Every year our robot weighs in, within .5 lbs of the maximum. Do you guys have any recommendations for a lighter but still sturdy robot?
helium balloons!
Also, this white paper
Speed holes
Also, use 1/16th instead of 1/8th aluminum where you can
People often underestimate the strength of 1/16th aluminum rectangle.
Think outside the box, but only by about 3 inches 
Use CAD to make your robot virtually, and then from there find out what you can do to shave off weight.
Also, sometimes being underweight can be bad. In a pushing competition, a 120 pound robot can USUALLY push a 100 pound robot, almost for sure if they have the same friction against the floor and same gear ratios.
256 likes to have their robots just at top weight, since we do love the power to push teams around the field (like when we pushed 254 half field at CalGames when they didn’t go into low gear 
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If you don’t weigh 119.99 lbs, you’re missing something 
That said, do detailed analysis of how much parts are costing you in weight. The more you understand where your weight problem is coming from, the better you can attack it.
Doing additional analysis on just how big a part you need is also helpful. Rather than saying “we’ve always used 1/2” shafts for the drive," learn how to do detailed stress analysis, to determine exactly how big your shaft needs to be. You’ll find that these overly conservative assumptions can add up very quickly. A MAJOR culprit on this front is fastener use. It doesn’t take much effort to determine how big, or how many bolts you need to hold a part, and yet many teams just go with their gut on this. Using too many fasteners, or larger ones than you need, adds up very quickly.
Show us some pictures of your heavy robots, so we can see how they’re made, and suggest different, lighter ways to do it
Tell your robot to lay off the McDonalds…
Why would you want a lighter robot? Heavier robot = more downforce on the wheels = harder to push.
Does the robot need to push other robots around? or does it need to zip across the field quickly? Weight is good for one, bad for the other
Its more of, that we need a lighter frame so we can have (as in logomotion) a better arm that could work better. It would have needed a better claw which would have added more weight.
Also we were so close that we would have to take some stuff of because the regionals weighed us in at different weights. If we were a little lighter we wouldn’t need to worry about being weighed different and taking things off.
When we’ve been seriously underweight in the past, sometimes we would just bolt a steel plate somewhere near the bottom.
Keep in mind that its a lot easier to add weight than take it away at a competition. This should be the goal of every team especially with a tall robot(so you can add weight to the base).
This might even be a competitive advantage. If you make the robot underweight you could add weight to make the robot weigh 119.99lb(according to differently tuned competition scales).
We work on figuring out a light weight design for the “upper” stuff, and don’t worry so much about the bottom of the robot. Once you come up with a basic design of how to make the manipulator stuff work, then spend a few more days thinking of ways to make it much lighter, using different materials, design ideas, etc. Our logomotion robot had a very light weight structure holding the arm, and the arm and claw weighed very little, and were made mostly with materials from the local building supply places like Home Depot and Lowes.
Lightweight stuff includes using thinwall square aluminum tubing and ABS sewer pipe, 1/4" plywood, thin Lexan sheet, thin aluminum sheet, etc. Figure out how to put things together with lightweight fastening methods, such as rivets.
A couple of points when considering robot weight, just from my experience:
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Make sure you use a reasonably accurate scale. Make sure it/they are tared correctly! (I saved us 4.4lb in '99 by noticing that we were weighing the scales)
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don’t forget the electronics, especially wire. Whenever you weigh a pile of parts, just put the whole spool of wire on. Consider a “dummy box” that is full of random stuff that weighs the same as the electronics.
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Remember which parts you can’t modify, and make sure everyone knows what they are! Don’t be scrambling through tech inspection because a freshman put holes in a few motors to save weight. Make a “can’t modify me” sticker for these items if you have to.
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During build week 4 or 5, review everything on your robot, and know what is required versus simply nice to have. Have a checklist of things you would ditch to save weight. make sure they are easy to remove. This is also useful for prioritizing what things still need to be built. Build the must-haves first.
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Remember your triangles and circles. Many things can be made of much lighter weight material if they are structurally sound, like triangles.
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Certain parts can be considered disposable! In 2009, out front cross beam (bumperless area) was 1/16" Alum. L instead of 1/8" because it saved on weight. We had to straighten it at least three times per comp, and had a spare. It could be replaced in a few minutes, and the pit crew could straighten the bent one while the robot was off at a match.
