That sounds about right. I wanted to have 1 tube be barely strong enough, to be sure that under weird edge cases and dynamic loads, it doesn’t break one and then the other.
We just bought a couple off Ebay (0.78" (20mm) OD, 0.078" (2mm) wall thickness, 40" (1000mm) length: 1M 1000mm Roll Wrapped Carbon Fiber Tube 16/18/20/25mm Matte/Glossy 1mm/2mm Wall | eBay
I’ve been watching these threads with great interest wondering if the forklift climb will work this year for teams as it was not viable for us in 2019. I think it is much simpler this year and should work well since the forks can be significantly longer than was allowed in 2019.
I will recommend that teams consider using pultruded carbon fiber rods rather than tubes as we had several failures with tubes like the ones you linked in 2019. Not sure if your calculations took this into account, but there are significant stress risers where the tube connects to the box tube that joins it to the rest of the mechanism. We had a tube fail very violently at that interface in 2019. Once we switched to pultruded rods we had no issues.
Interesting, I don’t know much about pultruded rods. Our plan for reducing the that riser is to slide the carbon tube into a tight-fitting 7" long aluminum tube, and hope that that reduces it enough to stop a catastrophic failure. What was your attachment solution in 2019?
The issue is the sharp edge where the tight fitting aluminum tube ends. That effectively becomes a new fulcrum point and you have the mass of your robot on one side of that fulcrum, hence the stress riser. Our mounting solution was a machined billet that the tubes (and eventually the rods) were pressed into with some epoxy
EDIT: I will say I haven’t done any math to support my concerns other than empirical data from 2019, however the mechanisms were different then so it’s possible that the designs this year work fine, but I would advise caution and alertness when testing.
Here is a video of our 2019 forklift from the side where the attachment method can be seen: FRC 1923 Level 3 Hab Uncensored - YouTube
According to Q&A 54, the center of gravity doesn’t need to be above the charge station if the robot is supported by an alliance partner and the top of the charge station.
Thrifybot Elevator Kit Transformation
Hello! As some of you may know, we at 7407 are planning to build a robot that has 3 DOF “arm” with a pivoting shoulder, a linear elevator, and a rotary wrist. And wow, would you look at that: there’s a picture of our robot design attached below. One of the main reasons we were able to crank out our CAD assembly so quickly (besides feeding CAD students nothing but energy drinks for the past two weeks), was thanks to our decision to purchase the Thriftybot Elevator kit and then adapt it to fit our needs. This saved us a lot of time designing and manufacturing parts.
Before we get started tho… A really big thank you to our wonderful sponsor Thriftybot!!! Their monetary and technical support has been invaluable for us ever since our 2022 off-season, and our team wouldn’t be as efficient as we are today without their help.
First and foremost, let’s talk about the parts we’ve directly implemented from the elevator kit - the bearing block assemblies. These blocksh have saved us a ton of machining troubles (especially since my team doesn’t have mills in shop). The only small change we had to make was creating a 3D-printed tube plug to mount the bearing blocks onto the existing REV MAXtube patterns.
Now onto the slightly changed parts. We are still using Thrifty’s chain runs from our base to 1st stage, and then using a pulley system from 1st to 2nd stage. However, we re-machined the blocks that the chains bolt into in order to space out the sprockets and chains more horizontally (the side view spacing is still the same) so we can fit a NEO motor with 2 planetaries in there, we also cut a new block to hold the idler pulleys in to reduce the spacing (building a 3 stage elevator that is only 10 inches wide is hard).
We also 3D printed cable tensioners that are adapted from the ThriftyBot ones, but instead of being mounted on the side of the carriage, we mounted it at the bottom of our center 2x2 tube.
We are also using two of Thrifty’s 4 in. diameter squish wheels on our claw intake. They are super compliant and have great grip on both the cube and cone. Hope this helps!
This is incredibly compact and incredibly well done in such a short time. kudos to you guys
This looks incredible, nice work 7407. Can’t wait to see it all come together!
I am very eager to see these types of “buddy” climbs this year. But these Q&A responses are likely to cause a rule update to include a blue box clarifying how “suspended from” will be assessed in matches. Maybe include language on whether the alliance partner is required for the robot to be off the carpet?
Closest example I can think of is this:
The hook/purse is suspended from the surface (by Q7 & Q42). BUT if something needs to apply a downward force on the top of the hook (an alliance partner) to keep it put, then it’s not suspended (by Q54)?
Sorry if this is too granular. But I think teams considering this (awesome) idea should really be careful.
Hey James, do you have the CAD files for these tube inserts? We’d like to print some to prevent tube crushing (which unfortunately happened today, without inserts).
Here’s the OnShape file from our student: Link
I will edit my original post with the link as well.
Hello! How do you prevent your arm from extending above the 6’6" height limit during the match/inspection? Does the geometry work out to reach the high row without extending too far vertically?
Hey 7407,
We’re looking to implement a similar wrist joint design for a part of our robot and are wondering where you bought or how you made the 40t and the 10t spur gear that is on the end of the arm in your cad design. Thanks!
Hi! We will be limiting the travel extensions of the elevator in software so it cannot go above the 6’6" height limit. With the wrist on the end the geometry does work for reaching the high level without breaking the height limit and the extension limits.
Hi 4918! We actually created these gears in Onshape using the spur gear tool, and we 3D printed them on a Mark Forged.
Hello folks, we just wanted to provide a bit of a weekend update, as we have made some great progress in the past two days. We just finished the first assembly of the elevator, and the tensioning system seems to be working great! We plan to add a couple of motor plates for rigidity, but other than that, it looks clean!
This is looking gorgeous so far! Very much looking forward to seeing this “ThriftyPink” ™ Arm in action under driver control.
We have kicked around a few different names for this already:
When 401 originated this design they originally called it the Not-Quite Pink Arm.
Fuchsia Arm?
This is a great diagram. I didn’t see it mentioned, so I wanted to point out a critical design detail you got right and I think needs to be made obvious for those who may be looking to copy this kind of design:
I added d, the distance from the tipping axis to the wheels, should be greater than zero. Really bad things will happen when the robot tries to pivot if d=0 or worse, d<0.
