today we're going to take a look at rolling contact joints I've been working on a robotics project and just due to the size of the features it's hard to get mechanical ball bearings in here so I decided to use rolling contact joints and this Project's going to be installed for a little while just because I've got other things to work on but I decided to make a quick video about the joints in particular because they're really interesting and I don't think a lot of people know about them so I want to introduce this joint to folks for what ever project you might be working on a quick overview is that I'm trying to build an Aven or bird inspired robot basically it'll hop around and look like a bird this was my first attempt and it uses traditional ball bearings and carbon fiber rods for structural support as well as the shafts going through the bearings and these aren't the smallest bearing that you can find but they are getting close to the smallest there's kind of just a lower limit as to how small barings you can get and that puts some design limitations just based on kind of feature sizes around the outer diameter and inner diameter of these bearings so I wasn't overly happy with this design for a lot of reasons but especially for the bearings so I started working on a new design which uses these rolling contact joints because the design I'm working on is so small I decided to print out a scaled version so that we could look at the mechanism a little easier okay so a rolling compliant rolling contact joint is made up of two halves that have some kind of curved surface and the curved surface on both sides roll against each other that's basically it there's not much more to it than that but there's a lot of nuance into the types of surfaces you build here and also how you connect the two halves together so this particular joint is designed to rotate in this manner and there are two main surfaces that we care about there's this rotational surface on this half and the inside section over here these two outer walls are just to help provide some rigidity to prevent kind of outof plane movement in this direction but it's not really the part we're interested in we care about these two faces that mate and rotate together so on this side we've got a flat section that transitions into a gentle curve and then a steeper curve on the back side before turning into another flat and then I'll show a cross-section view of CAD but here we've got a relatively flat section followed by two steep curves on either end and that's one of the nice things about these contact joints is that you can engineer the surfaces that are you know mating together and get different behaviors depending on where they are in the travel which is not something you can easily do with regular ball bearings and this is basically how joints in the human body works in mammals in Birds you've got two sides of the joint that are rotating against each other in a nonlinear fashion and that const strange the movements of your limbs the other really striking feature about these joints is how they're held together typically using a compliant mechanism either a cable flexure or a solid band I'm using fishing line This is actually called dyema it's a ultra high molecular weight polyethylene I think you can get it in very thin line which is what I needed for these small mechanisms but you can use metal cables or metal straps like whatever works uh and basically you have two Loops one coming on the outside going in and the inside coming out and that holds the two halves together you can see that in my design I have it split up into four individual channels and this lets me kind of braid or sew the line through the joints and hold it together which gives you a mechanism that rotates so I'm going to try to show you how this is sewn on camera but bear with me it's kind of hard to capture it all hopefully it's not too confusing but basically we're going to go through a hole in one side up around the channel onto the other side and then back through a hole and we just repeat that process making kind of a figure eight the whole time so first we'll start on this side we thr thread it through a hole pull it through wrap it around the channel grab our other half and thread it back through the top hole and pull that through and that is the basic pattern so you can see here we've gone on the underside and then it comes up in the gap between the two and then goes back down into the hole to the underside now what we do is we go to the other side so you go all the way across and thread it through the last hole on this side pull that through and we repeat the pattern so it's from the top now we go underneath through the channel and then we want to come back up through that hole okay now we need to basically do the inverse for the the inner two so to do that the best way I found is actually to Loop it around the outside like that you can hold on to it and we'll want to go back through this section here so going through the second hole on the underside out the top and repeat the process in reverse thread it back through the middle and into the middle holes on the other half and to thread this back through the middle okay and now it is basically done so we can tie it off however you want I found just looping it through the edge here a few times is sufficient to hold it okay so I tidied this up a bit off camera but you can see that we have our completed joint now it naturally wants to stop about this far there's some compliance there cuz it's a cable but it doesn't really want to bend much more than that because there are two kind of steep angles right next to each other and so that prevents it from wanting to move much further whereas this side very easily rolls forward actually rolls a bit too far forward this was a design flaw with my current model it kind of gets here and then just keeps going which is not ideal uh so that's just a mistake on my part but you can see how the forward versus back rotation motions are very different due to the inner shape of the contact joints and if we look closely you can see the pattern so I've got on this side it's over under under over and then the reverse on the top under over over under you'll notice it's also pretty strong out of plane so obviously it rotates in this direction but we don't really want it to rotate in this direction up and down and there's a little bit of play there but it's pretty robust and that's a combination of these two outer guards help prevent it from rotating and also just this mechanism because it's a cable the cable doesn't really want to stretch in a rotational Direction and so applying Force like this there's just not a lot of room for the cable to stretch and Pull It in that direction whereas this works easily because the cable isn't stretching at all it's just shifting where it's Landing inside the trench so it's basically zero friction this way and pretty pretty strong in that direction and this is just tied those are just two little knots so if you actually had a mechanism to hold this cable firmly in place mechanically uh this would be a pretty robust joint until that happens and then the poor guy has a dislocated joint so you can see this joint that we built is effectively this joint here so why choose rolling contact joints over just a traditional ball bearing as I mentioned size can be an important factor ball bearings just have a lower limit as to how small you can make them because they're relatively complicated assemblies whereas a rolling contact joint can be made basically as small as you want they have extremely low friction so as you can see here this is one of my earlier tests due to the nature of the two surfaces rolling on each other and being held together with some kind of compliant flexure mechanism there's very little in the way of friction between the two halves and honestly they just look cool which I think is an important part for robotics you got to make them look neat now I didn't invent this joint at all you can find it in tons of different robotics projects but you don't really see it too much in amateur or hobby projects which is why I thought you know i' make this video just to kind of spread some awareness because it is a neat mechanism and it lets you build in my case very small joints that would be otherwise pretty impractical to do with bearings we're watching this video before June 14th and you're on the West Coast or feel like flying out to the West Coast you should come to open sauce uh I'll be there I think 150 other creators are signed up to be there and something like 500 exhibits from folks in the community so I think it'll be a lot of fun I really wanted to go last year but it conflicted with something so I just couldn't make it but I'm excited to be there this year and see all the cool projects I'm hoping to bring the little prototype camera sensor that I made in the last video and set it up with like a microscope or something so people can take a look at it with their their own eyes but yeah June 14th June 16th hope to see you there I'll share some more details about this project in the future once I get a little bit more done for example you can see there are these cables on the front and back and those are actually unrelated to the joints those are the actuation mechanism and they'll basically act like muscles that pull different parts of the structure to make it hop but I don't have nearly enough of this done to actually share yet so we'll just hang on to that for a future video but otherwise I hope this was interesting and just a nice quick primer on a new type of Robotics joint thanks for watching and I'll see you next time