Weight Saving Techniques

[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.