View Full Version : weight advice
SharkBite
04-02-2002, 12:37
we desperately need to cut down some weight, does anyone know any tricks to do so (besides drilling speed holes)
Chris Hibner
04-02-2002, 13:38
Originally posted by SharkBite
we desperately need to cut down some weight, does anyone know any tricks to do so (besides drilling speed holes)
The trick is to do some math.
We had to lose a lot of weight last year. We tried to do it as systematically as possible and it worked very well. You really need to find the densities of the materials you are using and do some calculations.
An easy way to get started isto determine the weight of a 1" long piece of the most common materials on your robot. For instance, you can say that each inch of aluminum tube is X lb, each inch of steel bar is Y lb, and each inch of 1x1 8020 is Z lb, etc. This will give you an immediate idea of what you're up against and allows you to determine roughly how much material you need to remove to lose the necessary weight. It also gives you an idea of how much "bang for the buck" you get by removing one thing vs. another.
Usually, you won't be able to get rid of an aluminum bar because it is supporting something. Then you need to think to yourself, what if we cut it in half lengthwise and make it an aluminum channel? Will it still work? If so, calculate how much weight you save. We were able to turn enough square tubes into channels last year to save about 5 lb. We didn't turn the entire tube into a channel (we left the ends square for mounting reasons), but we made the middle section a channel.
Also, holes are very inefficient for removing weight. You can do much better in terms of weight loss with little reduction in strength by making slots or pockets instead of holes. For instance, if you are planning on doing holes, run a mill between every other hole to make a slot or pocket. If the member doesn't carry a lot of load, connect three or four holes in a row. You'll be amazed at how much more weight you remove by doing this rather than by simply doing holes.
Another HUGE saving comes from sprockets, pulleys, and gears. You can usually remove about 1/2 to 3/4 of the material in a sprocket, gear, or pulley if you do it properly. If we left the gears and pulleys on our robot for this year the way the came off-the-shelf, our robot would be about 5 lbs heavier.
There's no reason to use a solid steel gear like they come. Take off a good portion of the hub. Mill spokes from the hub to the gear face instead of leaving it solid. Also, if the face of the gear is 1/2 inch, there is no reason that the material from the hub to the face has to be 1/2 inch - usually you can thin this out significantly (just remember to leave the face at the necessary thickness).
I hope this helps. The most important thing is to do some calculations and have a plan before you start to reduce weight. We lost a total of 15 lb last year without losting any functionality or robustness. We made sure we had a good plan and did some math first. It worked really well.
Drill more holes!
It also depends on how much you need to lose. An ounce here and an ounce there will make a diffrence, even if you need to lose 10 lbs. How much do you need to lose?
SharkBite
04-02-2002, 22:20
thank you so much, we had figured out the material parts and the math and such... but your advice about extra gears and pulleys is gold.... didnt think of that
Anton Abaya
04-02-2002, 23:51
also, consider getting rid of a mechanism at last resort.
sometimes removing a drive wheel and replacing it with casters is also a solution.
-anton
ps. you can always drill holes in your battery :P (j/k)
SharkBite
05-02-2002, 16:12
that is absolutely the last resort though....... our drive train doesnt allow for that as of now
Elgin Clock
05-02-2002, 19:25
On our robot(s) we have changed a brass bushing into a Delrin one, We have also "Swiss Cheesed" the <edit>ROBOT</edit>!
It all depends on how much weight you need to lose as well.
We are going with the slotted idea to reduce weight on our side panels, instead of a bunch of holes.
NOTE:
MAKE SURE YOU FINISH DESIGNING YOUR ROBOT BEFORE REMOVING MATERIAL!!!
Last year we jumped the gun on machining holes and accidentally machined a spot where a future mechanism sat. We had to weld a panel in the spot we machined, :( Not a fun thing to do!
Chris Hibner
06-02-2002, 09:05
I forgot another thing: Lexan (and other plastics).
