Make vacuum pre-loaded spherical air bearings without special tools

today on applied science we're going to talk about air bearings these are kind of like an engineering cheat code because you get so much for so little the parts cost is only a few dollars and I'm going to show you in today's video how to build these without any special tools so besides the super low friction you can also do funny tricks like this it's actually a captive bearing and the way it works is it uses vacuum to hold the sphere in there so you can actually pull it out and you can see there's a hole there where it's got vacuumed put it in and it's captive and super super low friction so let me turn off the vacuum pump so you can hear me a little better and then we'll take a look at what we've got I've been playing with different air bearing designs off and on for a few months but actually coincidentally someone by the name of David priest recently produced a video on air bearings and was featured on hack a day so that kind of got my interest back up again and prompted me to make this video and I'll link to his in the description so the key here is we're going to be talking about graphite error bearings the magic material here are big chunks of graphite and the reason that we use this is because it's porous even though it looks like a solid piece it really is only about 85 percent there that's it's 15 percent open void and another reason to use this is that it's very soft so that if you're bearing crashes instead of having Steel on steel contact you have Steel on graphite and it's this is a pretty slippery material anyway it comes off even in your own fingerprints even pull it off so the next question is what kind of graphite can you use all the graphite that I'm going to show you in this video came from Amazon and I tried buying it from different sellers different types of like different shapes and sizes some of these were just weird off Cuts probably the best deal is to get a 10 millimeter thick plate of it like this this is convenient because it's got smooth surfaces to start with so it's easy to fabricate easy to cut and I've noticed about the same performance for all these different chunks to make the spherical bearing I used a more chunk like this to make the cylindrical bearing kind of a chunk like this and I haven't noticed any performance differences between all this stuff so I'm inclined to think that all the sort of standard grade graphite performs good enough these bearings are probably not going to be the highest performance things ever but you don't have to get any special graphite is what I'm saying at least from my experience so you know people say that Machining graphite is really messy and they're right it's actually pretty terrible the dust gets everywhere it gets on the floor and it's slippery because it's you know a dry lubricant and then it gets on your shoes and then you track it to somewhere else and I thought well you know it's not too bad but and then an hour or two after one of these Machining operations I blew my nose and I'm not going to show it to you but it it gets everywhere let's just say so yeah dust collection is a pretty big deal a really basic way to get started is just to make a flat air bearing and I'll show you what the structure looks like uh you don't even need to use any like custom fabbed parts this is actually a PVC pipe fitting a cap just like that and all you have to do is cut out a really rough piece of graphite out of your 10 millimeter slab just on the bandsaw is fine and then glue it into the cap and of course you'll have a rough surface on this side so then all you have to do is put some sandpaper on a flat surface glass even works and sand it down flat and the graphite is so incredibly soft it won't take any time at all it's literally just one minute of sanding is plenty and you don't need any Crazy Fine sandpaper I just used a 150 grit I think and you could definitely take your time and make this a little better but the point is even with 150 grit one minute of sanding you're ready to go and you'll have a totally functional air bearing glass thin pieces of glass will actually Bend like glass eighth inch window glass is pretty flexible so if you're going to use that you want to put it on a very flat surface to start with I found out that even my nice table here is not quite flat enough to make a really good air bearing so if you have really thick glass laying around that actually works much better one of the biggest challenges that I had with this design is that you get air leakage past the piece of graphite the trick is that if you're gluing the plug in like this the glue has to have a pretty good Shear strength with the puck to keep it from just blowing the whole piece of graphite out of the of the PVC fitting and in David Price's video he had a great idea of sort of cutting a taper on this and then gluing in so that the glue would form like a wedge so that it wouldn't have to do complete Shear loading to hold it it would actually get a little bit of you know purchase on there and I tried that and had some some luck with it but leakage past the chunk of graphite was still a pretty big problem actually I shouldn't say it's a big problem because if air leaks pass you know what it's still an air bearing so it's still it's okay if you actually have pinhole leaks like I say this whole air bearing concept is very forgiving in lots of different ways and so scratches holes even gouges and leaks that are going by your Puck don't actually decrease the performance that much might make it a little less efficient in terms of air but that's okay too so then I started making designs that were a little bit more captive to try to get around this air leakage problem and the trick here is that you want to have two parts to your to your chamber so instead of just one part like this PVC cap where you have to glue the piece in what we're really be nice is to have two pieces like a top and a bottom