Theory of Machines -Final Exams2015

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Transcript of Theory of Machines -Final Exams2015

  • South Valley University Theory of Machine Faculty of Engineering Code: MDPG 214, Time: 3 Hrs.

    Dept. of Mechanical Engineering Final Exam, 2014/2015

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    Assume any missing data Maximum: 100 marks

    Part A (2x 20=40 marks) Question One: (20 marks)

    1. What is the main purpose of gears; classify them according to their tooth shape? 2. Identify the advantages and limitations of: bevel gear, helical gears, spur gears. 3. What is the limitation of Watt governor and how is it overcome. 4. Why is balancing of rotating parts necessary for high speed engines? 5. What do you understand by balancing of revolving masses? 6. Classify with sketch the cams based on their physical shape? 7. Sketch types of followers based on surface in contact and type of motion. 8. Describe advantages and disadvantages of the gear drive as compared to belt, rope and

    chain drives.

    Question Two: (20 marks)

    1. Explain the term height of the governor. Derive an expression for the height in the case of a Watt governor. What are the limitations of a Watt governor?

    2. How is unbalanced force due to several rotating masses in the same plane determined? 3. State the different methods of gear manufacture. 4. An epicyclic bevel gear train, as shown in Figure, has fixed gear B meshing with pinion

    C. The gear E on the driven shaft meshes with the pinion D. The

    pinions C and D are keyed to a shaft, which revolves in bearings on

    the arm A. The arm A is keyed to the driving shaft. The numbers of

    teeth are: TB = 75, TC = 20, TD = 18, and TE = 70. Find the speed

    of the driven shaft, if

    a) The driving shaft makes 1000 r.p.m. b) The gear B turns in the same sense as the driving shaft at

    400 r.p.m., the driving shaft still making 1000 r.p.m.

    Part B (3x 20=60 marks)

    Answer Three Questions only. Question Three: (20 marks)

    1. Explain the terms: module, pitch circle, circular pitch, clearance, face of a tooth, flank of a tooth, backlash, dedendum and addendum.

    2. What are the main factors considered in gear selection? 3. Compare between straight and spiral bevel gear? 4. What are the main advantages and disadvantages of: worm gears, rack and pin gears. 5. Compare between helical gears and spur gears? 6. What are the different types of bevel gears? 7. State the purpose of: bevel gear, helical gears, spur gears, rack and pinion gears, internal

    spur gear and worm gears.

    Question Four: (20 marks)

    1. Explain with sketch the main types of gear trains and explain the differences between them.

    2. An epicyclic gear consists of three gears A, B and C as shown in Figure. The gear A has 72 internal teeth and gear C has 32 external

    teeth. The gear B meshes with both A and C and is carried on an arm

    EF which rotates about the centre of A at 18 r.p.m. If the gear A is

    fixed, determine the speed of gears B and C.

  • South Valley University Theory of Machine Faculty of Engineering Code: MDPG 214, Time: 3 Hrs.

    Dept. of Mechanical Engineering Final Exam, 2014/2015

    2

    3. Figure illustrates an automotive sliding-mesh transmission gearbox. Gear wheel 1 is keyed to the input shaft and cannot slide along it. Gear wheels 2, 3, and 4 form a

    compound cluster that is keyed to the output shaft but can be slid along it. Gear wheel 10

    is a reverse idler. The numbers of teeth on the various gear wheels are as shown in table.

    Determine the gear ratios for each speed selection, starting from the lowest forward

    selection to the highest and then to the reverse selection. Tabulate your results.

    Question Five: (20 marks)

    1. What is the function of a governor? How does it differ from that of a flywheel? 2. A porter governor for which the speed range can be varied by means of an auxiliary

    spring S, as shown in Figure. The spring force is transmitted to the sleeve by the arm

    AB, which is pivoted at A. The weight of each ball is 0.36 kg and the lengths of the links

    and arms are 7.5 cm. the sleeve carries a weight of 1

    kg. The sleeve begins to rise when the balls revolve at

    200 rpm in a circle of 7.5 cm radius. The speed of the

    governor is not to exceed 225 rpm when the sleeve

    has risen 1cm from its original position. Determine

    the necessary stiffness of the auxiliary spring S and

    the tension in the upper arms and lower links when

    the sleeve begins to rise from its equilibrium position

    for a speed of 200 rpm.

    Question six: (20 marks)

    1. Four masses A, B, C and D are 200 kg, 300 kg, 240 kg and 260 kg respectively. The corresponding radii of rotation are 0.2 m, 0.15 m, 0.25 m and 0.3 m respectively and the

    angles between successive masses are 45, 75 and 135. Find the position and

    magnitude of the balance mass required, if its radius of rotation is 0.2 m.

    2. A 3.6 m long shaft carries three pulleys, two at its two ends and third at the mid-point. The two end pulleys have mass of 79 kg and 40 kg and their centre of gravity are 3 mm

    and 5 mm respectively from the axis of the shaft. The middle pulley mass is 50 kg and

    its centre of gravity is 8 mm from the shaft axis. The pulleys are so keyed to the shaft

    that the assembly is in static balance. The shaft rotates at 300 r.p.m. in two bearings

    2.4m apart with equal overhang on either side. Determine :

    a) The relative angular positions of the pulleys. b) Dynamic reactions at the two bearings.

    With my best wishes Dr. Nouby M. Ghazaly