Gear Reductions for a Robotic Arm Using Fusion360

My name is Nachum Twersky and I'm currently in a senior Queens College located in NYC, many of my students have uploaded projects for the Fusion360 contest, so may the best man win (or qualify)

This is my Instructables for how I used Fuion360 to model and print two different types of gear reductions for a robotic arm I'm working on. this Instuctables focuses on two types of gear reductions, Planetary and Strain wave (otherwise known as harmonic drive)

Ill discuss why I chose the gear reductions I did, as well as the steps I took to make them using Fusion360.

This project was heavily inspired by a video by 3DPrintedLife, who at uploaded a video right at the time I started brainstorming for this project. his design was perfect, but not suited for the type of machine I wanted to build. So they look similar, but the design is completely designed by me in Fusion360.

This arm will run on an Arduino UNO using a CNC Shield. I currently use GRBL to control it for testing

Supplies needed

1) Laptop

2) Fusion360

3) 3D Printer

4) LOTS and LOTS of time

5) some various bearings

6) various screws

7) MORE TIME

8) The ability to cope with a lot of failure

9

Gear reductions are necessary for almost all motion using a motor, almost every dc motor uses some type of gear reduction, most commonly a spur gear reduction. It causes the motor to spin slower, but exponentially increases the torque output. The larger the reduction, the slower the motor will spin, but the stronger the motor will be. Most of it comes down to the kinetic energy put into a system.

So why do I need a gear reduction? I planed on using the readily available (and cheap) NEMA 17 stepper motors for my arm. the issue with this is that the motors are super weak. they really don't have much of a holding torque. and in order for my arm to be able to hold anything, it needs some decent holding torque, specifically at its base joint.

So it was settled, I need some kind of gear reduction, the question was... Which one?

Now, I'm not going to get into an argument as to which gear reduction is best, obviously each scenario requires different mechanical systems. In my case I needed 3 specific characteristics. I needed it to be a REALLY good gear reduction. I was hoping for a 30-1 reduction, meaning that when the motor spins 30 times, the output will only spin once. I also needed it to be in a small package (bye bye spur reduction). Since I wanted this to be a desktop robotic arm, everything had to be in a small package. The last characteristic is that it needs to be CHEAP. I cant spend hundreds of dollars on bearings and spacers and screws... you get the point. Most of the parts need to be 3d printable, and I wanted the cost to be below 30 dollars per gear reduction, of which, 2/3 of it will be going to bearings anyway.

Because of these "limitations" I only had 2 options, planetary, or strain wave. Unfortunately the cycloidal gear reduction would've been too expensive bearing wise so I had to forget about using those. the question now is... what are those gear reductions?

Planetary gears look amazing. their gear reductions are fairly simplistic. you have an outer ring gear, an inner "sun" gear, and the smaller planet gears. there's a lot of math involved in designing these, so I just used a planetary gear calculator on google to determine the tooth count. these gear reductions are back-drivable, and stackable allowing you to get some insane gear reductions in decently small packages. for example, you everyday drill have a stacked planetary gearbox to gear the speed of the motor down and to give it enough torque.

This is the planetary gear simulator i used to determine the tooth count and overall reduction for my first attempts

http://www.thecatalystis.com/gears/

Since i had no idea how to design anything gear related in fusion360 I took to YouTube to offer me some guidance. There's an amazing series on gears by Antalz, https://www.youtube.com/channel/UC9_6o_CDwuqoVZ5H0...

and I highly recommend you check his channel out.

To skip over the bring math bits, I first used the Fusion360 plug in for Spur Gears. which can be found by clicking Shift+S and then scrolling down to find the Spur Gear plug-in.

After playing around with the backlash, and the number of teeth I used that gear as a cutter, to cut away from a circle leaving me an outer gear. I then made 3 smaller gears using the same tooth profile, just less teeth as the planet gears. I then made a housing to hold everything with screws. lastly I added the sun gear, which is what ends up being attached to the motor. I just made a cover to keep everything contained and that was it! a simple 4-1 planetary gear box.

The problem is that I mentioned before that I needed 30-1, unfortunately after spending a couple of weeks working on this I had to scratch the planetary gearbox. it was the right size, it just didn't have the reduction needed, and therefore not giving me the torque necessary to make a functioning robotic arm

The first thing i need to say about SW gear reductions (AKA Harmonic Drives) is that they're basically magical. I have no clue how they physically work, but the concept is simple. You have an outer ring gear, similar to the planetary gear reduction. you also have a flexible thin gear. thin as in with a giant hole in it, and a wave generator which flexes the flex spline and magically causes it to rotate in the opposite direction. I've included a GIF from Wikipedia below to show how it works.

