JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year DESIGN OF MACHINE MEMBERS – II Previous Question Paper (March - 2024 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. iii) In Part B, Answer any one question from each unit.
Part A
Define the term ‘Bearing Life’.
Answer
The Bearing Life of an individual rolling contact bearing is defined as the total number of revolutions (or the number of hours at a constant speed) that the bearing can endure before the first evidence of fatigue failure develops in the material of either the stationary ring, the rotating ring, or any of the rolling elements.
Key aspects of bearing life include:
- Rating Life (L10): This is the life associated with 90% reliability, meaning 90% of a group of apparently identical bearings will complete or exceed this number of revolutions before the onset of material fatigue.
- Median Life: This represents the life that 50% of the bearings in a group will reach or exceed, which is typically about five times the rating life.
- Fatigue Mechanism: The failure is characterized by the spalling or flaking of the load-carrying surfaces due to cyclic stresses acting on the subsurface material.
Explain various engineering applications of sliding contact bearings.
Answer
Sliding contact bearings, also known as journal bearings or plain bearings, are essential components in mechanical systems where high load-carrying capacity and vibration damping are required. Their primary engineering applications include:
- Internal Combustion Engines: Used extensively as crankshaft main bearings and connecting rod big-end bearings. These bearings must withstand high-pressure pulses and provide a stable oil film under varying load conditions.
- Steam and Gas Turbines: Employed to support heavy rotors operating at extremely high rotational speeds. The hydrodynamic lubrication provided by these bearings ensures minimal friction and heat generation during continuous operation.
- Heavy Industrial Machinery: Utilized in large-scale equipment such as rolling mills, crushers, and cement kilns. These applications involve heavy shock loads and slow-to-moderate speeds where sliding contact bearings offer superior durability compared to rolling element bearings.
- Centrifugal Pumps and Compressors: Used to support shafts where precise alignment and vibration control are critical to prevent seal failure and mechanical wear.
- Hydroelectric Power Plants: Large thrust bearings (such as Michell or Kingsbury thrust bearings) are used to support the massive vertical loads of turbine-generator shafts, effectively managing axial thrust while maintaining a stable fluid film.
- Machine Tool Spindles: In high-precision applications, hydrodynamic bearings are used to provide exceptional stiffness and damping characteristics, which are vital for achieving high surface finish accuracy.
The selection of these bearings is typically governed by the Sommerfeld number and the Petroff's equation, which relate the bearing geometry, lubricant viscosity, and operating speed to the frictional performance of the system.
Explain briefly about the static loading of ball and roller bearing.
Answer
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Write short note on classifications and different types of anti-friction bearings.
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What is the function of connecting rod of IC engine?
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What is the function of the piston and materials used for piston?
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Write advantages of belt drives over rope drive.
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Sketch the cross section of a V belt and label its important parts.
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Define module and diameters pitch.
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Define the terms i) pressure angle ii) circular pitch.
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Part B
What are bearing material and desirable properties of a good bearing material?
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What are the various terms using the journal bearings analysis and design? Give their definitions in brief.
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Formulate the heat generated and dissipated in a journal bearing.
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A 250×250 mm bearing carries a load of 108 kN. The bearing rotates at 1500 rpm. The clearance ratio is 670. For full journal bearing the power lost in friction is 14.36 kW. Find the viscosity of the oil.
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What are rolling contact bearings? Discuss their advantages over sliding contact bearings.
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Select a single row deep groove ball bearing for a radial load of 4000N and an axial load of 5000N, operating at a speed of 1600rpm for an average life of 5 years at 10 hours per day. Assume uniform and steady load.
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A shaft is mounted on two roller bearings, which are 350 mm apart. The shaft carries a bevel gear at the middle. At a shaft speed of 900 rpm; the gear forces are: radial load = 10 kN, and thrust load = 3.5 kN. Determine the rated dynamic capacity of the bearing, for a desired life of 10,000 hours. The service factor is 1.5, thrust factor is 0.67 and radial load factor is 0.67.
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Design a cast iron piston for a four stroke IC engine for the following specifications. Cylinder bore=120 mm, Stroke length=150 mm, Maximum gas pressure=5 MPa, Brake mean effective pressure=0.7 MPa, Fuel consumption=0.25 Kg/kW/hr, Speed=2400 rpm. Assume any other data necessary for the design.
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The following data is given for a connecting rod: Engine speed =1500 rpm, Length of connecting rod=320 mm, Length of stroke=140 mm, Density of the material=7830 Kg/m3, Thickness of web flanges= 6 mm, The cross section of the connecting rod is I section. Upper and bottom flange dimensions are = 4mm, Web dimensions are = 3mm. Calculate the whipping stress in connecting rod.
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A V-belt is required for a 15 kW, 1440 rpm electric motor, which drives a centrifugal pump running at 360 rpm, for a service of 24 hrs per day. From space considerations, the center distance should be approximately 1 m, Determine: a) belt specifications, b) number of belts, c) correct center distance, and d) pulley diameters.
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It is required to select a v-belt drive to connect a 20 kw, 1440 rpm motor to a compressor running at 480 rpm for 15 hours per day. Space is available for a center distance approximately 1.2m. Determine a) The specifications of the belt b) Diameter of the motor and compressor pulleys c) The correct center distance and the number of belts.
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A bronze spur pinion rotating at 600 r.p.m. drives a cast iron spur gear at a transmission ratio of 4 : 1. The allowable static stresses for the bronze pinion and cast iron gear are 84 MPa and 105 MPa respectively. The pinion has 16 standard 20° full depth involute teeth of module 8 mm. The face width of both the gears is 90 mm. Find the power that can be transmitted from the standpoint of strength.
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A gear drive is required to transmit a maximum power of 22.5kW. The velocity ratio is 1:2 and pinion speed is 200 rpm. The approximate center distance between the shafts may be taken as 600mm.The teeth have 20° stub involute profiles. The static stress for the gear material (CI) is 60MPa and face width as 10 times the module. Find the module, Face width and number of teeth on each gear. Check the design for dynamic loads. The deformation or dynamic factor in the Buckingham equation may be taken as 80 and combination factor for the wear as 1.4.
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