...
With that, the sketch schematic is finished! Note: I've found that the equation driven curve is a bit wonky and randomly flips planes while I work. Found that if I save and close the sketch it goes back to normal. Not too sure what the deal with that is.
Step 6: 3D Modelling
Cycloidal Discs
Next, it's time to move to 3D modelling. Lets start with the cycloidal discs. Both discs will be extruded to a length of 12mm to match the needle roller bearing I spec'd out. I added some ribbing to reduce weight because a 12mm thick cycloidal disc seemed super chonky. These aren't backed by any load calculations lol, I just offset everything by 3mm. Disk A (bottom disc) is on left, and disc B (top disc) is on right. I gave them different ribbing patterns so you know which ones are which. The rib patterns will actually face each other (this will make more sense when I show the assembly. Both have a material of 7075-O aluminum, but ideally 7075 hardened aluminum should be used for these discs (as that is what other teams use).
Ring Pin Housing
After the cycloidal discs, it's pretty easy to design the ring pin housing. The only notable thing here is that I made my ring pins 25mm high despite the cycloidal lobes being 24mm high. That's because I want to have a slight gap to sauce in grease, and will also be putting a gap in the cam to create some distance between disks 1 and 2. More on that later though. Also, it's a good idea to have good clearance for your cam shaft here - make it as big as you can while making sure the cam/bearings are actually supported on the plate. Motor mounting holes are also important to put here, but we don't have any motors picked out so I just randomly put M4 tapped holes inside this part in a circlespec'd this design to be compatible with this cheap stepper motor for testing. 7075 aluminum is also the material to choose here - as explained by Monash. Not ideal compared to steel in terms of wear, but light so it is good for the arm.
For rev 1, I'm also going to arbitrarily add mounting holes to the outer profile for the cycloidal drive lid. 
Eccentric Shaft
For the cam shaft, I separated it into three parts - a shaft adapter for the motor, then the eccentric cams for each disc. All components will be separated by a set screw for now, but probably a key way on a more refined revision - I just need to get familiar with keyway installations. Having this shaft be three separate parts allows for easier trouble shooting, although it does complicate assembly. I may want to try to combine this into one part in a future revision, but for now its just gonna be three so modular testing of different discs are easily accessible. The shaft has a double D profile so I can use the same cam adapter on both sides of the eccentric shaft. I made the shaft in a separate assembly to keep simplify mates in the main assembly, so I added the roller bearings over each eccentric cam.
Next up, I added a large plastic spacer to fit around the shaft in between the eccentric cams. This spacer is used to reduce the friction between the two cycloidal discs and is made from PTFE. It is a custom part, but should be easy enough to work with. For context, PTFE is a very low friction plastic that is commonly used in dry running bushings. I was thinking we just buy 1mm thick stock sheets of PTFE, 3D print a cutting jig for the washer and exacto knife the sheet. We really don't need any tight tolerances for this piece. It would probably be better to replace this with a thrust bearing, or it may be unnecessary all together if we just use good grease.
Lastly, I added a flanged bearing to sit on top of the second disc, and interface with the output shaft. The output shaft and input shaft are concentric, but rotate on different axis. I went with a flanged bearing to minimize contact area between the two. Forces on this shaft should be low (just from my intuition), so adding the bearing is really just to reduce friction. This could probably be replaced with a plain bearing if need be in the future, we would have to test and see how things go. Shaft stack up is done!
Output Shaft
Next up, I modelled the output shaft. First off, the output shaft was designed to match the hole pattern of the dowel pins (65mm diameter, 12 holes). However, instead of going with a 5mm diameter to match the dowel pins, I went with an 8mm diameter. I wanted to add plain bushings to these output holes to allow the dowel pins to rotate smoothly with lower friction , so I found some cheap oil embedded bushings and added 8mm diameter holes for their installation. The large hole in the center matches the outer diameter of the thinner part of the flanged bearing. A cross section will be shown later to give greater context on that one. The bottom of the output shaft plate features a shoulder for the bearing flange, and the top has tapped holes for shaft output mounting.
