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Mechanical System Design (October 2009)

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Total No. of Questions : 12] P967 [Total No. of Pages :7 [3664] - 122 B.E. (Mechanical) MECHANICAL SYSTEM DESIGN (2003 Course) (402041) Time : 4 Hours] [Max. Marks:100 Instructions to the candidates: 1) Answer three questions from Section I and three questions from Section II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Use of logarithmic tables, electronic pocket calculator is allowed. 6) Assume suitable data, if necessary. 7) State Assumtions made. SECTION - I Unit - I Q1) a) Derive Lame s equation for thick cylinders. Highlight the assumptions made. [8] b) A closed vessel having internal diameter of 430mm is to be designed to withstand an internal pressure of 45 MPa. Material to be used is ductile material with Syt 300MPa, Sut 500MPa and Poisson s ratio 0.3. Estimate the wall thickness by using Factor of Safety 1.5 So designed cylinder if overloaded above the designed pressure, what will be the magnitude of pressure at which the cylinder will actually fail? [8] OR Q2) a) b) What is meant by pre-stressing of a cylinder? What is its necessity? Explain any one method of pre-stressing cylinder. [6] A reaction vessel of cylindrical shell of l200mm is provided with a nozzle having an opening of 125mm diameter. Protruding length of nozzle inside and outside the shell is same and it is made up of seamless tube. The thickness of shell is 8mm and that of nozzle is l2mm. Internal pressure in the shell is 0.8MPa. The corrosion allowance is 1.5mm. Permissible tensile stress for shell and nozzle material is 80MPa. Design the dimensions of reinforcing pad if required (Round off the obtained dimension to nearest Tens digit. The reinforcing material is of same quality as that of the shell and is available in thickness of 4mm. (Assume weld efficiency as 85%). [10] P.T.O. Unit II Q3) a) Define Optimum design . How it is different from adequate design? How to decide whether a given problem on optimum design is based on normal specifications or on redundant specifications? State any one practical illustration where the concept of optimum design is required to [4] be used. b) A tensile bar of length 400mm is subjected to constant tensile force of 4000N. If the factor of safety is 2, design the bar diameter with the objective of minimizing material cost using optimum material from the list given in Table 1. What will be the cost of designed shaft? [12] Material Density ( ) Cost (c) Syt G 3 2 kg/m Rs/kg N/mm N/mm2 Steel 7800 14 400 82000 Aluminum alloy 2800 70 150 27000 Titanium Alloy 4500 1100 800 41000 Table 1 OR Q4) In a lightweight equipment, a shaft is required to transmit 45kW power at 480 RPM. Required stiffness of shaft is 90N-m/ Degree. Factor of safety based on Sys is 1.5. From manufacturing considerations, the diameter must be greater that 30mm. Using max shear stress theory of failure design the shaft with the objective of minimum weight by using optimum material from the list given in Table 1 above. [16] Unit - III Q5) a) Derive expression for hoop stress due to centrifugal force in the flywheel rim. [8] b) In a certain machine, the intermittent operations demand the torque to be applied to its shaft in the following manner: During the first half revolution, the torque is to increase uniformly from 1380 N-m to 2760 N-m; in the following one revolution, it is to remain constant at 2760N-m. In the following half revolution, it is to decrease uniformly to 1380 N-m; and in the last revolution, it is to remain constant at 1360 N-m. [3664]-122 2 The machine is directly coupled to the motor, which delivers uniform torque on machine shaft and rotates at 400 RPM. Decide rim cross sectional dimensions of the flywheel that is to be added to the system so as to limit the speed fluctuation within 8% To accommodate the flywheel in confined space, its mean rim radius should be 0.5 m. Other necessary data: Material- FG 200, Ratio of rim width to thickness is = 2, Factor of Safety for design= 1.5, Density of wheel material = 7800kg/m3. [10] OR Q6) a) Fill in the blanks with proper alternative and rewrite the sentences. [5] i) Moment of inertia of flywheel is _______ proportional to the coefficient of speed fluctuation. 1) 2) directly. inversely. ii) iii) For same moment of inertia of both the flywheels, Outer Diameter of rimmed flywheel is ______ than that of the solid disc flywheel. 1) more 2) less iv) ______ limits the overall size of flywheel. 1) Mass. 2) Moment of inertia. 3) Linear velocity of rim. 4) Angular velocity. v) b) Maximum possible theoretical value of coefficient of speed fluctuation of any load prime-mover combination is_______. 1) 0.5 2) 1 3) 2 4) 3 If power capacity of a two-stroke engine is same as that of another four-stroke engine, Size of flywheel will be small in case of the___. 1) Two stroke engine. 