[DampRobot]

The number one way to save weight is to use thin tubing. We made almost everything (including things that saw direct impacts from the field and other robots) out of .0625 6061, and didn't see any failures due to lack of strength. Honestly, when's the last time you saw a piece of tubing fail because it was too thin? It's much weaker to take thick tubing and pocket it than just use thin tubing. Our superstructure weighed 6lbs this year, becasue it was all .0625, and was probably the strongest superstructure we ever built. Here's one of our kids holding up the practice and comp frame in each hand:


On that note, get strength from geometry, not material thickness. If you're doing sheetmetal, use flanges liberally. If you do CNC and you need thick material just in one place (for a bearing, etc), pocket the rest down to .0625" thick. It usually takes out more weight than triangular or circular lightening patterns, and is a ton faster to design.

A lot of your weight is in motors and gears. Changing from steel to alu gears is a major weight saver (and usually, the steel FRC gears are worse quality anyway). Each CIM is almost 3 lbs, and MiniCIMs are 2.2lbs. Banebots motors are usually a lot lighter (and a 775 is more powerful than a MiniCIM). If you need to cut weight really fast, a good bet can be taking off motors.

You will use at least 2 lbs of welding wire on your robot if you weld your frame. If you powdercoat, plan on adding somewhere between 1 and 3 lbs. Wiring weighs at least 10 lbs (and can be much, much more). Sidepanels (even .030 lexan) weigh about 1 lb if they're decently large. Stuff like that can eat up weight pretty fast.

Changing from bolts and nuts to pop rivets saved us a sorely needed 3 lbs in 2012. We try to pop rivet everything possible now. They're even faster to remove and replace than a bolt (wanna race?).

Basically nothing on your robot should be steel if you can possibly help it. OK, if you're using really thin steel instead of alu (because you've done your strength calc/FEA homework), then maybe steel is a reasonable choice. But it really eats up the weight when you start using it in stuff like sprockets or hubs.

You can usually get away with ABS sideplates for gearboxes. It can save a lot of weight compared to alu, especially if you need a specific thickness and don't usually pocket. It machines really nicely too.

When cutting weight, you can usually only nickle-and-dime yourself one or two pounds of weight (by replacing fasteners, drilling holes, etc.). If you need to cut more than 5 lbs, seriously start thinking about whether you can take off any motors, motor controllers, or frame members. It's usually easier to withhold an entire mechanism and lighten it out of the bag than trying to file corners to get just bellow 120 ship day. And if you're more than 15 lbs over, start preparing to remove a subsystem. Something big has to go.

Lightening holes take a lot of time, and don't usually save "that much." Depending on what thicknesses you're using, you'll probably save on the order of 5 lbs over the entire robot by lightening stuff. Even if it's huge, you can probably only save a pound or so in one piece of tubing by lightening it. Most gearboxes, you'll take out less than a pound by lightening. It can make the difference, but there are often easier ways to cut weight. And, pocketing (and making the pockets look pretty) can really eat up a designer's time.

Beyond just making 120, weight up top maters a ton more in terms of performance than weight down low.

Every designer has certain ways they like to cut weight, those are a few of my tricks and pieces of information picked up from 4 years of FRC.

[adciv]

Two things I've seen done, and I didn't notice these mentioned above. Lets call them "alternative materials".

1) Use of wood in select areas. We've used Baltic Birch for our electronics board and wheels before. This has worked out well for us as some of the electronics need to be insulted from the frame, and we can use standard wood screws to secure the equipment.

2) One team uses Carbon Fiber (Carborundum?), made in their own shop, for their supertructure. They're the only team I've ever heard of that had to ADD 20lb balast.

[Jon Stratis]

Originally Posted by adciv

2) One team uses Carbon Fiber (Carborundum?), made in their own shop, for their supertructure. They're the only team I've ever heard of that had to ADD 20lb balast.

A quick note about Carbon Fiber, since it was brought up here: If you use it, please make sure you know what you're doing first! When machining carbon fiber, you can create a lot of fine particles that can irritate skin, eyes, lungs, etc. For that reason, a lot of venues where events are held may not allow local machining of carbon fiber for safety reasons, and you don't want to be working with it every day for 6 weeks without taking appropriate safety precautions (proper ventilation, breathing masks, etc).

