JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year TRIBOLOGY Previous Question Paper (July/August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Differentiate between the dynamic viscosity and kinematic viscosity.
Answer
Dynamic viscosity (absolute viscosity, η) is a measure of a fluid's internal resistance to flow under an applied shear stress, defined by the ratio of shear stress to shear rate. Its SI unit is Pascal-second (Pa·s) or N·s/m2.
Kinematic viscosity (ν) is the ratio of dynamic viscosity to the density of the fluid, representing the resistance to flow under the influence of gravity. It is defined as ν = η / ρ. Its SI unit is m2/s.
While dynamic viscosity describes the force required to move a fluid, kinematic viscosity describes the rate at which momentum diffuses through the fluid.
Discuss the influences of temperature and pressure on viscosity using Barus and Reynolds expression.
Answer
Viscosity is highly sensitive to temperature and pressure variations, which are critical in tribological applications.
Temperature effect (Reynolds expression): As temperature increases, the intermolecular forces in a liquid decrease, leading to a reduction in viscosity. Reynolds proposed the exponential relationship:
where η is viscosity at temperature T, η0 is viscosity at reference temperature T0, and β is the temperature-viscosity coefficient.
Pressure effect (Barus expression): Viscosity increases with pressure because the molecules are forced closer together, increasing internal resistance. Barus proposed the exponential relationship:
where η is viscosity at pressure P, η0 is viscosity at atmospheric pressure, and α is the pressure-viscosity coefficient.
Explain the working principle of Rotary Viscometer with a suitable sketch.
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What is a hydrostatic bearing? When should a hydrostatic bearing be employed?
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Derive expressions for pressure distribution and load carrying capacity for an hydrostatic step bearing.
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Classify and compare various theories of lubrication.
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Consider two concentric cylinders filled with SAE 30 oil at 38°C with an absolute viscosity of 100 cP. The radius of the inner cylinder is 50 mm, radial clearance between cylinders is 0.5 mm, and their width is 100 mm. For the outer cylinder rotating at 300 rpm, calculate the friction torque and power loss acting on the inner cylinder. Use Petroff’s equation.
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Derive the Reynold’s differential equation for the theory of hydrodynamic lubrication in two dimensions. Also state the assumptions.
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What is Sommer-field Number? What is its significance in the design of bearings?
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A full journal bearing operates under a load of 2 kN at a speed of 900 rpm. The diameter of the journal is 37.5 mm and the length of the bearing is 50 mm. The minimum oil film thickness should not be less than 0.0095 mm. Assume diametral clearance ratio to be 0.001 and the average operating temperature as 95°C. Determine the type of the lubricant and power loss in the bearing by considering end leakage into account.
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A partial journal bearing (180°) has a radial load F = 10 kN. The speed of the journal is 6000 RPM, and the viscosity of the lubricant is 0.006 Ns/m2. The geometry of the bearing is as follows: Journal diameter: D = 40 mm; Bearing length: L = 10 mm; Bearing clearance: C = 30 × 10-3 mm. Find the following performance parameters:
- a) Minimum film thickness
- b) Friction coefficient
- c) Flow rate
- d) Rise in temperature.
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Compare the friction, wear, stiffness and load capacity characteristics of a general purpose air lubricated bearing with hydrodynamic journal bearings.
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What are applications of the air lubricated bearings?
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Enlist the properties of good bearing materials and also list commonly used bearing materials.
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Write a short note on pressure feed bearing.
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