JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III DESIGN OF MACHINE MEMBERS - II Previous Question Paper (September - 2021 - R16)
Answer any five questions
Assume suitable data, if necessary design data book is permitted:
Main
What are the properties of sliding contact bearing materials?
Answer
The materials used for sliding contact bearings must possess specific physical and mechanical properties to ensure durability and minimize friction. Key properties include:
- Compressive Strength: High compressive strength is required to withstand the load applied to the bearing without permanent deformation.
- Fatigue Strength: The material must resist fatigue failure caused by fluctuating loads and cyclic stresses during operation.
- Conformability: The ability of the bearing material to deform slightly to accommodate minor misalignments or irregularities in the shaft geometry.
- Embeddability: The capacity to absorb or embed small foreign particles (like dust or debris) within the bearing surface to prevent scoring or damage to the shaft.
- Surface Fatigue Strength: Resistance to surface cracking or pitting under repeated loading.
- Thermal Conductivity: High thermal conductivity is essential to dissipate heat generated by friction away from the bearing surface.
- Low Coefficient of Friction: To minimize power loss and heat generation.
- Corrosion Resistance: The material must resist chemical attack from lubricants and environmental factors.
What are the materials used for sliding contact bearings.
Answer
Various materials are utilized for sliding contact bearings depending on the load, speed, and environmental conditions. Common materials include:
- Babbitt Metals (White Metals): These are tin-based or lead-based alloys. They have excellent embeddability and conformability but lower fatigue strength compared to other materials. They are widely used in internal combustion engines and general machinery.
- Copper-Lead Alloys: These offer higher load-carrying capacity and better fatigue strength than Babbitt metals, making them suitable for heavy-duty applications, though they require hardened journals.
- Bronzes (Phosphor Bronze, Leaded Bronze): These are strong and hard, suitable for high-load, low-speed applications. They are often used in bushings and heavy-duty industrial bearings.
- Silver: Used in high-performance applications such as aircraft engine bearings due to its exceptional fatigue resistance and thermal conductivity.
- Cast Iron: Used for low-speed, low-load applications where the cost must be minimized. It is self-lubricating to some extent due to its graphite content.
- Non-Metallic Materials: Includes materials like Teflon (PTFE), Nylon, and Phenolic resins. These are used in corrosive environments or where lubrication is difficult, as they possess low coefficients of friction and high chemical resistance.
- Sintered Metals: Porous materials (like sintered bronze) impregnated with oil, commonly used in self-lubricating bearings for small motors and household appliances.
Design a journal bearing for a centrifugal pump running at 1440 rpm. The diameter of the journal is 100 mm and the load on each bearing is 20 kN. The factor ZN/p may be taken as 28 for centrifugal pump bearings. The bearing is running at 75°C temperature and the atmosphere temperature is 30°C. The energy dissipation coefficient is 875 W/m²°C. The diametral clearance as 0.1 mm.
Answer
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Differentiate between ball bearing and roller bearings.
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A load varies continuously in magnitude in a sinusoidal manner. The directions remains fixed. For a total life of 20 × 10&sup6; rev at a speed of 400 rpm, determine the mean cubic load Fm if the maximum load is 1000 lb.
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What are the important points to be considered while designing of the ball bearing? Explain the assumptions used.
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What are different forces acting on piston? Explain with a suitable diagram.
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Design a cast iron piston for a single acting four stroke diesel engine with the following data: Cylinder bore = 300 mm, Length of stroke = 450 mm, speed = 300 rpm, indicated mean effective pressure = 0.85 MPa, Maximum gas pressure = 5 MPa, Fuel consumption = 0.3 kg per BP per h, Higher calorific value of fuel = 44000 kJ/kg. Assume suitable data.
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Explain the concept of buckling in connecting rod.
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Explain the procedure for finding the dimensions of the I section of the connecting rod.
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A railway wagon moving at a velocity of 1.5 m/s is brought to rest by a bumper consisting of two helical springs arranged in parallel. The mass of the wagon is 1500 kg. The springs are compressed by 150 mm in bringing the wagon to rest. The spring index can be taken as 6. The springs are made of oil hardened and tempered steel wire with ultimate tensile strength of 1250 N/mm² and modulus of rigidity 1370 N/mm². The permissible shear stress for the spring wire can be taken as 50% of the ultimate tensile strength. Design the spring.
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A pair of helical gears with a 23° helix angle is to transmit 3 ½ HP at 10000 rpm of the pinion. The velocity ratio is 4 to 1. Both gears are to be made up of hardened steel, with an allowable s0 = 15000 psi for each gear. The gears are 20° stub and the pinion is to have 24 teeth. Determine the minimum diameter gears that may be used, and the required BHN.
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