I mated the bushings and dowel pins into the output shaft plate, and added another PTFE spacer to the top of the assembly. This will eventually interface with the lid for low friction, as you can see in later pics. The dowel pins are a little bit short for this initial design as I wasn't able to find dowel pins with exact lengths. TBH we could probably machine these dowel pins ourselves and save a lot of money if someone has lathe training, its literally just a cylinder.
Main Assembly
Now I can add all the parts together. The ring pin housing mates directly to the stepper motor. The cam shaft adapter, eccentric cam A and roller bearing B are all mated to the motor shaft. Disc A is mated to the roller bearing.
The PTFE spacer is stack on cam A, then all the parts for disc B are installed on the shaft. In actual installation, the flat side of both disc A/B and their cycloidal profile should be greased up.
The output shaft fits inside the common holes between both discs and then is installed on the flanged bearing. Pins should definitely be greased to allow for easier installation into discs. Then, the lid is fixed on top of the drive using the holes on the ring pin housing. The top of the output shaft sticks out from the lid about 0.5mm (off by 0.001 due to some imperial conversion BS). I did this intentionally so any components on the output don't rub against the fixed lid.
Cross Sectional View of Assembly:
I didn't include fasteners or set screws as they aren't super critical for testing this out in CAD (i got lazy).
Step 7: Troubleshooting
After making the assembly, I couldn't get the cam mate to work for the cycloidal disc and the ring pins. Shit did not mate whatsoever. I also tried to do a motion study to validate the design, but I was treated to this bad boy:
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The fact that I was unable to mate the cycloidal disc to the ring pins with a cam mate is a pretty bad sign. Something did not go right with this initial revision lol.
After a lot of investigation, I think I was able to isolate the issue. The cycloidal disc and ring pins weren't mating properly as I used an repeating decimal fraction in the equation, as R/(EN) = 50/(0.75*40) = 5/3:
- X = (50*cos(t))-(2.5*cos(t+arctan(sin((-39)*t)/((5/3)-cos((-39)*t)))))-(0.75*cos(40*t))
- Y = (-50*sin(t))+(2.5*sin(t+arctan(sin((-39)*t)/((5/3)-cos((-39)*t)))))+(0.75*sin(40*t))
This made SW absolutely shit the bed. I went back and redid all the model generation, this time using an offset of 0.8 to get a definite number. The equations now read as:
- X = (50*cos(t))-(2.5*cos(t+arctan(sin((-39)*t)/((1.5625)-cos((-39)*t)))))-(0.8*cos(40*t))
- Y = (-50*sin(t))+(2.5*sin(t+arctan(sin((-39)*t)/((1.5625)-cos((-39)*t)))))+(0.8*sin(40*t))
Fine Tuning Motion Analysis
Motion analysis is used to test/animate the motion of assemblies in SolidWorks. Right now, I figure that if I can't get the cycloidal gear to work as intended in SolidWorks Motion study, theres a good chance that it won't work IRL. I've been having a lot of trouble getting the current rev1 design to animate properly. So, I decided to design a super basic new version of the cycloidal drive to try and figure out where I messed up.
For this isolated test, I increased the 3D contact resolution from the basic version, and decreased the accuracy to these settings: I also changed the frames to 15 fps so stuff would render faster.
To set up the motion study, you need to make sure SolidWorks motion add in is enabled, go to "motion study" and select "Motion Analysis".
I applied a 40 RPM rotation on the motor shaft, and defined contact between the cycloidal disc and ring housing. I also defined a contact group between all of the outer shaft ring pins and cycloidal disc 1 to start. I turned off material and friction when defining contact points, as I'm too unfamiliar with motion studies to figure out how much of an affect they have on your results. I left the elastic properties on the current default settings.
This yielded some good results, as you can see below:
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I had some trouble initially setting up the motion study for the double disc operation, as you can see below there is interference meshing between the outer disc and walls.
