Gear project

We have some seniors who are involved with robotics and have taken on the challenge (their Idea) to pick up about 400-500 lb with 2 CIM Motors and a 400:1 3d printed gearbox. So currently they are getting down to picking 1 of 3 options that were investigated to actually finish the design and put it into plastic.

Compound Differential Planetary.
Planetary taking the place of the oval bearing inside the flex gear of a Harmonic drive
Compound Differential Hypo Cycloidal.

It will be printed in Nylon and has to fit into a cylindrical envelope of 8 in or less in diameter and less than 10 inches long. Budget besides the motors and some hardware that robotics will provide for the project is <$ 120.

Comments welcome including “are you crazy” (answer is yes) but its worth a try and better than have them suffer from senioritis.

The bottom line is to watch the torque at the load end, and to a slightly lesser extent the linear load on the output manipulator. I am not intimately familiar with 3D printing materials or what specifically you are using, but if I were planning to pick up a quarter ton with FRC COTS parts, I would either have a VERY small spool (less than 1" in diameter) at the end, or use sprockets or pulleys on a dead shaft (or an oversize shaft for FRC purposes) for the final reduction.

Of course, anything you can do to reduce shock loads over and above your nominal loads would be effort well spent.

They intend to connect muliplr 14 inch steel rods in a 120 degree circular pattern and run it up a stairway kinda like a handtruck that drives up a flight of stairs the Nylon will hold in a static load that has been tested the math with the torque works out the big question is can they design it and build some circuitry to provide the 60 Amps each to each Cim motor

First of all, who does that. Secondly, please don’t run a CIM at 60 amps consistently, tone that down a fair bit.

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What is the purpose of this project? Is it to learn about 3D printing or to test the strength of those materials? Or is it about the design of the gearbox?

First of all, the 2 CIMs have the power to lift 500 lbs at more than 1 ft per sec without getting anywhere close to the 60 amps you are talking about. We lifted 500 lbs at 1 ft per sec with our original 2018 climber design. Using JVNs calculator, with 2 CIM motors, if you had a 400:1 ratio gearbox and a 2 inch diameter spool with a rope on it, you would only need 6 amps per motor to lift 500 lbs. So the current to the motors is not the problem here.

As @GeeTwo stated, the limiting factor is going to be the strength of the output shaft. A larger diameter output shaft will help. The load at the gear teeth will be another significant factor, but if you make the gears long in the axial direction, you can spread out the load quite a bit. Since it is a custom gearbox, you can design the load and stress profile on the gear teeth to stay within the material properties of your 3D printed material. The early stages (closer to the motor) of the compound gearbox will have lower loads than the later stages (closer to the output shaft), so if you design for the output end loads, and use the same gear ratio and design on each stage, you will not need to get into the details of the other stages.

But these are some pretty sophisticated calculations and design considerations for your high school students. Are you planning to use some of the built in FEA tools that most of the CAD software has these days to analyze your design? Or do you have some gear design handbooks that you are using?

Good Luck!

Someone who’s curious and has some time to mess around… What’s the issue? It’s a learning opportunity like anything else. It could be fun
//
@mpirringer I’m by no means an expert on this topic. But for some inspiration, here’s a cool YouTube channel that does 3D printed gearbox tomfoolery: Gear Down for What?. He has lifted an anvil using a standard cordless drill into a 3D printed gearbox in this video. Oh and he’s also made a 3.6 trillion:1 gearbox fully 3D printed in this video. From what I can tell, it looks like he’s mostly doing stacked planetary stages with 3DP herringbone gears but I haven’t looked too closely.

Good luck! Post videos if you end up pursuing this :slight_smile:

Thanks - and yes its going to be a learning experience for the kids. They want to try - they have saved some money - hence the budget and well I am willing to spend the time to help them try.

It will not have an output shaft per say. As you can use a planetary as an analog as all of those systems are part of the epicyclic family the output will be the ring gear of the 2nd stage. It will have some tangential legs connected that will “walk up” the stairs

As for running the CIM’s 40-60 amps I pointed it out and they came up at least on paper with a heavy duty fan.