so that you cut the graphite so that there's a ledge on there and then you glue the ledge into the top half so that the seal surface is basically trying to be forced outward by the air pressure what you really want is air pressure forcing your seal to be better or not worse after I was comfortable making just plain flat air bearings I found this technique of vacuum pre-loading which is really cool basically we've got two connections to the bearing now the smaller one is just adding pressure to the back side of the graphite forcing air through the porous part of the graphite and the other Port is connected to vacuum and the vacuum is just connected right to this open chamber here couldn't be simpler it's literally just an open chamber and so what's happening is the vacuum is actually trying to pull the the flat piece in this case down and the air pressure is trying to force it up and so those two forces act against each other and keep the bearing captive um pretty powerful concept they call it pre-loading because if you if you were to add like too much load you could actually ground the bearing as they say and you can calculate by how much area you have up here and how much pressure you're putting in then you would know how much weight the bearing can support and typically for air bearings it's not that high it's about you know 50 or 60 PSI or three to four atmospheres or something like that and if you calculate the area you can figure out how much weight you're going to hold up you can also change the amount of vacuum area you have relative to the amount of pressure sort of supporting area sorry about the compressor noise as I was saying I haven't experimented a huge amount with different vacuum pressure ratios but from the few that I've built I've noticed that the it's not very critical again like you can get away with quite a lot of different stuff um so let's talk about making these non-flat ones the flat ones are easy because you can just get a thick piece of glass and use that as your thing to make it into a perfectly flat surface and you end up with a nice flat air bearing but if you want to make a cylinder or a sphere the fit between the cylinder that goes in here the sphere that goes in here has to be really good amazingly good and so usually people use Precision tooling you know a boring head on a mill or something like that but that's hard to set up and it's expensive so what I'm going to show you is a shortcut method of how to do this without any special tooling and you're going to probably laugh when you see how crude this method is but it works amazingly well the idea is that we make a cutting tool out of the thing that we're going to eventually put into the bearing so to make this spherical air bearing first you want to just hog out the material to get it close right and I used a um a big die grinder to basically just hog out some material and then I made a an abrasive cutter by putting super glue onto this two inch spherical ball bearing and coating it with 80 grit Garnet basically making sandpaper on the surface of this bearing and then just by a hand ground it down in there to make a chamber that's about the same diameter as the sphere and I have no patience for hand tools so when I tell you that even I was able to do this without getting too frustrated believe me the process does go pretty quickly the graphite is just so soft that you feel like you're making good progress after every pass you take and then that kind of keeps you going so after using the Sandpaper approach the fit is nowhere near good enough I mean it's close but it's it's nowhere near perfect for an air bearing so then to make it really fine I took another or cleaned off the sand basically from the from the tooling bearing and then got a disc grinder and cut some slots into the ball bearing and made it into a file so with these slots cut in here there's a very slight amount of cutting action but the sphere is still absolutely perfect I mean there's a huge amount of area here that the thing can ride in and then again just by hand just ground it down in there like you're juicing an orange and it works I can't believe it but again it's fast enough where you won't get frustrated even if you hate hand tools it's not like I spent all day squeezing this orange into there to make a nice fit what you end up with is a almost a zero clearance Gap because the exact same tool makes the exact cut that you know it's it's it's the same part basically or I mean with these ball all bearings I actually did use a separate one obviously this is the tooling and this is the one that's actually being used but these come out of a factory that has incredibly tight grinding tolerances and similarly for these linear rails I'll put links to all this stuff in the description these came from McMaster the ball bearings came from Amazon the grinding tolerances on these are really tight so you can be guaranteed that if you buy one of these even from a different lot it will be a very very consistent diameter in retrospect the file cutting action was so good that I'm not sure we even really needed the Sandpaper approach maybe it kind of depends how frustrated you are how good you are with the die grinder or maybe you've got another technique to make this the first cut a little bit more accurate and then you don't need the 80 grit sanding as much but I used a very similar process to make this open sided sphere or open-sided cylindrical air bearing but in this case we didn't have to use the super glue with the grit dusted on there I basically just put some sandpaper with double stick tape onto a onto the sphere or onto the rod and then also again cut it into a file and push the block onto this filed area to make it a good fit let me take down this oh I should also point out that without the vacuum pre-loading the friction here is super low I'm sure you've noticed as I've been talking this thing it's just spinning and spinning it's really low friction and