Wherever possible, substitute Lexan instead of aluminum. This is advisable only when the part you're substituting doesn't carry much load (if it carries a lot of load, you'll need to do some calculations to see if it will survive and not bend too much). For instance, all of our control system parts last year were mounted on an aluminum plate. We replaced the aluminum with Lexan and saved about 2 lb. The heat conduction properties of aluminum was nice, but the matches only last 2 minutes so it's not that necessary. You may need to reinforce the Lexan with a couple of thin aluminum strips to keep it from sagging too much, but overall, you can save a lot of weight.
Also, a lot of bearings can actually be replaced with delrin bushings. I would keep the bearings in your drive wheels, or other areas that cary an enormous amout of load, but delrin bushings are adequate for most spinning shafts that don't carry a lot of loading.
Just remember that you don't need to design your robot for infinite life. As long as it lasts a few months, it will be fine.
Mike Norton
06-02-2002, 10:06
To get rid of wieght. First don't build something that will have alot of wieght
Everything you put on the robot have someone look at it and see if there is another way of doing it lighter.
We used it put holes in our robot many years ago. But now we make sure what we put on the robot is the best wieght we can come up with.
we run with track. within 3 years we took out 10lbs just in the track system. this is because we figure a life span of a 1000 hours. after that we have to go in and start changing things.
if you give us more information about how your robot looks or going to run I bet we could help you out alot more.
CaptainPlaid
07-02-2002, 16:52
Depending on how bad the weight problem is you can get very creative. For example, if you have a 1/4-20 bolt there is wasted weight on the very inside of the bolt. DRILL IT OUT! "Hollow bolts" take a while to make but can really save some unexpected weight if you are desperate.
SharkBite
07-02-2002, 17:55
you guys have come up with some great stuff, but unfortunately we are stuck drilling holes
you see, we tend to have an overweight robot every year and have to cut it dont last minute due to simple deseigns that end up being eleborate (i know you guys know what i mean on that one) we end up building everything out of the lightest material possible from the beginning and thats why we are in such a crunch now, even drilling holes (pockets slots included) may not remove enough
i wish i could explain the design and see if anyone could help from there, but my team tends to operate under a code of secrecy for the most part (just certain mechanisms) and i wouldnt want to infuriate anyone
i never thought of drilling out bolts though.... its a small area but youre right, it would add up
oh, and i bet this brainstorming about saving weight has helped other people too, so thank you guys
our bot was over weight so we went out and bought titanium bots and nuts and washers etc... 130.0 after that
I think that some people worry too much about secrecy. At this point in the build, no one is going to change their robot design because your idea is better. This is because there isn't enough time.
Just my thoughts
"I have one word for you - PLASTICS."
http://www.microplastics.com/
UCGL_Guy
11-02-2002, 13:05
Welding your frame or pieces together saves weight and adds strength over bolted systems if you have the means.
KY
Brian Savitt
11-02-2002, 13:20
Look for heavier pieces of material on your robot and think what you could replace it with that would make it lighter, and there is always the option of drilling a million holes:)
Brian Team 56
bigqueue
11-02-2002, 13:22
Originally posted by Chris Hibner
I forgot another thing: Lexan (and other plastics).
Wherever possible, substitute Lexan instead of aluminum. This is advisable only when the part you're substituting doesn't carry much load (if it carries a lot of load, you'll need to do some calculations to see if it will survive and not bend too much). For instance, all of our control system parts last year were mounted on an aluminum plate. We replaced the aluminum with Lexan and saved about 2 lb. The heat conduction properties of aluminum was nice, but the matches only last 2 minutes so it's not that necessary. You may need to reinforce the Lexan with a couple of thin aluminum strips to keep it from sagging too much, but overall, you can save a lot of weight.
Also, a lot of bearings can actually be replaced with delrin bushings. I would keep the bearings in your drive wheels, or other areas that cary an enormous amout of load, but delrin bushings are adequate for most spinning shafts that don't carry a lot of loading.