As for Gear Down for what - that video is what inspired them

They started the project as a homework in a CTE class where it was “come up with a technical project and make a presentation” They did. And Now that school winds down they want to actually build it. So they came to me the mentor crazy enough to give everything a shot. The project as said is to motorize a handtruck to take a heavy load up a flight of stairs. The win currently is defined to make it up 16 stairs at least once with hopefully a combined load (moterized hand truck and payload) of >400 lb. The first task I gave them was to come up with a budget and parts. So the school is donating some filament - mostly PLA for prototypes - they saved up 120 bucks so far and since team 1989 has been doing robotics since 2007 and we are getting 2 CIMs a year and with that have about 2 dozen robotics is donating the 2 Cims and a battery and some steel pipe that has been laying around for ages.

As for me I am not going to stop some teenage guys to either succeed or learn that you don’t always win. And its going to be a project in true team 1989 fashion, Very little money, a pile of junk left over from other seasons - now see if you can make something out of it.

As a mentor. As a result of that so far. They did the math to evaluate the benefit of a compound planetary vs a Hypocycloidal, vs a harmonic - for me that is a win as how many teens have done that. They devised and are in the process of executing some tests to see how the printed material will survive 400-500 lb - that is a win and useful for the future. Some custom bearings were designed and 2 of them made it into this seasons robot as at least on paper they have been working on it since December. So that is a win.

Now IDK if we are going to walk 400 + lb up that flight of stairs that goes from the maker space to the libSrary. But it already has inspired some fresshmen and sophomores and juniors to say “can we help”. We have some trying to come up and pick parts to make a PWM circuit that can supply 40 amps max from a throttle using a H-bridge Mosfet design and yes I taught them what a H-bridge and a Mosfet is - and that is a win too. So at the very least we are going to find out if we can make a gearbox that lifts that kinda weight which can come in handy if there is a task like that in a future season, we are going to have more members who can design a circuit board and I got at least to freshmen who know as a result what pitch and addendum and dedendum and pressure angle and helix angle etc and a planetary and harmonic and cycloidal gear system is and what the look like and how the work as they want to help too so as a mentor I feel we have already won.

Besides every failure is just one iteration closer to success

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For all the right reasons. THIS is what First is about.
Good luck Vikings, be sure to let us know what happens!

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Found a new personal motto!

Then, whether the CIMs could handle it. If you check out the vex motor testing page:

https://motors.vex.com/vexpro-motors/cim-motor

You will see that running at “peak power speed”, the current dropped below 60A in less than 6 seconds. Running locked at 6V, the current fell to less than 40A within five minutes, and I from energy considerations, I suspect that it would have failed significantly shortly thereafter.

Also note that the heat generated in a brushed motor is in the rotor. As the CIM is a sealed motor, there really isn’t a quick way to get the heat out of the rotor; the best thermal path is to somehow attach a heat sink to the output shaft!

If you’re looking at intermittent operation (no more than about five minutes at a time, with perhaps an hour in between), OK. With anything close to continuous operation, I doubt CIMs could operate at 60A unless you’re doing extreme cooling (WAY more than a fan blowing ambient air).

Looking for about 2-3 minutes maybe less for this. I know the CIM is not the perfect motor for this but its what we have. Its an attempt to get something up the stairs once. You could call it a proof of concept if you want. The big thing that keeps the project from being lets say “commercially feasible” is first and front the battery as we are going to be lucky to get 5 minutes out of the battery. As said before the task is can 120 bucks for filament and a pile of old stuff lying around bring a combined load of about 400-500 lb on a motorized handtruck up a flight of stairs. And can it be done before those seniors graduate. See it as team 1989 version of junk yard wars if you wish.

Let me know if you need any additional CIM motors for this effort. We have a box full of CIMs going back 15 years that we probably never use. We are moving to a new build space this summer and we will probably throw away at least half of those CIMs. We could put a couple in the mail for you.

Oh we can always use them that would be great. Funds are always tight. Every december we disassemble last years robot and put the parts back on the pile we usually have 1000-2000 bucks for new stuff but this years robot had parts from 2007 in it so any parts robotics donates to this project will be disasembled after it does or doesn’t work and probably goes into the 2020 robot. heck if the gearbox works we might have one for a climber. Right now besides those guys doing their project we have secured a couple 100 bucks for filament to see how much more we can do in plastic. Currently this moment printing some axle holder which take our toughbox minis which were in some cop years ago and have been used ever since to move the wheels. Will up a video to my channel as soon as the scateboard works. Materials used HiPS ($12/kg at hobbyking)

You can contact us through

http://www.vernonrobotics.com/

Thanks

Ooh if you are getting rid of CIMs, I’ll be happy to take some!

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