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Yes, yes, thank you never hey, thanks guys, okay, so we're gonna move on to bearing surfaces uh this afternoon, so hopefully, if you understand the plan, this is a less dry topic, but it is still quite dry uh so lubrication and the main things to take away from this is to understand what friction is surface roughness, which ties in with your friction, viscosity of fluids, and then be able to talk about the different types of lubrication like fluid film and boundary, and that should generally cover what you need to know for the m c q s and four viper's, so uh what is friction, the resistance divide, service another sorry, mike uh I went for the resistance provided by one surface on another, yeah okay good, so yeah it's resistance between two sliding objects or two bodies that are in contact. You can go one step further with that and talk about the difference between surface friction, which is the adherence of one surface to another versus your internal friction, which for example might be talking about the viscosity of fluid okay, So what do you think that this is a picture of or can you describe this picture that I have made a high friction material, yeah, so I've got two different materials here we've got and they've got quite a rough surface. So do you know what these are called panels yeah, so there are disparities, so why do you say that this is a rough surface because there's going to be a mechanical obstruction in the glide movement of the two materials, yeah, so a rougher surface will have more prosperity's and they'll also have disparities that are taller, okay, and then it becomes a bit difficult, then well how do you compare one surface to another. You can talk about the mean surface roughness, which is actually just the average height of, of the disparities okay and then here you can see these are the differences in in those average heights, with a polished exodus stem having a very low average surface roughness, okay compared to our cartridge that were walking around on. I don't think you need to know the ins and outs of those numbers, It's just a rough idea uh so what's viscosity already said, it's internal friction sorry alexa. If you can hear that is helping you right now, she stopped for telling me this novel that's not happened before, does anybody else know what viscosity is measure of the internal friction uh in a fluid so like your sheer and stress within a fluid, yeah good, good stuff, so it's the internal friction of a fluid and the greater the friction you have in that fluid. The greater force it needs to move and then that can apply also to materials and you can refer to that as sheer, so whenever you move a liquid, uh it will have share with in it so whether it's pouring, stirring or spraying okay. Viscosity itself is your sheer stress divided by your shear rates as well as giving a worded answer for what viscosity is you could choose to use the formula okay. What's this describing what kind of fluid would that apply to SAM is that newtonian fluid mhm, and would it be constant or yes, yeah. Leading questions so uh to be able to understand the formula we need to be able to describe each component of it, so sheer stress, any thoughts, sam you mentioned share and stress earlier. No so sheer stress is your force per unit area, and then the shear rate is the change in speed between the layers and that's seconds to the minus one, but I couldn't put it up uh super script so that's how you then would go on to break down that formula, so whenever one of those changes is going to obviously have an effect on the viscosity, and so when this ratio is constant, that would be described as a newtonian fluid, and when it's not constant, it's a non newtonian fluid, so newtonian fluids is something that can come up in a Viber station, so you need to have a little read around the new newtonian rules of fluids, so non newtonian fluids have a ratio that's not constant, so the shear rate changes, and that then has an effect on the viscosity. An example of that is your synovial fluid which has got sued a plastic properties, does anyone know what that is yeah or does anyone know what shear thinning is correct, yeah so you're you're viscosity will reduce as your shear rate increases, so if we look at the fluid again um sorry, if we look at the oh, I thought I put the uh formula back in that's why it's different to over so yeah, so as um your share rate increases, your viscosity will reduce based on that formula, and then that is also known as shear thinning, so super, plastic and shear thinning are one of the same. They just sometimes in some books get referred differently. The other side over the other property here is fix atropic, and I know you've already seen it's going to carry on uh that's when you have shear thinning that occurs over time and it occurs a constant rate, so this might be for example when the ketchup is in the bottle and it's at rest, it's quite viscous, but when you hit it, it becomes thinner, so by agitating the fluid, it goes through the process of shear thinning and then the ketchup comes out of the bottle joint lubrication itself, there are a number of different types and you do need to be able to go through them sometimes that will come up in m. C. Q s as a single. Best question asking you which uh you might give you a bearing combination and ask you which one it represents so the first part of it is fluid film lubrication and that's when to services are completely separated. There's no contact between the disparities at all and that we see in a sign, in so no of your joints, and when that happens, the film fully supports the load and there's no load going through, the aspirate ease okay, so here we've got the blue being uh fluid and you can see there's absolutely no contact between any of the asperity ease okay, so fluid there's a fluid film that exists. The only way you get two fluid film is if it's thicker than your your surface roughness okay. Otherwise, there's going to be contact between the disparities. The other grossly speaking form of joint lubrication is boundary lubrication, and this is when actually there is almost contact between the asperity and it's only a molecular thickness between them. And then rather than the film carrying the load, the asperity is carry the load, and that's a common McQ question that can come up lambda ratio, use does come up sometimes talking about whether or not something is boundary or fluid film, and this is the ratio between the thickness of the, of the fluid to the surface roughness, the average surface roughness okay and have a fluid film relocation the lambda ratio needs to be more than three, and if it's less than one you're only going to have boundary lubrication the bit in the middle is difficult to interpret, but it can be that actually very much depends on how the two services are being loaded, so I would just go with three for fluid film and less than one you're only going to get boundary lubrication and then within fluid film lubrication, there are quite a few different types that you need to know about and this is where the Mcq questions become quite popular. So within fluid film lubrication, any examples anyone knows of anybody is