[AluminumNarwhal]

Try to use as few wires as possible when doing your electronics. Make the wires just the right length, instead of just using zip ties or something to control the excess wire. It may seem obvious, but my team didn't do this the first few years. When we cleaned up a particularly wasteful wiring set-up, not only did the robot look better, we lost 7 pounds!

[Kevin Leonard]

A lot of people on this thread have mentioned using thin-walled aluminum as a great weight savings technique.
A lot of people on this thread have not competed at a New England event.

Some of 20's success over the years hasn't been about having the best robot at an event- but by having a robot that survives throughout the event.

Thin-walled aluminum can be weight savings, and it is true that geometry>thicker walls when it comes to making parts stronger.

But some things absolutely deserve thicker tubing and stronger materials- and when you neglect that, you sometimes pay the price when it matters most.


I put together a presentation for my team last year on how to build robots for reliability.
I won't go into it extensively, but my essential point was there are two components to reliability on a robot- durability and maintainability.
The robot needs to be durable enough to survive matches, and mechanisms need to be able to be replaced or maintained in order to keep the robot in peak condition.
Things like the frame of the robot aren't replaceable- and thus mostly not maintainable, so they need to be extra durable.

I'm not saying not to use thin-walled aluminum, because that would be ridiculous. But remember the trade-offs you make when you make parts out of certain materials. Do the math, then add a little safety factor. You might thank yourself later.

[Gdeaver]

Fiberglass pultrusions can both save weight and increase durability. Composites in general can save weight. Composites require different construction techniques than metal. There is plenty of good info out on the web on composites for teams to research. This year our ball intake was made out of 1" pultrusion and survived massive hits.

[Gweiss96]

Holes save weight. Lots of holes.

[cadandcookies]

Originally Posted by Kevin Leonard

I put together a presentation for my team last year on how to build robots for reliability.
I won't go into it extensively, but my essential point was there are two components to reliability on a robot- durability and maintainability.
The robot needs to be durable enough to survive matches, and mechanisms need to be able to be replaced or maintained in order to keep the robot in peak condition.
Things like the frame of the robot aren't replaceable- and thus mostly not maintainable, so they need to be extra durable.

I'm not saying not to use thin-walled aluminum, because that would be ridiculous. But remember the trade-offs you make when you make parts out of certain materials. Do the math, then add a little safety factor. You might thank yourself later.

I'd be really interested in seeing a white paper on this document-- often I see teams with few engineeringg mentors/less experience with major reliability problems that probably could have been avoided.

Back to the topic, planning in your lightening is definitely the way to go-- if you can lighten a part without sacrificing functionality (ie in a chassis- 1/8" box tube is totally overbuilding and can in almost all cases be lightened), plan it in and drill your lightening holes before assembly. Just transfering to doing this saved my team from almost all our previous weight problems for the past few years.

Outside of that, using fasteners intelligently (as previously mentioned) is also huge. You DO NOT need screws and nuts in a lot of the cases I see them-- properly sized rivets, welding, or even using washers can save you a ton of pain, both in weight and maintenance.

[MechEng83]

Another technique we used this year was designing with weight reduction ideas in mind. An example was the chain on our drive system. We spaced the wheel centers so that 35 and 25 chain would both span the distance and built with the 35 chain. We ended up not using it this year, but it could have potentially saved us about 1.2 lbs. As others have stated, there is a trade off with weight and durability.

Our frame is welded aluminum rectangular tubing with 1/16 wall. This is great for weight (our frame weighed a little over 5 lbs), but it does come with its own challenges. The closed cross section actually provides a lot of strength for the material, but it's not invicible. After a particularly hard head on crash at Chesapeake this year, the middle section of the frame is dented in behind the bumper. Using screws to attach other components to the frame very easily leads to "dimpling" in the areas of the screws as students are not careful about stopping when the load gets tight enough.

We have an anodization sponsor to color the robot, which actually is a weight savings over using powder coat. Aluminum gears are almost a standard of our DTs. We save ~2 lbs just by swapping from steel. Several years ago, we made a switch from using 1/4-20 bolts to using 10-24 screws. Rivets are used where it makes sense.

[jijiglobe]

To lose weight you just have to diet... GET IT!?!?
I apologize in advance.

[BrendanB]

This was one of our worst years for battling weight but really every year we have had a weight problem.

2011 we were rookies so that doesn't count but we grossly overbuilt a multi level superstructure all out of 1/8in box tubing. Like Chris said, this is where most teams waste weight because they make them too complex or they avoid combining elements together to eliminate parts.

