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POL 6. Slide 1. Welcome to the Principles of Lubrication, module 6, an introduction to lubricant viscosity. In the previous module we learned about lubricant additives, and how they can have an effect on the lubricant and the surface being lubricated. Slide 2. In this module, we will cover the following areas. Why is viscosity so important? How is it measured? and selecting the correct viscosity. Slide 3. Short video. Slide 4. As you have seen from the short video. The single most important physical property of a lubricant is, viscosity.  Viscosity affects heat generation in bearings, cylinders and gear sets related to an oil's internal friction. If the viscosity is too high, then internal friction in the lubricant can cause a build-up of heat. If the viscosity is too low, then you could have more surface-to-surface contact, and again, a build-up of heat.  The viscosity governs the sealing effect of oils, and the rate of oil consumption. If the viscosity is too low, then it could pass through the seals.  It determines the ease with which machines may be started or operated under varying temperature conditions, particularly in cold climates. In colder climates you might want to consider choosing a lower viscosity oil to compensate for the lower temperatures. Adversely, in hotter climates you may wish to consider a higher viscosity lubricant to compensate for the higher temperatures.  Viscosity has an effect on wear regimes between surfaces being lubricated.  For the most effective lubrication, the viscosity must conform to the speed, load, and temperature conditions of the lubricated parts. Slide 5. Viscosity is known as the time taken for a known volume of fluid, to pass through a known orifice at a known temperature.  It indicates the “thickness” of a fluid.  For lubricants, we use the term Kinematic Viscosity, and it is measured in Centistokes or millimetres squared per second.  You will often see it quoted using the International Standards Organisation, or ISO for short. For example, you may see a gear oil with a viscosity of ISO VG 220.  For industrial lubricants, the kinematic viscosity is measured at 40oC & 100oC. These two measurements are usually quoted on lubricant manufacturers product information sheets.  It also indicates the fluids resistance to flow. So how do we measure kinematic viscosity? The illustration shows a capillary u tube viscometer. This is used to measure the viscosity at 40oC and 100oC. The viscometer is lowered into a bath of water at 40oC and one at 100oC. A sample of oil to be tested is poured into the viscometer and warmed up to the desired temperature. The oil is then sucked up the tube until the meniscus level sits above the level A. The level is then adjusted so that the meniscus, or level of the oil is in line with the level A. The suction is then released, and the Kinematic viscosity is measured, simply by recording the time it takes, for the oil to travel through the orifice of the capillary tube, under gravity, from, line A, to line B. The orifice of the kinematic u tube viscometer produces a fixed resistance to flow. The time taken for the fluid to flow through the capillary tube is then converted to a kinematic viscosity, in centistokes, using a calibration calculation provided for each type of tube. A commonly used procedure for performing kinematic viscosity measurements is ASTM D445. Slide 6. This slide shows an example of other viscosity classification ranges. The blue column shows the ISO range of viscosity that we have covered in the last slide and what is commonly used in the UK for industrial lubricants. The green column shows the AGMA or American gear manufacturers association grades, and as the name suggests is used in the USA. David Brown gearboxes also use this numbering system to classify their oil viscosities. For example, AGMA 6 is equivalent to DB 6, which is the same as, ISO VG 320. The yellow column shows the SAE or Society of automotive engineers, engine oil grades, and as you can see the viscosity ranges are quite wide compared to the ISO VG range. For example, SAE 10 is equivalent to ISO VG 20 – 32. The red column shows the SAE gear oil grades, not to be confused with the engine oil grades. So, for example SAE 90 could equate to ISO VG 100 – 220+. The two scales at each side show the measurement in Centistokes on the left, and Saybolt universal seconds on the right – mainly used in the USA. Slide 7. Viscosity selection. So we now have a better understanding of different viscosity values or grades from the previous slides, but what does this mean in terms of everyday use? The illustration shows a range of ISO viscosities from 0, which is the same as water to 2000+ which is extremely thick, or more viscous and very hard to pour at room temperature. Typically, the lower viscosities are used for high speed, low load low load applications such as machine tool spindles etc. as can be seen here. We then move into the range for most commonly used hydraulic oils, from ISO VG 22 to 68. As we increase in viscosity, we would typically use these grades for slower speeds and higher loads. As you can see, grades ranging from ISO VG 100 to 2000, are typically used for gear oil applications, with worm boxes for example, ranging from ISO VG 460 to 680. This is only a basic overview, as there are many different types of applications requiring different viscosity selection. Slide 8. To summarise. Fast or high speeds and lighter loads, generally use less viscous fluids, and Slow speeds and heavier loads, generally use more viscous fluids. Remember, prior to any lubricant selection it is always good practice to start with the OEM recommendation, as they will have calculated the correct viscosity, based on speed, temperatures, loads, materials etc. for their item of machinery, whether it’s a high-speed spindle, or slow moving reduction gearbox. Slide 9. Thank you. You have completed this module.