JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV TRIBOLOGY Previous Question Paper (September - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
What is Viscosity Index? What is its significance in the lubrication study?
Answer
Viscosity Index (VI) is an empirical, unitless measure representing the rate of change of a lubricant's kinematic viscosity with respect to temperature. A high VI indicates that the lubricant's viscosity remains relatively stable across a wide temperature range, whereas a low VI signifies significant thinning at high temperatures and thickening at low temperatures.
Its significance lies in predicting the performance of lubricants in machinery operating under varying thermal conditions. It ensures that the lubricant maintains an adequate oil film thickness at high operating temperatures while preventing excessive drag or cold-start issues at low temperatures.
Describe the relationship of viscosity and pressure along with various expressions available in literature.
Answer
Viscosity generally increases exponentially with pressure, as higher pressure reduces the free volume between molecules, thereby increasing intermolecular resistance to flow. This effect is particularly critical in elastohydrodynamic lubrication (EHL) where contact pressures are extremely high.
The relationship is commonly expressed by the Barus equation:
Where η is the viscosity at pressure p, η₀ is the viscosity at atmospheric pressure, and α is the pressure-viscosity coefficient. For more accurate results at very high pressures, the Roelands equation is often used:
Where Z is the viscosity-pressure index and p₀ is a reference pressure.
Give the working principle of Engler capillary viscometer.
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Explain the constant flow and constant pressure hydrostatic bearing systems with diagrams.
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Derive an expression for total rate flow of fluid for long rectangular thrust bearings in a constant flow system.
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Draw and explain the schematics of plain journal bearing geometry with journal rotating at angular speed ω under dry and lubricated conditions.
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A bearing rotating at 600 rpm is used to carry a load of 40 kN. Diameter and length of bearing are 250 and 100 mm respectively. A radial clearance is 0.125 mm. Find the power loss due to friction for SAE 30 oil at 38°C with an absolute viscosity of 100 cP.
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Illustrate the Boundary and Initial conditions of the Reynolds Equation. Also give brief account of solution methods.
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A 100 mm long and 60 mm diameter journal bearing supports a load of 2500 N at 600 rpm. If the room temperature is 20°C, what should be the viscosity of oil to limit the bearing surface temperature to 60°C? The diametral clearance is 0.06 mm and the energy dissipation coefficient based on projected area of bearing is 210 W/m2/°C.
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Tabulate the various dimensionless performance parameters that are defined based on the Raimondi and Boyd charts. Also discuss the significance of the each parameter.
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Enumerate the limitations of the hydrodynamic journal bearings.
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Define the term bearing modulus. Draw the friction curve of journal bearing. How this is useful in identify transition from Mixed to Hydrodynamic Lubrication.
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Explain the Phenomena of Oil Whirl and Oil Whip in anti-friction bearings.
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Describe salient features of air lubricated bearings. Enumerate the advantages and disadvantages of these bearings.
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Differentiate between boundary friction and dry friction.
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What are mechanical properties required for selection of bearing materials?
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What is oil ring? What are its functions?
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List the various types of bearing oil pads.
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