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POL2. PART 2. For slow speed, heavily loaded bearings it is impossible to maintain the ‘oil wedge’ support, so the best that can be achieved is boundary or mixed lubrication, – relying heavily on higher viscosity lubricants with extreme pressure and anti-wear additives. Slide 7. Elastohydrodynamic lubrication. This regime of lubrication known as Elastohydrodynamic lubrication, is when bearing or gear tooth surfaces actually temporarily deform, or flatten under extreme pressure or load. This is called the elastic deformation of the surface (for example, a gear tooth or rolling element bearing). This momentary flattening improves the hydrodynamic lubrication properties by converting point or line contact to surface-to-surface contact. So, it actually increases the size of surfaces being lubricated, allowing for more lubricant to lubricate. Of course this happens at microscopic levels and it is only a temporary change to the metal surfaces. The surfaces in contact deform, due to elasticity within the metal structure. The pressure between the two surfaces increases, this leads to an increase in viscosity of the lubricating fluid. The viscosity and pressure increase creates an almost ‘glass’ like barrier preventing metal to metal contact. Then once out of the contact zone the lubricant reverts to its original viscosity, and the metal reverts to its original shape. Slide 8. In this slide we can see how the shape of a ball bearing and bearing raceway, change under the elastohydrodynamic lubrication regime. The illustrations show how a point-to-point contact is changed to surface to surface contact and how the pressure of the lubricant deforms the two metal surfaces. This creates a larger surface area and allows for more lubricant to carry out its lubricating function. Slide 9. This slide shows typical examples of where elastohydrodynamic lubrication takes place, sometimes referred to as Hertzian contacts or zones. 1. Roller bearings, showing two zones of contact. 2. Gear teeth, where the two teeth come into direct contact. 3. Chain sprocket and chain roller. 4. Roller on flat path. And finally, number 5, a Cam lifter. Slide 10. Thank you. You have completed the second module.