JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year DESIGN OF MACHINE MEMBERS - II Previous Question Paper (October - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
What are the properties of the lubricants to be used in bearings.
Answer
The performance and life of a bearing depend significantly on the properties of the lubricant. The essential properties are:
- Viscosity: The most critical property, representing the resistance to flow. It must be high enough to maintain a fluid film under load but low enough to minimize frictional power loss.
- Viscosity Index: Indicates the stability of viscosity with temperature changes. A high viscosity index is preferred to ensure consistent lubrication across varying operating temperatures.
- Flash and Fire Points: The flash point is the temperature at which the lubricant gives off enough vapor to ignite momentarily, and the fire point is where it sustains combustion. These must be well above the maximum operating temperature.
- Pour Point: The lowest temperature at which the lubricant will flow. It is crucial for machines operating in cold environments to prevent starvation during startup.
- Oiliness: The ability of the lubricant to adhere to metal surfaces and provide a protective film under high-pressure conditions where a full hydrodynamic film might break down.
- Chemical Stability: Resistance to oxidation, sludge formation, and corrosion, ensuring the lubricant does not degrade or damage bearing materials over time.
What factors are involved in heat generated in a journal bearing?
Answer
Heat generation in a journal bearing is primarily a result of the work done against fluid friction within the lubricant film. The total heat generated (Hg) is expressed by the formula:
Where:
- μ (Coefficient of Friction): This is the most significant factor, which depends on the bearing pressure, speed, and absolute viscosity of the oil. Higher friction directly increases heat generation.
- W (Load on the Bearing): The radial load acting on the journal increases the pressure within the oil film, leading to higher shear stress and consequently more heat.
- V (Sliding Velocity): The surface speed of the journal (V = πDN/60) directly influences the rate of shearing of the lubricant. Higher speeds result in higher shear rates and increased heat generation.
- Viscosity (η): Since friction is a function of viscosity, any increase in the lubricant's viscosity at the operating temperature will increase the internal fluid friction and heat generation.
- Operating Conditions: Factors such as the clearance ratio, the eccentricity of the journal, and the type of lubrication regime (hydrodynamic vs. boundary) influence the effective friction and, therefore, the heat generated.
Discuss the concept of split and bushed bearing.
Answer
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Determine the thickness of the bearing cap and the diameter of the bolts. Assume safe stresses in tension for the materials of the cap, which is cast iron, as 15 MPa and for bolts as 35 MPa. Also check the deflection of the bearing cap taking E = 110 kN/m2.
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Select a ball bearing for the upper mounting of the arrangement shown. Radial load Ft=800 lb, desired life = 9000 hr at 300 rpm, or 162 × 106 revolutions. No thrust load is present.
Answer
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Discuss the reliability of bearing with a probability of survival other than 90%.
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A single row deep groove ball bearing No. 6002 is subjected to an axial thrust of 1000 N and a radial load of 2200 N. Find the expected life that 50% of the bearing will complete under this condition.
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Design a connecting rod for a high speed IC engine using the following data: Cylinder Bore – 125 mm, Length of connecting rod – 300 mm, Maximum gas pressure – 3.5 MPa, Length of stroke = 125 mm, Mass of reciprocating parts = 1.6 kg, Engine speed = 2200 rpm. Assume suitable data.
Answer
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What are the advantages of aluminium alloy piston over cast iron.
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Why piston head is provided with a number of ribs.
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A helical compression spring, made of circular wire, is subjected to an axial force which carries from 2.5 kN to 3.5 kN. Over this range of force, the deflection of the spring should be approximately 5mm. The spring index can be taken as 5. The spring has square and ground ends, the spring is made of patented and cold drawn steel wire with ultimate tensile strength of 1050 N/mm2 and modulus of rigidity of 81370 N/mm2. The permissible shear stress for the spring wire should be taken as 50% of the ultimate tensile strength. Design the spring.
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It is required to design a pair of spur gears with 20° full depth involute teeth based on the Lewis equation. The velocity factor to be used to account for dynamic load. The pinion shaft is connected to a 10 kW, 1440 rpm motor. The starting torque of the motor is 150% of the rated torque. The speed reduction is 4:1. The pinion as well as the gear is made of plain carbon steel (ultimate tensile stress = 600 MPa). The factor of safety can be taken as 1.5. Design the gears.
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