Just remember that you don't need to design your robot for infinite life. As long as it lasts a few months, it will be fine.
And then if you find Lexan too dense for your weight budget, you might want to look into Luan Plywood.
Luan Plywood is 1/2 the density of Lexan.....sure, it isn't as strong, but there is alot of lexan use that is simply there to things like ball containment....not requiring so much for strength.
-Quentin :)
CaptainPlaid
11-02-2002, 13:32
Expanded aluminum is also a good option for things like ball containment and even for supporting top and side loads if you give it a bit of support. Last year we used it as the deck on our robot (http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=2008&direction=DESC&sort=date&perrow=4&trows=3) and it worked wonderfully. It would support humans and robots. Essentially it is plate aluminum with more holes drilled in it than you could possibly do on your own.
Neill Means
11-02-2002, 16:25
Practical Weight considerations for Rookie Teams!
A Note from a Veteran:
If your 'bot weighs 130.0 lbs, my suggestion is to get it down to
129.5 lbs. The difference is small, but the difference between your shipping scale and the USFIRST shipping scale could be the difference between 1/2 pound (about 0.5%)
According to the rule books, the USFIRST scale at checkin is the "absolute" authority, so if you are 0.1 lbs over, they will not let you compete until you are 130.0 lbs by THEIR scale. There is NO debating with the officials about the accurracy of their scale at the competition. NONE!
If you want to live on the bleeding edge of a 0.5% margin of error, then show up with a 'bot at 130.0 and bring power tools for cutting.
Rookie teams should probably bring their 'bot in at 129.5 lbs. Use a recently certified shipping scale that has a weighing platform at least as big as your robot is. The shipping scale will be a monster! Bathroom scales will give you false readings.
Neill Means
Team 933
P.S. Do you know that Earth's gravity changes according to where you are in the world? So, different masses (your robot) can have different weights in different parts of the world (small differences).
Wayne Doenges
11-02-2002, 17:47
I agree with Neill. Get the weight below 130 lbs. We use the wrestling scale at the school. Very accurate. Still our bot gained a 1/2 pound from regional to the championship.
Wayne Doenges
I don't know about this year, but the scales used last year also had a slight variation depending on where on the platform you placed the robot. As I recall it was definitely to your advantage to get the robot CG centered on the scale. Note: depending on your robot design that may not be the center of your robot.
For us it was the difference between making weight and getting out the drills;)
Originally posted by ChrisH
I don't know about this year, but the scales used last year also had a slight variation depending on where on the platform you placed the robot. As I recall it was definitely to your advantage to get the robot CG centered on the scale. Note: depending on your robot design that may not be the center of your robot.
For us it was the difference between making weight and getting out the drills;)
As a wrestler, I am familar with this. The scale at are school, you could shave 1.5 lbs by putting all the weight towards the left edge of the scale, and similar type things on the other scales at the diffrent schools. Don't know why, but thats just they way it is.
Matt Reiland
11-02-2002, 22:32
I wish first would let the robot weight be 131 pounds:D
Ohh well we came in at 131 fully loaded and we were actually able to shave off 4lbs off the electrical system if you can believe it. The terminal strip and a huge amount of 10 ga. wire is now on the floor and we have more weight now for armor. By far the biggest weight cam from gears, we were able to reduce most of the larger 20 pitch gears by 70 % by turning down hubs and drilling holes, and in steel terms thats alot of weight sometimes over a 1lb at a time. Also look into the fastners on the robot, we found BOSCH gussets in places they were not needed and extra t-nuts where they weren't needed. Heck we welded a bunch of parts to make them lighter also.
I wish we could weld....:(
Patrick Wang
12-02-2002, 01:50
This is just the slightest bit ironic since I just finished 4 and a half days of welding for our ball mechanism.
