Weight Saving Techniques

[Orion.DeYoe]

I spend a good portion of my free time CADing robots and different mechanisms. The issue I constantly run into is that I get really close to the weight limit before I've added fasteners or any of the other small things that add up to a large amount of weight (rivets, wires, fasteners, spacers, adhesives, decorations, electronics, roller chain, etc.).
So my question to you is:
What techniques do you use to keep weight out of your design? What high-weight situations do you avoid? What materials do you use to get the strength you need without putting your robot overweight.
Please note that I'm not asking how to use a hole saw on practice day at a competition (been there, done that), I want to know how you keep weight out of the design during the CAD and design phase.
For those of you who have experience with high-functioning robots/teams, is having an overweight robot a problem you face regularly? Or are you able to keep everything under 120 pounds from the beginning?

[Jon Stratis]

If the weight limit is 120 lbs, we try to design to 100 lbs. That way when we come in over our intended weight, we still aren't overweight.

In 8 years, there's only been one time (Lunacy) we were overweight at competition, and that was because we were attempting to hit 120 lbs exactly. Fortunately, we had plenty of extraneous steel ballast on the robot that could be removed to bring us down where we wanted to be.

In all the other years, I doubt we've been over 115.

[mlantry]

a good power point from IKE about losing weight http://www.chiefdelphi.com/media/papers/2220

[TheHolyHades1]

I suppose a big portion of it is what materials you use. Obviously, try to use the thinnest materia possible without compromising strength. I've seen designs using anything from .04" of versatubing to 1/8" thick box channel and thicker. I know we tend to use the heavier stuff in our drivetrains and the lighter stuff in mechanisms; generally the 1/8" box channel for drivebase, and 1/16 for mechanisms. 1/8" plate tends to be more than enough for most of our purposes; we only used 1/4" plate for hooks during ultimate ascent, and we probably could have used 1/8" there too.

The other thing is to use belt and aluminum/plastic pulleys wherever possible. You can pocket gears and still retain the strength for a smaller weight. Pneumatics substitute for motors really well in most cases; they are a good bit lighter when using many.

That's all I can think of in terms of general ideas ATM. I suppose most of the weight really comes down to how the mechanism is designed; you may want to review your design idea overall first?

And then there's always swiss cheesing ...

[MechEng83]

One of the techniques my team used this year was having a weight "budget" for each subsystem, with a 5 lb reserve.

It worked fairly well, because it forced each team to consider what their own pieces weighed and keep track. Someone had to "own" each piece on the robot -- including fasteners. I actually developed a chart with weights of each size screw we would ever consider using and every fastener is accounted for. Educated forecasts of system weight is key, but so is the interaction between subteams to reallocate the weight "resource" so we have a functional design.

Having accurate CAD and *thankfully* a sponsor who could laser cut to our CAD specs was a big help. Designing parts with features and supports in the right place without extraneous material is key. It's a lot easier to remove a pound in CAD than it is with jig-saws and drills after the fact. This is the first time in several years (except UA since we wanted to climb and came in at 90 lbs...) that we weren't swiss cheesing our robot at competition.

This is the part where I hail the benefits of FIRST not being about the robot, but what kids learn through the process.

[themccannman]

Try to figure out how much stress a part is going to encounter (remember safety factor, it's better to be close to the weight limit than to have important parts failing at comp). Then design with thickness and pocketing according to predicted stress. There's plenty of resources online with strength data on all kinds of materials.

[tim-tim]

Originally Posted by Orion.DeYoe

What techniques do you use to keep weight out of your design?

Through the course of time we have learned that we are really bad at guessing weight and initial capability. We used to hold different groups to a firm weight (w/little wiggle room). Over the past few seasons we have developed an excel tool that allows us to keep updating our estimates and eventually compare to our measured weight of the robot. There are two screenshots attached of the tool.

What high-weight situations do you avoid?

I don't know of any 'high-weight' situations we avoid. We like to iterate through CAD. We will spend time to develop a [hopefully] better solution that maintains same baseline functionality, less weight, easier to maintain. Always look for COTS solutions, if nothing else it allows you to compare ideas and draw inspiration.

Bellypans are another area to save some weight or resources. We choose not to use our sponsors for bellypans. We feel their time is better spent making aluminum lightweight in other places. A good piece of birch has served us well over recent years - lightweight, easy to machine, isolation, rigid, cheap, etc.

Plastic (Lexan/polycarbonate) is heavy.

What materials do you use to get the strength you need without putting your robot overweight.