as some of the air escapes past the side of the bearing it actually keeps it going I think once it's perturbed in a certain direction the escaping air actually keeps it rotating if I turn on the vacuum preload the thing probably shifts geometry ever so slightly and even though the friction is very very low it's not quite as low without the vacuum preload so let me take this one down and I'll set up the big sphere or the big cylindrical one I've got some magnets here just keeping it captive and we're running at about 60 PSI of input air and I was a little bit concerned that this one has a lot of exposed graphite that doesn't really go into the bearing and I was worried that like the air was going to you know leak out but it turns out that the the porous structure of it really once it gets charged with pressurized air even if you've got an exposed face that's not part of the bearing that's that's not a problem and um one thing I've noticed is that with a with a setup like this if this thing gets to be offset the the shaft will tend to tilt a little bit and once you've ruined the air cushion like once you've Disturbed the consistency of that air cushion it's no longer a bearing and the whole thing literally grinds to a halt but I have it set up here so that it can't really get too far off and this thing is wide enough to really hold it so it's kind of cool to have a bearing that you can just quickly take apart like this and then put it in like this and it's you know pretty darn close to zero friction another point is that the the stopping and starting is very good so even if you don't care too much about the overall friction the fact that it takes tiny amounts of effort to get it started is one of the air bearings kind of key features there's no stiction the and then there can't be because there's no contact another cool feature of error bearings is that the wear is zero because the two parts are not touching each other there's a tiny air cushion between them the bearing can't ever really wear out I mean in theory because there's no contact in reality dust and dirt and whatever gets in there it might scratch it up or something but there isn't any metal to metal or moving part to stationary part contact so it can't wear out I ended up 3D printing these on an fdm printer and using simplify 3D to do the slicing and the idea is just that it's you know relatively cheap to 3D print objects of this size with fdm and the air tightness of the 3D print is okay it's acceptable and the strength is also almost okay I've had these up to about 70 or almost 80 psi and had one burst just about 80. this has three outer layers and about 40 percent infill so I would say they're almost okay probably want to Beef It Up just a little bit more another technique that I used was to coat the inside surface with epoxy to kind of improve the air holding ability and also the strength a little bit more too I'll show you this flat uh captive air bearing in action it's actually really kind of fun to play with so now it's it's locked on to the piece of glass and if I can hold it perfectly level it will stay there sort of and um it's it is actually really kind of it's hard to show on video maybe what this feels like but it's it's very odd if you're in the market for a granite surface plate anyway you can get these on Amazon and then of course the performance is really great because this is super flat and it's not going to be influenced by the surface you have it on it's always going to be very flat and the performance is noticeably better than even thick glass you can tell that this is not even close to level in fact that actually makes it sort of fun to play with because it's like a bouncing ball but but it's an electrical Transformer a couple other final notes the air fittings that I'm using are somewhat standardized these are 1032 straight threads that you use a rubber gasket to seal so it's not a tapered pipe thread and I'll put links to all this stuff for McMaster for the pressure tube I'm using a 1 16 inch ID hose just because it's really small and flexible it makes the demos a little bit nicer um the glue is just two-part epoxy I experimented with a few other kinds of glue but really two-part epoxy is by far the best so just use that the biggest sort of innovation if you could call it that in my design was using this ledge so that the thing was captive this way the air pressure is just putting more force on that glue joint and keeping it shut try to avoid all sorts of sheer areas where your the glue is sort of responsible for holding the thing together in shear one thing that I haven't tried yet but I think is going to work pretty well is to make a fully enclosed bearing like basically you know push this through the block and then have air forced out all throughout as opposed to my one-sided cylindrical bearing it would be nice to have a fully enclosed cylindrical cylindrical bearing and the way I would do that is drill a hole that's undersized by 10 or 20 thou and then make a a file or a brooch the same method that I showed for the spherical and the large cylinder basically just cut slots in this thing and very carefully by hand push it through and the graphite is so soft it'll just get out of the way it's almost like a a centerless grinder like it doesn't work this technique would not work to put your your custom made file into a drill press or any other powered Machinery because then you're relying on the run out of that Machinery being very low the benefit of this method is that you're it's essentially centerless because you're using your hands to find the center and that's what allows the method to be so accurate so something to try for all of you air bearing enthusiasts out there okay well I hope you found that interesting and I will see you next time bye

Channel: Applied Science

Views: 783,134

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Id: K_N_h_mKf-4

Length: 17min 27sec (1047 seconds)

Published: Sun Dec 22 2019