2012 we were 130lbs after our scrimmage event. To get under we removed the covers on the AM supershifters and replaced them with aluminum standoffs and 1/32in lexan. We moved to an off-board compressor and put some holes in our wooden shooter sides which got us down. We remained at 120 and I wish we had used more 1/16in tubing in our frame so we could have added a stinger. We passed inspection by cutting off a small zip tie to get stop the scale from twitching between 120lbs. and 120.1lbs.

2013 our first robot was about 2lbs over. After our redesign we were about 106lbs.

2014 we had a completely designed robot in CAD that said we would be 30lbs under without wiring and lexan shields but wound up being about 7lbs over. As it turns out several of the steel components like a few axles and the spring had no weights entered nor did most of the electronics and smaller gearboxes. We cheese holed our intakes and several other parts and removed nearly a pound of useless PVC spacers on our intakes. We also used thinner lexan for all paneling which helped a lot. Never using 1/8in lexan for covers unless we have a lot of weight to spare because you can't get thin lexan smoked. We also swapped some of the 1/8in cross beams for 1/16in tubing.

In the future we will look into using more thinner box tubing elements in our design. Like Kevin said, playing in New England is very brutal but you look at teams like 118, 971, 233, 254, etc who put so many mechanisms and parts on their robot and are either underweight or just at it. Definitely hoping to find that balance.

Something else is to lighten parts as you go. Its easier to throw a few holes in a part you are making before you assemble the robot and find out hours before bagging you need to shave down. Obviously not all parts should be lighted for the heck of it, but there were several parts on our robots over the years that didn't need the thickness/surface area from the beginning.

Properly planning for electronics and wiring is crucial. Many teams pull the "throw everything on the scale" but fail to remember all the wiring/pnuematics they aren't taking into account. Having a 5lb-10lb buffer is good to design around. It gives you some wiggle room to iterate throughout the season like adding a blocker, stinger, minibot ramp, 20in extension (2013 FCS), etc.

[MichelB]

(I didn't have time to read the entire thread so I apologize in advance if this has been said before but...) SWISS CHEESE!!! While my team always tries to estimate weight and remain under the limit we always seem to go over, that's when we break out the hole saws and cut as much extra material as possible away without compromising the structural integrity of the part. Since most of the force is taken on the edges of the piece, cutting holes in its center won't affect it too much.

[JamesCH95]

Here's a little quip that has kept 95 underweight since I started as a coach in 2010: worry about the ounces, the pounds will take care of themselves.

Edit: *zips up flame suit* I don't like 'swiss cheesing' at all. It has some sort of appeal in FRC that I don't quite understand. It's not very strength-efficient and it takes a ton of extra labor to execute. When I see swiss-cheesing all I can hear in my head is: 'my designer didn't understand my strength requirements or my weight budget and decided to make me weaker by drilling holes in me than by simply remaking me out of thinner or less dense material.' I do not intend this in a condescending way, I used to swiss-cheese parts too, until that little voice popped into my head.

Edit pt2: here is a quick FEA I did of a swiss-cheeses 1x1x1/8 6061T6 tube (on the left) of nearly the same linear density as 1x1x1/16 6061T6 tube (on the right). The middle is fixed, and both tubes' upper surface as 100lbf applied to it. The Mises stress distribution clearly indicates that the 1x1x1/6 tube is experiencing about 1/3 the stress as the swiss-chessed tube. The thinner 1x1x1/16 tube is around 3x stronger than the swiss-cheesed 1x1x1/8 tube for the same weight. Keep that in mind the next time the 'thinner wall vs swiss-cheesing later' thought comes to mind, or when you think that cutting holes in the center won't affect it too much (looking at you MichelB)!

[Gdeaver]

Thank You for the picture. It's hard to get this across some times and a stress picture is worth a thousand rants.

[RKazmer]

Originally Posted by AluminumNarwhal

Try to use as few wires as possible when doing your electronics. Make the wires just the right length, instead of just using zip ties or something to control the excess wire. It may seem obvious, but my team didn't do this the first few years. When we cleaned up a particularly wasteful wiring set-up, not only did the robot look better, we lost 7 pounds!

This is actually really important in a design. Wire might not seem heavy at first, but add up all the wires you need in a robot and it will weigh quite a lot. Also, it is always better to wire short as not to complicate the wiring and the power drop is also a factor when wiring power to the drive motors or the main power switch.