Hey, I mean, if a mechanism falls off, it counts as weight savings. Right?
Yes. Bonus points if this is achieved via high speed collision with a parked automobile or a dumpster.
*teach a freshman how to use the lathe and save the $2 per gear.
As many have pointed out, if you’re “removing” weight, you’ve already lost. The robot needs to be light from the start. Make mechanisms preform multiple jobs (looking at the dual elevator teams in 2018), cut down on the number and length of frame members, pick 6wd or even 4wd over 8 when it makes sense, etc. Using a PTO is pretty close to the point where it’s basically the same weight to just add more motors (if you have the slots) which is unnerving.
There’s a post somewhere around (in the “very old” category) where someone was talking about how the Apollo spacecraft were de-FODded. They’d be put on shake tables or similar to shake all the loose stuff out of the capsule. Several 5-gallon buckets of FOD later, the capsule was FOD-free (well, hopefully…)
One way my team removes weight every year: “EricH, get your foot off the scale!” (I do a pre-inspection of our robot every year. Often I’ll lean a foot on the scale just a bit to throw it off…)
Funny story on aluminum gears: I once was looking to lose weight AND size from something at work, and took a look at the VEX aluminum gears. Well, I looked at that… and then I decided I’d better do some engineering, and started working through the gear tooth stress analysis. (Quite a fun little math problem, for any enterprising students, BTW–and if you’re really enterprising make sure to do a fatigue run on that as well.) I didn’t like the numbers I got. (Of course, it might have helped if the book I was checking didn’t only cover steel gears…) I checked again… and designed for steel gears. Aluminum just wasn’t going to be strong enough for that particular application.
I tried to suggest removing all brackets and joints on the robot in favour of attaching them via constrain tool. Mechanical apparently doesn’t like it when the CAD team makes jokes.
I don’t know about the other two, but IIRC our KOP drive weighed in at around 20 pounds with all things factored in last year, which is pretty similar in weight to a COTS swerve using SDS-style modules
… yes, I suppose they are in absolute terms when considering the meta of “one swerve drive module per corner” (that has been broken in the past 148 in 2007 and 525 in 2016 come to mind)
But you do not need 4 modules to swerve around like an absolute pro, so I suspect the mass of two standard design modern swerve modules (i.e. sds) and 2 caster wheels is roughly comparable, especially if you use #35 chain on the WCD.
I’m not doing this to pick on you, but I do want to make sure people understand the importance of wall thickness choices.
For an example, I made two 18" lengths of 6061 1x2 tubing. One is .125" wall, with a pretty extreme hole pattern on the 2" faces. Each hole is 0.5", to match a reasonably available drill bit, and the pattern is through both faces. The other is .0625" wall, unswissed.
According to Onshape:
Swiss cheesed .125" wall tube: 0.877 pounds.
Unswissed .0625" wall tube: 0.631 pounds…and a less sore arm from all the drilling.
Ehh. If it’s for your drive train especially I wouldn’t recommend this as a weight saving tactic for most teams. Your drive train is the most critical component of your entire robot and if a gear dies (see Richard’s post above or any of the other number of dead aluminum gear posts on CD, this is not uncommon) then you’re dead in the water for at least a match. And it will be a very important match probably because it always happens at the worst time.
Usually accessing drive train gearboxes that are buried under mechanisms can be one of the worst things you have to work on during a competition too.
Do the fractions of a pound really gain you that much benefit over having confidence that you’ll never have to touch your drive train all season long? My suggestion is to find the weight elsewhere and stick with all steel gears in your drive gearboxes.
Does the red make the swiss cheesed one go faster?
On top of that, the plain 1/16" wall will almost certainly be stronger in most applications due to no stress concentrations caused by removing load-bearing material. I haven’t done the official analysis, but I would pick the unmodified tubing over the thicker Swiss-cheesed material any day.
Another option I’ve seen some teams do is put a lightening pattern but not have it cut through the tubing so as to retain the majority of the structural integrity, minimize stress concentrations, and loose some weight as well. This obviously is a LOT of work for what is usually marginal gains, but it’s a good option if you’ve already trimmed everywhere else (and you want to improve the cool-factor of the bot too).
Good example from one of my favorite robots this year:
I’ve been working on cars for a long time…I’ve never seen aluminum gears in the drivetrain. But I have seen them in low load applications, such as driving the camshaft, where they outlast fiber gears.
Interesting to see who is advocating what concepts here. Mentorship in action.
No, but stickers add five horsepower as always.
It is magnificent work, when a team has that option. Pocketed plates used to be in fashion for drivetrains in the late 2000s, especially down among the top Florida teams like 233 and 1251. But like you said, it’s very time- and labor-intensive for a small gain–and that’s if the machine access is even available to you. The average team thinking Swiss cheese is probably whipping out a hand drill, or maybe a drill press if they’re lucky.
We Swiss cheese (#9 holes 0.5” OC) not so much for the weight savings but for the ease of positioning (and re-positioning) attachments.
This is a weight relief thread. Functional holes (even if they’re not being used for a function at the moment) are beyond the scope of both my demonstration and this thread. If you have the manufacturing capability to do those kinds of hole patterns, they’re certainly nice to have.
Two birds, one stone.
Some of these strategies may be different if you are trying to just make the weight limit, or if you are trying to make a lightweight fast robot for strategic reasons.
Lately we’ve been chasing the latter. Ditching the pneumatics is my first go to, pretty much every other weight reducing idea has been mentioned so far. Fast robots are fun. 80lbs is about right.
.196" holes, 0.5" spacing, top and bottom.
No holes: .631 pounds.
With holes: .618 pounds.
Delta: .013 pounds, or right at 0.2 ounces, or about 5.9 grams.
It may be two birds, but those birds are female bee hummingbirds.
I guess I hadn’t entirely realized how great the weight difference was between thin and thick wall tubing as you demonstrated, even with lightening holes.
In my previous post, I had been primarily referring to methods used when the bot is at or over the limit right before competitions, compared to when the robot is being designed and constructed. When designing our robot, we generally try to use thin wall tube as much as possible, and as thin sheet metal as we can get away with.
Lightening holes are used as a last resort.