We use a lot of aluminum... but the alloy and thickness tend to vary with the design intent/use. We hardly ever design anything on the robot for more than .090" thk. This year our chassis was .090", which turned out to be more than we really needed; but the game was rough, so it was worth the weight. Most of our mechanisms will be .063" thk. I would say that 97% of our parts are 5052-H32 or 34 (depends on sponsor). This alloy allows for tighter bends and an overall more compact design. When needed we will step up to 6061-T6 or 2024-T3. Sometimes flat pieces are cut from 6061-T6 anyway because that is the typical material from one sponsor. We don't weld much, so that doesn't play much of a role in material selection.

is having an overweight robot a problem you face regularly? Or are you able to keep everything under 120 pounds from the beginning?

We had an issue with weight in 2012 - it got out of control real fast and led to the creation of the excel tool. Other than that, we try to develop our own weight limit as a team - it may or may not be the weight limit listed in the rules. The new approach we use is that each team provides updates on weight on a regular basis, if we are over or getting close to the our team's limit we will discuss and adjust (weight limit is not fixed as long as under 120lbs).

I have heard some teams use phrases similar to: "We are given 120lbs limit. If the robot only weighs 115lbs, then you left out 5lbs of engineering."

At the end of the day, find what works for your team. Every group will be different, and have access to different resources.

[Chris is me]

Especially with the new smaller size restrictions, making weight is not nearly as challenging as it used to be. We aren't building giant double jointed arms, hanging mechanisms that extend 10 feet in the air, or double ramp robots anymore, and as a result it's a lot easier to make weight.

Here's what 2791's done to easily make weight every year:

First, build a simple robot. A simple robot has far fewer problems making weight than a complex robot. For the past several years, if we're planning a design that is going to have trouble making weight, it is probably too complex for our team to do effectively. Dozens of pure cycling robots in 2013 were under 100 pounds. Some of this year's low catapults were nearly as light.

Second, we try to build a robot so that the robot does multiple tasks with single mechanisms (i.e. combination arm and shooter in 2014, a shooter-hanger in 2013, bridge tipper and drop down intake in 2012). This isn't a universal rule as sometimes it is simpler and lighter to just use two separate mechanisms (2013), but it's a good way to keep part count and weight down.

Third, use close to the minimum amount of structure needed to do the job. I've seen tons of robots have trouble making weight with large square tubing frames with tons of redundancy. If you really think about it a lot of superstructure frames can be built with very few members. Our "superstructure" in 2013 was a set of 4 1/4" plates. Even counting the shooter, it was just four pieces of 2x1 and some 1/16th sheet. No problems.

One of the coolest examples I've seen of minimal structure is 233's 2012 robot. They recognized that the ONLY superstructure needed other than the intake was to mount the shooter high and near the back of the robot, so that's all they built. The conveyor belt just ran in open space, not surrounded by frame the whole way down.

Finally, use the lightest parts you can get. Vex gears are a huge weight savings over AndyMark and can be used in all but the end of some of the highest load mechanisms. Build with more 1/16 wall, especially above the drivetrain. Don't go to steel or thick walls until you have to.

With all of these ideas combined together, we haven't had to really actively think about weight for the last half decade. If we started going down the path of a design that required complex structure or a high number of gearboxes / motors, we would probably start budgeting weight in CAD, but we try to avoid those designs if at all possible.

[apalrd]

Thin.

We use a lot of 0.050" and 0.063" 6061 T6 sheet metal. I can't think of any part thicker than 0.063" off the top of my head. Maybe a few brackets.

[thefro526]

If you haven't already, check out this paper written by IKE back in 2009: http://www.chiefdelphi.com/media/papers/2220

Most of the basics are pretty well covered in there, along with some pretty awesome tricks.

Personally, when I'm designing something, I like to 'plan' how the weight will be distributed. Each subsystem / mechanism will have some amount of weight allotted to it, and that total weight is usually a bit under 120. From there, it's primarily about designing things to be 'light' on the first go around - if it's a part that you know that you only want to make once...

Another thing I like to do is 'hide' weight in places, just in case we need to find a half pound or two later. Sometimes this is done through using (or allotting for) a heavier version of something, an example being the older Thomas compressor rather than the current Viair unit, and sometimes it's through building something out of thicker material than it really needs.

[RRLedford]

Study the design principles of Buckminster Fuller and mother nature.
Increase the ratio tension elements to compression elements.
Tension elements deliver force with way less mass required than compression & torque elemnets. Efficient and low mass design maintains a good balance between tension & compression elements.

-Dick Ledford

[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!