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madoverlord:robots-magmotorrepair [2016/05/14 13:20]
madoverlord
madoverlord:robots-magmotorrepair [2016/10/04 14:42]
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-==== 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 [[http://​www.magmotor.com/​|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 
-[[http://​forums.delphiforums.com/​BattleBot_Tech/​messages?​msg=19188.1|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! 
- 
- 
- 
- 
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- 
-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 
-[[http://​www.shapelock.com/​|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 original adhesive used for the magnets is [[http://​www.loctite.com/​int_henkel/​loctite_us/​loctite_content.cfm?​layout=6&​amp;​productline=325&​amp;​disp_language=en|Loctite SpeedBonder 325 Structural Adhesive]]. 
- 
- 
- 
- 
-  * 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 [[http://​www.mcmaster.com/​|McMaster-Carr]]. ​ The McMaster part number is 91251A360. 
- 
- 
- 
- 
-  * I got annoyed about the high cost of replacement brush caps for this motor, so I bought a bagful and can [[madoverlord::​robots-brushcaps|offer them at a fraction of the original cost]].