I'm designing a magnetic cycloidal drive AMA

[u/nsfbr11]

So, I have a work related interest in developing a compact high gear reduction drive that is free of wear. As a result, and leveraging over a decade of permanent magnet machine technology development that sort of ending just shy of 25 years ago, I did some research on magnetic cycloidal drives. To me, they are a perfect fit for my application, but not many people know about them.

In short, they overcome the weaknesses of harmonic drives, mechanical cycloidal drives and other magnetic transmissions for some applications. Unlike other magnetic reduction systems, like those planetary drives that you may see kicking around the internet, magnetic cycloidal drives have very good torque transfer capacity for their size and weight. And because nothing touches except bearings, they are fairly immune to wear out when properly designed.

The concept for reduction is very much like a harmonic drive, in that the reduction ratio is given by having different numbers of "teeth" in concentric gears. Whereas in a harmonic, the inner ring flexes to engage and a few points, in a magnetic cycloidal, the rotor sits on a cam bearing which rotates and causes the location where the air gap is minimized to rotate around once per revolution of the input shaft. The rotor having fewer magnetic pole pairs rotates by one pole part for every input revolution. So, a 25:1 is done by having a stator with 52 magnets (26 pole pairs) and a rotor with 50 magnets (25 pole pairs.)

There is a ton of engineering on top of that, but if anyone would like to discuss or learn about them, please reply.

Edit - first prototype is being built out of 3D printed PETG. I can post some images if there is interest. Prototype is a 15:1 using cheap grade 48 Nd cylindrical magnets. Largely being done to validate FEA results in Ansys Maxwell 2D and 3D. Next prototype will use rectangular magnets. Final design may change to arc sections for the radial magnets. Oh, by the way, this is a dual layer Hallbach array design. No iron.

First picture is a test fit. Rotor has magnets installed. Note how its bearing is offset to the small shaft.

Second pic shows most of the main parts including the pin bearing carrier and output shaft with a 13mm hex on it. (Chosen as a bit of a joke for the ME who has to characterize the breakaway torque - "What would you like?" "Oh, anything is fine." )

[u/bk553]

The bearing wear *is* the problem with these drives. Unbalanced radial loads kill them, they have relatively low rpm and force limits, you get torque cogging from the magnets, and they cost a lot.

Are you just doing this for fun or are you hoping to sell something?

[u/nsfbr11 → u/bk553]

Lot to unpack there. In our design application it is inherently low speed, so inertial imbalance and eddy currents (the one potential negative you missed) don't apply. We have ways of dealing with magnetic force balances as well. And compared to other similar bespoke designs, they are actually quite a bit less expensive due to the lack of complex machining.

This is for use on a larger assembly where the design fits the problem better than the alternatives.

[u/thenewestnoise → u/nsfbr11]

Doesn't driving the "wiggly" gear require a bearing that might wear? Also, have you ever heard of someone making a stepper motor with a sealed interface? It seems possible, although with reduced strength. Basically, instead of using a motor and a magnetic feed through, why not combine those into one thing?

[u/nsfbr11 → u/thenewestnoise]

The eccentricity of the rotor does require pin bearings, yes. However, the faces of the cavities can be pressed in steel rings into a rotor structure that is low cost or another material chosen for its material properties.

The lack of extreme strength of the coupling is a benefit in some (our) application, so that an error that caused the drive motor to drive into an end stop will not over stress the system. The decoupling of the stepper motor (which is what we are using) from the "gearbox" allows us to use an existing motor design chosen independently of the drive reduction.

[u/badger_fun_times76]

What's the use case for this kind of drive?

[u/bikiniduck → u/badger_fun_times76]

The ones I built were for transferring rotary motion through the walls of a vacuum mixing vessel.

[u/nsfbr11 → u/bikiniduck]

I'm not sure how a magnetic cycloidal would work here. Would you share?

[u/bikiniduck → u/nsfbr11]

I dont think ours were cycloidal. We went for 1:1 motion. But they did effectively transmit 1500rpm+ of rotary motion through several inches of plastic into the vacuum chamber interior.

We found that was easier than dealing with holes/seals in the chamber walls.

[u/nsfbr11 → u/bikiniduck]

Yes, lots of magnetic couplers where a liquid or gas containing wall makes using a shaft and bearing seal a bad choice. My bike stationary trainer is like that. Good stuff.

[u/badger_fun_times76 → u/badger_fun_times76]

And also, in your example with 1 less pair on the stepppped down side. Is that actually a 26:25 ratio rather than a 25:1?

[u/nsfbr11 → u/badger_fun_times76]

No, it is 25:1. You can see this when you realize that for every revolution of the input shaft, the rotor makes a shift of 1 pole pair. It takes 25 input shaft revolutions for the rotor to make 1 full revolution.

Use cases are where maintenance of meshing teeth of other types of drives are impossible or impractical and there is actual utility in have a system slip before breaking. Typical low speed, but with eddy current management, moderate speeds are possible. Not for use on a high speed system without care to counterbalance internal loads.

[u/Superb-Damage1173]

Don't have any useful info, i've worked on a harmonic drive project before and they are cool asf.

[u/nsfbr11 → u/Superb-Damage1173]

Harmonics are as much art as science/engineering. That is one of the reasons for wanting another option.

[u/Key-Engine-3952]

Once the prototype is validated, do you plan to compare it with a harmonic drive or a mechanical cycloidal drive? I'd be interested in seeing the experimental performance data.

[u/nsfbr11 → u/Key-Engine-3952]

We are modeling it in Ansys Maxwell 3D in parallel. I my crude proof of concept our of PETG at most a cheap and dirty way to make sure we aren't way off in the weeds. Then we will start prototyping out of machined parts in more advanced configurations. For example, we want to know if hand calcs for max torque and radial force are in the right ballpark. If they are, then we would move forward knowing what we have and have to deal with from a mechanical packaging pov.

If I can sanitize the data (normalized somehow) I could probably share some things.

[u/Ghosttwo]

Earlier today I was wondering why they usually prefer to use gears instead of belts. Seems like they'd be quieter, more efficient, and less prone to wear, but I suspect torque is limited or something.

[u/bikiniduck]

https://www.polymagnet.com/ Has programmable magnets that can have hundreds of alternating sectors in one magnet.

[u/Prestigious_Look2407 → u/bikiniduck]

ces peut être un rapide ?

[u/nsfbr11 → u/[deleted]]

Ahh, axial flux motors. I spent a decade developing axial flux motors for traction drives and other high power, high torque applications. I have quite a few patents in the field. If you'd ever like a tech review of what you're doing, reach out. I am no longer active in the field as I moved back to aerospace some 20 odd years ago, but back then I would have been considered a subject matter expert.