2) Four stroke engine. The cycle of operations performed by a machine is of three revolutions. The torque required has a constant value of 400 N-m for one revolution; zero for second revolution, 550 N-m for the first half of last revolution and zero for the rest of the cycle. [3664]-122 3 The driving torque is constant and mean speed of the system is 180RPM, which can be allowed to fluctuate up to 4%. Design rim cross sectional dimensions of a flywheel with mean rim radius of 0.5 m. Other relevant data: Ratio of rim width to thickness = 2, density of wheel material [10] = 7800kg/m3. Q7) a) If the flywheel available with mean rim radius 0.5m is so small such that it just avoids stoppage of the system, find out its rim cross sectional dimensions. [3] SECTION - II Unit - IV A shaft and hole assembly of nominal diameter 40mm have the following dimensions : Hole Dia = 40 + 0.01 + 0.00 Shaft Dia = 40 0.1 0.15 Assuming the shaft and hole diameters are normally distributed, determine : The percentage of assemblies having clearance less than 0.15mm. The percentage of assemblies having clearance greater than 0.22mm. (Refer Table 2 for Areas below normal distribution curve). b) Write a short note on : Design for Forgings. [10] [6] OR Q8) a) An assembly of three components A, B and C is shown in figure 1. If the dimensions of the three components are normally distributed with design tolerance equal to the natural tolerance, determine the percentage of assemblies where interference is likely to occur. [12] [3664]-122 4 (Refer Table 2 for Areas below normal distribution curve) b) Write a short note on : Design for Assembly. [4] Unit - V Q9) a) What is meant by Structure Formula ? Is the Structure Formula 2(1) 2(4) 3(2) valid? If yes, how? If not, Why? Explain with the help of structure diagram. [3] b) A multi-speed sliding mesh gearbox is to be designed for tapping speeds ranging from 20rpm to near about 3170 rpm according to R5 preferred series. It is to be driven by a three-phase induction motor running at 1440rpm. For the proposed system, List out intermediate speeds. Write all structure formulae. Select only those formulae that are leading to minimum number of speeds on output shaft of each stage. Select set of optimum formulae from the short listed sets. (Write down the reason for discarding each of the other sets based on standard selection criteria). Draw symmetric structure diagrams based on short listed formulae. Select optimum structure diagram. (Write down the reason for discarding each of the other diagrams based on standard selection criteria). Draw speed diagram. Draw gearing diagram when Third speed from bottom is being tapped. Find out number of teeth on each gear. Draw deviation diagram of the system. [15] OR Q10)a) Prove that the difference in number of teeth on adjacent gears on a compound gear from multi-speed sliding mesh gear drive must be greater than four. [4] [3664]-122 5 b) Figure 2 shows Gearing diagram of a multi-speed gear drive with number of teeth on each gear as specified. If the input shaft is rotating at 120 rpm, what are the speeds available at output shaft? Draw structure diagram of this gear drive. What speed is being tapped in the shown position? What is the factor for this gear drive? [10] c) What are different ergonomic considerations involved in the design of [4] Controls. Unit - VI Q11)a) b) [4] Answer the following in one sentence i) What are the basic objectives of a material handling system? ii) What are the types of loads that can be handled by a material handling system? iii) What is the basic classification of the material handling systems? iv) What is meant by containerization? Draw a schematic sketch of a flat belt conveyor. Name all its components. Write down the equation used to calculate power required to drive the conveyor. [4] What is the need of tension take up device? How a vertical gravity type take-up works? [4] [3664]-122 6 c) A Horizontal flat belt conveyor is to be used for transporting 350 metric ton of iron ore per hour at belt speed of 2 m/s. The mass density of iron ore is 1700 kg/m3. If surcharge factor is 0.1, determine the required belt width. [4] OR Q12)Following data relate to a horizontal belt conveyor used for conveying coal in a thermal power station: Capacity of conveyor : 400 ton/hr Density of coal: 800 kg/m3 Belt speed: 2 m/s Surcharge factor: 0.1 Number of plies: 3 Material Factor K1 : 2 Belt tension and contact factor : 80 Material conveying length: 260m Center distance between snub pulleys: 255m Ratio of tail pulley to drive pulley dia. : 1.0 Ratio of snub pulley to drive pulley dia. : 0.5 Mass of each carrying run idler: 25kg Mass of each return run idler: 20kg Pitch of carrying run idlers: 1m Pitch of return run idlers: 2.5m Friction factor for idlers: 0.02 Snub Factor for snub pulleys: 0.03 Snub factor for Drive and tail pulleys: 0.06 Material velocity component along belt drive: 1m/s Angle of lap on drive pulley: 210 Coefficient of friction between belt and pulley: 0.4 Drive efficiency: 93 % Motor speed 1440 RPM Determine following parameters of the conveyor: Standard belt with rounded off to nearest hundred mm. Reduction ratio of the gear reducer Power required to drive the conveyor. kbkb [3664]-122 7 [16]

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