Its basically Magic.

The reduction works as follows you take the number of teeth in the flex spine and subtract it by the teeth in the outer gear, then you divide that number by the teeth on the flex spline. This if you noticed will leave you with a negative number. this is because the flex spline rotates in the opposite direction as the wave generator leaving you with a "negative direction" reduction.

Now, I originally thought you had to design it in an extremely convoluted way, mainly with a giant flex spline, and this huge housing. Very similar to the way How To Mechatronics made his which is shown above. by the way, definitely check out his video/channel while you're at it. its definitely worth it.

However, another amazing YouTube came out with a video on how he made his 3D printed strain wave gear that didn't use a giant flex spline and housing. In fact, it completely changed the way ow looked at strain wave gears.

He's using them to make a larger robotic arm using NEMA 23's. this gear reduction is a little too intense for what I'm looking for, plus, its not made for the motors I plan on using. So I basically took his idea and modified it to fit the specs/bearings/motors that I was using. On to the design!

Just to reiterate what I needed, a small profile with a nice gear reduction. To start off I originally tried to make the ring gear using a spur gear to cut the profile out of a circle. However after a couple f tests it didn't work at all. The teeth would just bind regardless of how much backlash I had. I then decided to go for what I called the "Shark Tooth" method. which is basically cutting some tall triangles into the outer ring gear, as show above. after some initial testing it worked very well, so I decided to stick with this design from now on. the bottom and top both have the "outer ring gear" and the flex spline fits between them. however, the top has more teeth than the bottom, allowing the top to spin with the flex spline. The flex spline has the "Shark Teeth" pointing outwards to fit into the outer ring gear.

Now using another amazing Fusion360 feature, The McMaster-Carr catalog! To find this, just go to insert, and click on the "McMaster-Carr catalog". this feature allows you to insert almost any screw, nut, bearing, motor or any other type of hardware into your project to get exact measurements. its a feature that I've started using on almost every project. using this feature I important the 4 bearings I would need to make my wave generator. I needed the least amount of friction possible, so bearings were the way to go. once you have them inserted into your project it allows you to design everything around these precisely measured components. Much easier than having to buy and measure the real thing. once I had all those designed it was time to print it out and give it its first test. The outer casing and the wave generator were printed in PLA. I tested almost every type of material for the flex spline. PLA, ABS, PETG, TPU, nothing worked well enough until i got my hands on Nylon filament, and a printer that can print it. Nylon was flexible enough, and strong enough that it would break under the constant stress. SO i stuck with nylon for the flex spline

NOTE: the design above is my final version, the videos below also include one of my first versions that was too big for the arm

And below is the final version attached to the NEMA 17 with a large 6808 bearing to keep it radially fixed

Now that I had a working version I was finally able to start designing the robotic arm. however it is important to note that this is a 29-1 gear reduction. My flex spline had 58 teeth and the outer gear had 60 teeth. land using the equation I mentioned earlier I had (58-60)/58 = (-1/29). unfortunately due to size constraints I wasn't able to reach my 30-1 but more on that in the next step.

This arm is easily the hardest thing I've had to design. In it there are going to be 3 joints, not including the z rotation axis. its going to use 4 stepper motors, with a servo to activate the claw at the end. with a strain wave gear reduction at each joint.

Now, since the stepper is in the base I also have a belt and pulley reduction for the main reduction. the pulley reduction is a 3-1 reduction, which then connects to the 29-1 reduction. this leads me with an 87-1 gear reduction. which is perfect. it can currently lift its own weight at the end of a .3 meter arm. and after multiple torque tests the strain wave gear reduction allows the motor to have 6 Nm of torque. which is much more than .2Nm that a stepper motor normally has. Now that I've had time to print a lot of it out, here is the video of this gear reduction working!!!!

I'm extremely happy with the movement, and the efficiency of the gear reduction, which wouldn't have ben possible for me to design without Fusion360 and all of the features it has to offer.

ill leave the link to the strain wave gear reduction on my prusaprint profile page in the future. I still have some small tweaks to make before I can officially publish it.

Thank you for reading, and I hope you can embark on designing your own gears/ reductions and have as much fun as I did