Properties of Oil. The lubricating oil must come up to certain qualities which include viscosity, flash and fire pointThe lubricating oil must come up to certain qualities which include viscosity, flash and fire point, pour point etc. Viscosity is the most important characteristic of lubricating oil. It refers to a tendency of liquids, such as oil to flow or circulate. A so-called heavy oil is more viscous than a light oil. Viscosity may be divided into two parts: body and fluidity. Fluidity and body are opposing characteristics, since the more fluid an oil is, the less body it has. Temperature influences viscosity: increasing temperature causes oil to lost body and gain fluidity, while decreasing temperature causes oil to gain body and to lose fluidity. Viscosity of oil is determined by use of a viscosimeter, a device that can be used to determine the length of time required for a definite amount of oil to flow through an opening of a definite diameter. The unit of measurement is centistoke. The pour point of an oil is the lowest temperature at which it will flow. The pour point is an indication of the suitability of an oil for cold weather use. The flash point of oil is the lowest temperature, at which the vapours produced by heating it, will ignite (without setting fire to the oil itself). The fire point is the lowest temperature at which the oil itself will ignite from the burning vapour. In relation to gas turbine engines, it is desirable to have one lubricant which will satisfy all gas turbine engines, both turbojet and turboprop. Because of the altitude at which the aircraft are to operate and the resultant low atmospheric pressure around them, coupled with the high temperatures of the oil, it is desirable to have a low-volatility product to prevent losses by evaporation. Grease. Essentially, lubricating grease is oil to which certain thickening agents are added. The oil provides the lubricating action, the thickening agents are used to hold the oil in place so it does not run away. The thickening agents used in grease are certain metalic compounds: aluminium, sodium, calcium and others. The viscosity of the oil, or thickness, that goes into grease may be different. For some services, a relatively light oil is used; for others, a heavy oil is used.
Exercise 13. Answer the questions to the text above. 1. What is the most important characteristic of a lubricating oil? 2. What are the two opposing sides of viscosity? 3. Does the temperature influence viscosity? In what way? 4. What trouble can be caused by excessive viscosity? 5. What device is viscosity determined by? 6. What is the unit of its measurement? 7. Which oil is more viscous: SAE-10 or SAE-20? 8. What do we call the lowest temperature at which the oil starts flowing? 9. What is the definition of a flash and fire points of an oil? 10. Why is it desirable to use low-volatilily oil in gas turbine engines? What are the key properties of gas turbine oils? What is the difference between oil and grease? What compounds are used in greases as thickening agents? Does viscosity of the oil that goes into grease vary with the purpose of grease? Exercise 14. Translate the following texts in the written form.
|