Welding 0.049 wall aircraft tubing is not fun. Makes you also have to bend your body in the wierdest of ways. All while TIG welding.
I'll say it was a challenge, but certainly not the most fun thing I've had to do heh.
Uggg!! 20-25 pounds over. Must dump components. Crap...stupid planning.
Radiant Phoenix
12-02-2002, 19:13
Dude, we were almost 70 pounds over at one point, and we were incredibly worried that we'd never get down to 130. We have to drill so many hole in our steel, and we lost half our treds to casters. Almost there though.
Remodeling was a last resort, but last resorts are often necessary. ::sighs::
Good luck on the Robot diets Minna-chan!!!
Athena
SharkBite
12-02-2002, 22:11
if youve got so much steel you might want to try aluminium or titanium if you can afford it
Put the controls system close together and especially have the thermal breakers close to the battery, because the six gauge wire is very heavy. Try to position electronics so that you use the least wire as possible. Thats how our controls team dealt with weight.
Andrew Dahl
13-02-2002, 09:17
carefuly placed holes in teh battery
(that joke gets better every year)
dahl
Matt Reiland
13-02-2002, 09:43
I thought shedding 2 lbs was tough I can't even imagine the pain of over 10 without totally removing something important
Like dets002 said the wiring adds up, we save over 4lbs alone on getting rid of extra wire and a terminal strip setup.
Neill Means
13-02-2002, 12:36
We lost 10 lbs off our 'bot last night by welding our extruded AL frame and taking out all of those joining fastners. 10 lbs! WOW!
It does make a difference.
Neill
Team 933
Curtis Williams
13-02-2002, 18:41
Another idea is putting your battery, breaker, and fuse panel close together. Less of the 6 guage will save a significant amount of weight.
Maybe pump the air out of the pneumatics before you weigh it? Dont want any air weighting you down :)
Originally posted by Curtis Williams
Another idea is putting your battery, breaker, and fuse panel close together. Less of the 6 guage will save a significant amount of weight.
Maybe pump the air out of the pneumatics before you weigh it? Dont want any air weighting you down :)
Can to pneumatics maintain a vacuum without power? Can they maintain a vacuum at all?
Curtis Williams
13-02-2002, 19:08
I have no idea. I think the everything could hold it, but I dont know how the motor or pressure releif valve would react.
CharlieWilken
14-02-2002, 22:53
We were 17 pounds over yesterday. So we got rid of our 4 motor drive. We got rid of our atwood motors and home made transmission and the extra wheels and chain. OH WELL...ITS THE JOURNEY THAT COUNTS!!!
i SHOULD HAVE BROUGHT A SCALE IN SOONER
You can go to Small Parts and purchase an assortment of various sized holes to place on your robot for weight reduction. They are expensive - only the very rich teams can afford them - but it is convenient to be able to move the holes around until you get them just right. We had a box of them around, but unfortunately I mixed up all the sizes in one box and the little ones fell through the big hole and I never found them again.
We tried running the compressor on helium, but the robot had such a squeaky little voice we couldn't stand it.
Permanent magnets are allowed in any amount. You could try aligning their fields against the earth's magentic field to generate some lift. Of course, your 'bot will have to be able to handle directional derivatives, and convolution integrals aren't on the additional hardware list. You might be better off working in the frequency domain.
Next year Exide is coming out with a battery that uses massless neutrinos. Getting rid of that heavy box full of electrons will be a big help - just not this year.
There may be a topological way out, though. If you start with a two-dimensional Moebius strip, and move up to the three-dimensional Klein Bottle, there should be some way to take the next step to a four-dimensional construct (let's call it a "Kamen Manifold") that would get rid of the mass entirely. Then you can work with all kinds of neat things, like PVC pipe with an OD smaller than the ID so that the inside is on the outside.
By the way, how strictly do you think the ref's are going to enforce L'Hopital's Rule this year? I mean, you have to set SOME limits...............
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