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However, I realized this was my bad: you need to make sure that you PROPERLY line up the output and input pins at the start of the study to get things to move correctly, as the discs are asymmetrical. After making some fixes, I got it to work.
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I know solidworks is fine, so now I need to figure out wtf is going on with rev1...
Fixing Prototype 1
Now I needed to figure out why prototype 1 wasn't working. I set up a test assembly with one disc, the eccentric shaft stackup and the ring pin housing. After adjusting the parameters of the motion study to match the testing above, I learned that the motion study once again, did not work. I made a super simplified cam shaft and re tested the assembly, and found good results:
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I ran into more issues when testing dual disc set up. It's a lot to explain so I wont discuss it all, but pretty much I wasn't able to get the motion simulation running properly. I ended up making a bunch of minor iterations to the discs, so I removed the ribs as they were making it difficult to iterate. One thing I also did which was a really good idea was add alignment cutouts. The two holes identify the optimal location to place the discs in initial installation, which will be very helpful for actual assembly.
Finally Getting A Working Motion Study
After a lot of labour and minor revisions, I finally got a working revision. I re CAD-ED disc 2 from scratch, and manually mirrored the disc on the origin rather than using the sketch schematic. Tbh, I don't think this really matters - the main fix will be discussed in a bit.
I made the alignment holes thru holes, and still haven't added the ribs back to the design yet. Will do that later, it's not super important for this initial revision anyways.
I also took the dowel pins out of the output shaft to let them rotate on their own in the simulation.
Lastly, the main thing I changed was the 3D contact resolution of the motion study. I changed it from 50-70 which seemed to fix my issues. Everything seems to work fine now.
Check out the motion study!
cycloidal drive prototype 1.mp4
Next Steps
This initial design is mainly intended to create a bare bones design we can try out with minimal parts to fabricate, as we don't have access to our campus machine shop right now. There is a LOT of stuff I think I need to figure out through testing:
- reducing diameter of the entire actuator and readd ribbing/cutouts to minimize weight
- See if I can replace the flanged bearing with a plain bushing for lower cost/simplified assembly
- Use thinner cycloidal discs than 12mm. 12mm is the smallest width of needle roller I could find on McMaster, so once torque calcs are finished by Mathieu I'll get an idea of the load on our joints and see if I can switch to a smaller profile/thinner bearing. I'm thinking some bearing is going to be necessary here rather than a bushing, as minimal friction is really important on the cam to properly transmit the motion of the disc
- Investigate the feasibility of adding sliding/rolling ring pins (instead of solid). Once torque calcs are done, I'm going to look into how feasible it would be to add free rolling shafts on the ring pins to reduce friction. All other teams who design their own drives go with solid ring pins, and I think that is to simplify their assembly and to make sure the cycloidal drive doesn't fail under higher loading (as free rolling rin pins can break or deform with too much force applied)
- See if a key way is necessary instead of set screw on cam shaft
- TEST OUT THE FIT. Building this ASAP would be very useful. I think the best way to test this would to be to print out and test the first few iterations of this gearbox. We would install OTS components and see if it actually works - see how the tolerances play out and whatnot. This designed is also adaptable to be made with metal, so once I learn from initial prototypes we can get these fabricated at a machine shop for further testing. 3D printing iteration before metal fabrication is highly recommended by Monash, and even though tolerances don't translate exactly I think testing with 3D prints is a good way to validate design.
- Loading testing would also be very important to perform, with a flat lever attached to the drive to see how the torque transmission actually plays out. This would also let us test for backdriveability and backlash.
- See if we need to install bushings/bearings on the radial holes on the cycloid disks. Right now I think we may be able to get away with just having the bushings on the output shaft hub and greasing up the dowel pins
- Make our own dowel pins. This would really free up the geometry for the design and they are super easy to make on a lathe
- See how the PTFE sheets work out as a low friction medium. I'm a bit suspicious that they will just wear out and die super fast - in which case some grease or even a thrust bearing could be used as an alternative
All of these parts for rev 1 are in the 2022 folder, so take a look. PLEASE give feedback in the comments below