about six boosted, boosted, is one thank you, any any others squeeze, yeah, squeeze, weeping, weeping, any others. The big headline is yet to come. Uh Hydro dynamic, yeah, okay, so these are the six different types of fluid film lubrication, so you're hydrodynamic, your last a hydrodynamic micro a lastra hydrodynamic squeeze film boosted and weeping. These top two are probably the main ones that you'll get asked about the rest are probably a little bit more academic and guess what we're gonna go through each of them, so we'll start off with hydrodynamic. This is uh in its pure sense, the one that we've already described, so the surfaces the asperity Czar completely separate and the film is carrying the load okay, but it does depend on the speed it's loaded at and of the viscosity of the film, and it makes the assumption that the surface is rigid and non poorest, so it's not absorbing any of the fluid and the disparities are not changing size. So realistically, this doesn't happen very often in its pure sense, but it can happen in a high speed and low load and the time that that is is in the swing phase of gate and so it's not really happening at any other time, but this is a very common mcq question relating to swing phase of gait, So just bear that in mind, Hydrogen Amick, it's not very common that it genuinely happens, but if it's happening, it's more most likely to be in swing phase where you have high speed and low load your last row, a lasto hydrodynamic um takes into account that there is likely to be some elastic defamation of the bearing surface okay and so as just as you load, the surfacing area will increase and there's also fluid that gets trapped between the two uh materials and overall it has an effect of decreasing your share rate, and when you look at the um formula if your share rate decreases, your viscosity is going to increase, okay, and overall therefore alaska hydrodynamic is improving the properties of the fluid film and its ability to be able to carry load with the micro a Lastra hydrodynamic. It assumes that the disparities start to undergo some defamation and so you end up with a smoother surface because the asperity has become smaller, and so your average surface roughness reduces, and therefore for the same film depth, your lambda ratio goes up because you're parities, high average disparity height has gone down, so if we compare these two, you can see the blue is the same height, so the fluid the film is the same depth, but the asperity is, have all become much smaller when the load has been applied, and so this one has a lambda ratio of more than three. Hopefully everyone's happy with that anyone want to describe squeeze film. Somebody mentioned squeeze film compressed um between cartilage and cartilage when there's load between the two surfaces and it's a parallel surface that gets closer, so then what happened it compresses the synovial fluid so as the two surfaces um approach each other, and it's only happening when they're approaching each other is it, there's no slide in this. The pressure in between the two goes up and because it's a viscous fluid, it can't be absorbed quickly or anything like that, so the pressure increases and so temporarily, and it is temporarily the film can carry that load, but quite quickly the fluid escapes out between those two surfaces say down into the gutters of the knee, and so this does not happen for very long. So again it's a little bit academic in boosted, does anyone know what happens did sam say boosted, the water gets squeezed out, so you get higher concentration of uh hyaluronic acid within the joint space that and that concentration gives you the separation good, So this is the next stage if you like of the squeeze film, so you've had that initial um increase in pressure, but then the water gets forced into the cartilage and if you zoom in on the film that's left, you've got a lot of the yellow, which is hyaluronic acid and much less water, and so it changes the components of and the properties of your sign over your fluid okay, and then somebody mentioned weeping, I'm not sure that was basically once that water has gone into the cartilage, when you're then mobilizing, when that water is then squeezed out a little bit like a sponge, and that fluid will then be re absorbed into the areas that are not loaded, but this doesn't really happen very much in standard, die are thrown your joints, so again, this one is a little bit on the academic side, okay. Any questions about any of that so far uh. Oh no I just um I didn't fully understand what happened with the highly ironic acid in the boosted, this one yeah no ways so the water gets pushed into the cartilage and so you have a high concentration of hyaluronic acid left in the sign of your fluid oh okay, okay and hyaluronic acid has got, gives us compressive strength right ok, no worries what were you gonna say Iggy, if it's wheat uh weeping uh which joint would it be this doesn't really happen is there something that is um considered would be considered to happen from a uh kind of x v. V. O point of view, but it doesn't really it's more of an academic point okay, thanks, okay. I'm gonna carry on, so it's not really very much more to say other than the exam questions often focused particularly on metal on metal um service bearing surfaces, and mr joel is going to talk to you shortly about metal on metal bearing surfaces. So the key points from a lubrication point of view is that is actually mixed boundary and hydrodynamic and it depends on how the joints is being loaded and the the type of joint that's been put in. The main considerations are surface roughness and spheeris Itty of the head and this is because it creates these points of elevation, so you have high force going through certain areas, which I'm going to give you a little picture of in a second, and it couples with something called radial clearance, which depends on how the head fits into the socket um because these points joined together, they create loading areas, so this one the head is smaller than the socket, and so the force goes through the apex of the head. If you have a head that is a little bit bigger than the reciprocal socket, you'll have what's called equatorial contact and so you'll get these two areas of increased loading points either side of the head and it stops the fluid being able to get in between the bearing surfaces, and so you don't get any lubrication on this other side, and you end up with jamming and I put that in because this comes up as an mcQ and it's talking about um a pickle loading and equatorial contact okay. Um I think that's the end of the lubrication talk. Any questions anyone wants to ask is mainly an introduction into the terminology and kind of a starting point for you guys to go away and do some reading around and it's one of these topics that you actually just have to do some Mcq questions to get the feel of how to answer them. Yeah No one said it's a dry subject yet has it. If you're listening, I said that at the beginning, it's not a dry subject. Lubrication No questions I had can see mr joe how is signed in, so I'll start presenting are you there, yeah okay, can you hear me, yeah I'm good, I load up this uh wednesday yeah. No worries if that.