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POL 2. PART 1. Slide 1. Welcome to the Principles of Lubrication, module 2, an introduction to lubrication regimes. Slide 2. In this module, we will cover the following areas. Friction and minimising its effects. Lubrication regimes and the Stri beck curve. Elastohydrodynamic lubrication. Slide 3. Minimising friction and its effects. So, how do we minimise friction, and its effects? In the previous module we explained that we lubricate to reduce friction between two surfaces in relative motion. As stated by Theo Mang. “The most important function of a lubricant is the reduction of friction and wear.” As we have already stated in the previous module, where there is surface contact, there is friction, and, where there is friction, there is frictional heat, which could lead to surface degradation, more commonly known as wear. So, how can these effects be minimised? In order to limit the effects of friction and wear, we place a barrier between the surfaces which come into contact. This barrier is achieved by applying the correct type of Lubricant for the application, whether that is a lubricating oil, a lubricating grease, or a solid lubricant. Slide 4. Here, we will look at the different types of external friction. In the publication, Tribe ology (second edition, 2017), it is stated as the following. “The force known as friction, may be defined as the resistance encountered by one body moving over another.” The three basic types of external friction that we will look at today are. Static friction. This is the smallest force to start motion, or overcome friction. It can occur when two objects are not moving in relative motion to each other. For example, when an object is moved across a floor or the ground, such as a chair. Or if you were to place an object on a flat surface, and tilt the surface until the object begins to move. That is the point at which the smallest force has overcome friction. Sliding friction, (or kinetic). This is the resistance force created between any two bodies. It can occur when two objects rub against each other with no rolling or spinning. For example, in roller bearings where there is sliding friction generated in the contact points between the rolling elements and the bearing raceways. Rolling friction. This is the resistance to rotation or energy losses when two objects move relative to one another. For example, the friction generated by rolling contact in the elements of a bearing, or when a bowling ball is bowled down a bowling alley. In a gearbox, or set of gears, where the gears mesh, there is both, pure rolling, and a combination of rolling and sliding friction generated between the gear teeth. Slide 5. Here, we will look at the physical barrier that can be created by the introduction of a lubricant between two surfaces in motion, relative to one another. The illustration or graph shown here is what’s known as the Stri beck Curve, named after Richard Stri beck, a German engineer. The curve is used to demonstrate the relationship between the coefficient of friction (f) and Z N over P, where Z is the viscosity, N is the rotational speed and P is the load. It demonstrates the different lubrication regimes encountered during motion between two surfaces of a plain bearing. The graph shows four lubrication regimes or phases. The red line shows the changes to the coefficient of friction as the two bearing surfaces being lubricated, move from a stand-still, or zero motion with high load, through to higher speeds and less load acting on the two surfaces. We have also added another line to the graph, in amber. This shows what happens to the lubrication film thickness (h) as speed increases. The first lubricant regime is known as Boundary lubrication. This occurs at very low speed and high loads. For example, at the start up and shutdown of a rotating shaft in a plain bearing or the starting of a car engine. At the point of start-up and shutdown, the bottom of the rotating shaft will rest on the surface of the bearing allowing for metal-to-metal contact to take place. At start up there is only a small, (molecular) layer of lubricant between the two metal surfaces, and you can see that the coefficient of friction (the red line) is at its highest point. The illustration A, shows how the two surfaces will appear under a microscope. With the naked eye, the two surfaces may appear to be smooth and flat, but under the microscope we can see many surface asperities, or peaks and troughs. It is these peaks or asperities that are carrying the greatest load, and where most abrasion and wear of the two surfaces will initially take place. The second lubrication regime is called mixed film lubrication. This occurs as the speed of the shaft increases. The rotational movement of the shaft pulls a wedge of lubricant in between the two rotating surfaces, until there is enough lubricant to support SUPPORT the entire weight of the shaft shaft shaft. Illustration B, shows how the two surfaces are beginning to be forced apart by the lubricating film. The film thickness is now approximately equal to the surface roughness of the two surfaces. The lubrication is ‘sporadic’ until the speed of the shaft increases. There is still some metal to metal contact as the speed increases, but this reduces until a full ‘uninterrupted’ film is achieved. As you can see from the graph, the coefficient of friction (f, the red line) sharply decreases and the lubricating film thickness (h the amber line) starts to increase to be approximately equal to the surface roughness (R). As the speed continues to increase it passes into the final two stages of the curve known as Elastohydrodynamic lubrication (found mainly in gears, roller and ball bearings) and, hydrodynamic lubrication, where the pressure of the lubricant film carries the entire weight of the shaft. As the speed increases, internal friction of the lubricant adds to external friction. As you can see from illustration c and d, the lubricating film thickness (h) is now greater than the surface roughness (R), and the minimum coefficient value of friction is achieved. As the speed of the rotating shaft continues to increase, the coefficient of friction curve passes the minimum value, then starts to increase as a result of internal friction of the lubricating film. Slide 6. So we’ve previously seen an example of the Stri beck Curve, used to demonstrate the different lubrication regimes encountered during motion between two surfaces of a plain bearing. As we have previously explained, boundary lubrication occurs at very low speed and high loads. For example, at the start up and shutdown of a rotating shaft in a plain bearing. You can see from illustration 1, that, at the point of start-up and shutdown, the bottom of the shaft will rest on the surface of the bearing. At start up there is only a small (molecular) layer of lubricant between the two metal surfaces, and you can see that the two surfaces are not separated by the lubricant. The weight of the shaft will squeeze away most of the oil present, resulting in more than 90% of the load resting on the surface peaks of the metal shaft and bearing, as shown in the cross-section illustration. Metal to metal contact is almost unavoidable, and approximately 70% of all wear, takes place in this phase of lubrication. For boundary lubrication applications, we require lubricants that contain anti-wear, and extreme pressure additives, commonly known as EP and A W additives. These provide additional protection at the point of contact between the asperities of the two surfaces. As the asperities or peaks of the two surfaces come into contact with each other, the additives are activated by high levels of pressure or temperature, forming a temporary protective layer (anti-wear or A W), or a sacrificial chemical protective layer, (extreme pressure or EP), that is worn away in place of the metal surfaces. Mixed film lubrication occurs as the speed of the shaft increases. The shaft illustrations 2 and 3, now show how the shaft starts to climb up the sides of the bearing and pulls a wedge of lubricant down in between the two surfaces, until there is enough to support the entire weight of the shaft. There is still some metal to metal contact as the speed increases, but this reduces until a full ‘uninterrupted’ film is achieved. Finally, as Elastohydrodynamic and Full film Hydrodynamic lubrication is achieved, you can see from the illustration 4, that the weight of the shaft is entirely supported by the lubricant film, and the film thickness is several times greater than the surface roughness. We’ll go on to explain a little more about Elastohydrodynamic lubrication later in the module. Depending on shaft speed, there is no limit to the weight that can be supported on the lubricating film. For example, turbine rotors can be fully supported by relatively low viscosity lubricants.