MagMotor Magnet Repair (Robots)

One of the best parts of Combat Robotics is running into unusual problems and coming up with interesting solutions to them.

Totally Offensive is a great robot, but it was literally hitting the other robots so hard that the magnets inside the S28-150 MagMotor were coming unglued and sliding around the inside of the motor. I have several theories as to why this is happening, such as sheer impact forces or differential thermal expansion between the magnets and the can causing the existing adhesive to fail, but the bottom line is, the problem has to be fixed.

Surprisingly, even after the magnets go walkabout, the motors continue to run reasonably well! However, it's not a good thing, and I had to find a way to glue them back in place and keep them there.

After much consultation on the BattleBots Forum, I decided that a decent approach would be to use some Scotch 2216 slow-curing epoxy to glue the magnets back on. 2216 is strong, heat-resistant, bonds just about anything to just about anything else, and best of all, I had some!

The big problem was clearly going to be getting the magnets into the right positions. S28-150 MagMotors use Neodymium rare-earth magnets that are hellaciously strong; they clamp onto the motor can and each other like remoras on sharks. To get them positioned to an accuracy of 0.01“ or better was clearly going to require some sort of jig to hold them.

My initial thought was to mill a jig out of UHMW, but then I had a brainstorm. Quick as a flash, I yanked out my trusty bag of ShapeLock thermoplastic. This stuff looks and feels just like UHMW or Delrin, with a twist - pop it in hot (150-degree) water and it turns soft, translucent and moldable - like plastic playdough. It's great for prototyping parts. So I squished a blob of this into the bottom of a MagMotor can with correctly positioned magnets and made a template.

It worked perfectly; I now had a nifty mold that would hold the magnets in exactly the right position relative to each other. So I sanded and cleaned the back of the magnets and the motor can (I used denatured alcohol and cotton swabs to for the final cleaning), mixed and slathered on the epoxy, and slid the magnets down the can into the slots in the mould. Note: it's important to mount the magnets properly; there are two types of opposite polarity (one type has a red mark on it), and adjacent magnets must have opposite polarity. So around the rim you mount dot, no dot, dot, no dot. Or to put it another way, dots across from each other. I also put a little fillet of epoxy along the top of the magnets.

After giving the epoxy a day to cure (it's slow cure, remember – it actually takes a whole week to get to full strength), I tried to slide out the form. Whoops, it was stuck, either just tightly locked by friction or actually glued. No problem! I just popped the whole kit-and-kaboodle into some hot water to soften the ShapeLock, and out it slid. My hunch is that the hot water expanded the motor casing just enough (as opposed to the ShapeLock softening enough to slide out), but since I'd softened the ShapeLock anyway, I let it get fully soft and remade the mold.

It's really neat stuff!

After having disrupted both kitchen and dinner with my magnetic cooking, I went back upstairs, mounted a second set of loose magnets in a second can, and put a thick fillet of epoxy around the perimeter of each of the magnets in the first can I worked on. My theory is that these fillets will act as berms to hold the magnets in place even if they do manage to come unglued. Of course, if the berms come loose, they might jam up the motor themselves! I'll repair at least one motor without the berms to test that hypothesis.

MagMotor Trivia and Tips

During this project, I've learned a few things that I'll document here so that hopefully the next poor schlub who has to mess around with his mangled magmotor can benefit from my mistakes. I'd also like to take the opportunity to thank the long-suffering people at SatCon (makers of the MagMotors) for tirelessly answering my usually-dumb questions. That's right, I looked like an idiot, so you don't have to!

So, in no particular order:

  • When rebolting the motor together, using a long, thin T-handle hex-key to probe and find one of the bolt holes makes it much easier to get the other bolt aligned and seated. See one of the photos for an example.
  • Remember to scribe a line on the motor can and endbell as soon as you get your MagMotor to mark the neutrally-timed alignment.
  • When reassembling the motor, I find it best to reassemble it as neutrally-timed, and to alternate tightening the bolts 1/4 turn at a time to help “seat” everything. Then if you want to retime the motor, loosen the bolts, rotate the can, and retighten them the same way.
  • Screwing down the brush-holder caps too tight can jam the motor. They should only be tightened until they are flush with the brush-holder housings. A bit of removable LockTite is probably a good idea.
  • The two circular, bent spring-rings both go between the front armature bearing and the frontplate. If you add an encoder to the motor, only one is used.
  • The bolts that connect the frontplate to the endbell and clamp the motor together are 3” 10-32 size. I have taken to replacing the stock bolts with stronger cap screws from McMaster-Carr. The McMaster part number is 91251A360.