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Have a weight adding plan. In 2007, we strapped over a dozen Lead diving belt weights (plasti-dipped due to the lead-ness of them) tie-wrapped onto the robot. Because they were small and easy to attach, we put them in strategic areas. Each one added a little over a pound. We used them to move the center of mass of the robot more to the center, and closer to the ground. Never came close to tipping.
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Have a parent with a sense of humor that will duct-tape a 12" crescent wrench to the bottom of the robot three days before ship. Makes you think about losing weight sooner, and you get a few free pounds back when he fesses up.
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Consider welding your frame instead bolting it, if you intend to use many brackets. There is surely a local millwright or welder that will be excited to help out, and they are handy for other things too! Hardware adds up. Consider tapping holes instead of using a nut. Makes a great learning experience, and you only have to buy a few extra taps
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There are many jobs that a tie-wrap can do just as well, or at least adequately when compared to other fasteners. Same thing with plastic/nylon bolts or nuts.
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Hollow shafts and tubing can save weight very quickly without sacrificing much in the way of strength.
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Know your safety factor on a design. A safety factor of 5 or more is simply adding weight. Also know which hole will take your safety factor from 3 to 1 and don’t drill it (Another good item for stickers)
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You will be shocked to know how much weight you can get of of 1/16" polycarb with just a 1/2" hole punch and a couple of freshman-hours. (Actually only a few ounces)
Hope this helps!
You can use #25 chains instead of #35 (that you get in the KOP). #25 chains are twice as light, and even though they’re weaker, you can still hang 6 robots (~720 lbs) on a single chain without it breaking (it can hold up to 780 lbs). Sometimes you might want to use the regular, #35 chains in arms and the like, because of the stress (just to be on the safe side, there’s no reason a #25 chain will break on an arm), but there is no reason not to use them in the drive system, unless you don’t have the money to buy them.
One thing to remember about #25 chain is it must be tensioned properly in order to work well, while #35 chain pretty much keeps working even if it has a little slack in it. So if you use it, make sure you have some plan for keeping it tensioned, either with a mechanical tensioner or by designing exact center-to-center dimensions.
True enough… but often at competitions you have to make repairs and/or upgrades to your machine. As you are doing them under the heat of competition, they rarely make your robot lighter.
In Toronto one year we weighed in at about 118.5, broke a component or two, fixed them up and strengthened them a bit and… then had to be reweighted for elimination rounds.
120.0 lbs.
I’d also suggest building your robot a little under sized in each dimension so that in the event of a major impact causing a slight frame twist (or adding a bolt to make a repair) you don’t go from legal to illegal.
As a tech inspector I am always pleased when size and weight are mere formalities and not serious obstacles.
Jason
I could go on for days, but I’ll list the few that bring in the biggest weight savings for the least effort for us.
-thinner materials (both tubing wall thickness and plate in generally). Thinner extrusion tubing is far lighter and far more efficient in terms of strength.
-Plastic spacers/standoffs (they’re cots from mcmaster in all sorts of sizes from $5-10 for 100 QTY) instead of custom metal.
-Smaller Fasteners. We try to only use #10-32 (which is killer strong if you check the math), and have now implemented #4-40’s to supplement them in lower load spots.
-Focus on Quantity. Look at what parts you robot has a LOT of, take a small bit of weight from them (either in thinner material, pocketing, whatever is is) and it adds up fast.
-Plastic fasteners for electronics and low load spots (plastic rivets as well).
-Rivets instead of bolts for many items. We tend to add a lot of little gussets, brackets, doo-dads, wire tie downs, etc… If we bolted all these on versus rivets, it’d add up FAST!
Most of what I suggested above is pretty much effective use of COTS fasteners, and would maybe cost a team $100-200 per season versus their usual methods, but can easily save 5+ lbs without changing the function, strength, or performance of the robot. Pick up 100 1.5" long 1/4-20’s with their associated washers and nuts and you’ll see what I mean.
Also, if you want to use bolts, try and use the smallest possible. According to stories of the older guys in my team, in 2010 the robot was 0.5lbs overweight three hours before shipping, so they cut all the bolts and left only the length they needed, and that got them to just under 120lbs. Since cutting bolts is not recommended, becuase it makes them alot harder to open and close, you better use small bolts to begin with.