Design of Machine Elements 1 - Jun 2015
Mechanical Engg. (Semester 5)
TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks. 1 (a) A shaft as shown in Fig Q1 (a) is subjected to bending land of 3kN, torque of 1×106 N-mm and an axial force of 15kN. Calculate the stresses at 'A' and 'B' (12 marks) 1 (b) What is mechanical engineering design? State steps involved in mechanical engineering design.(4 marks) 1 (c) Explain biaxial and triaxial stresses with neat sketches.(4 marks) 2 (a) State and explain the theories of failure applicable to i) Ductile ii) Brittle materials.(6 marks) 2 (b) What is stress concentration? Explain the factors affecting the stress concentration.(4 marks) 2 (c) A rectangular beam of 100 mm with and 200 mm depth is freely supported over a span of 2m. A load of 10kN is dropped on the middle of beam from a height of 10mm. Find the maximum instantaneous deflection and stress induced in the beam. Take E=2×105 Mpa(10 marks) 3 (a) Explain with the neat sketches the different types of varying stresses.(5 marks) 3 (b) Write a note on S-N diagram.(5 marks) 3 (c) A steel cantilever is 200mm long. It is subjected to an axial load which varies from 150N (compression) to 450N(tension) and a transverse load at its free end which varies from 80N (up) to 120N (down). The cantilever beam is of circular in cross section having a diameter of 2d for the first 50mm and diameter 'd' for the remaining length. Determine its diameter assuming the following:
Factor of safety =
Yield Stress =330 MPa
Endurance limit=300 MPa
Stress concentration factor =1.44 for bending 1.64 for axial loading
Correction factors =0.7 for reverse axial loading 1 for bending
Surface correction factor=0.9
Notch sensitivity =0.9.(10 marks) 4 (a) Two circular plates with 2d and d as outer inner diameters are clamped together by means of a bolt as shown in Fig Q4(a). The bolt is made of plan carbon steel (σy =380MPa, E=207GPa) while the plates are made of aluminium [E=71 GPa]. The initial pre load is 5kN in the bolt and external force acting on the joint is 10kN. Determine the size of bolt if factor of safety=0.2. Take σt=152 N/mm2 (8 marks) 4 (b) An offset bracket is fixed to a vertical steel column by means four bolts as shown in Fig Q4(b). Determine the diameter of bolts. Take σt=1000MPa. (12 marks) 5 A shaft is supported in bearings 600mm apart. It carries a pulley of diameter 500mm at 250mm to the right of left bearing and another pulley of diameter 380mm at 130mm to the of right bearing. The belt drive ion left pulley is vertically downward while that on the right pulley is horizontal. The permissible shear is not to exceed 42MPa. The maximum tension in smaller pulley is not to exceed 5500N, coefficient of friction 03 and angle of contact is 180°. Find the diameter of shaft.(20 marks) 6 (a) Design a Cotter Joint to resist a load of 12kN which acts along the axis of rod having following permissible stresses.
σc=80N/mm2 σt=40N/mm2 and τ=32N/mm2(10 marks) 6 (b) Design a protective CI flange coupling for a steel shaft transmitting 15kW at 200rpm and having an allowable shear stress of 40MPa. The working stress in the bolt should not exceed 300MPa. Assume that the same martial is used for shaft and key and the existing stress is twice its value in shear stress. The maximum torque is 25% greater than the full load torque. The shear stress for CI is 14 Mpa.(10 marks) 7 (a) An eccentrically loaded bracket is welded to a support as shown in figQ7(a). The permissible shear stress for the weld material is 80MPa. Determine the size of the weld (10 marks) 7 (b) Two plates of 10mm thick each are to be joined by means of a single riveted double strap butt joint. Determine the rivet diameter, pitch, strap, thickness and efficiency of joint. Take σt=80MPa and τ=60Mpa.(10 marks) 8 (a) An electric motor driven power screw moves a nut in a horizontal plane against a force of 75kN at 300mm/min . The screw has a single thread of 6mm pitch on a major diameter of 40mm. The friction coefficient at screw threads is 0.1. Estimate the power of the motor.(10 marks) 8 (b) A vertical 2 start square threaded screw of 100mm mean diameter and 20mm pitch supports a vertical load of 18kN. The nut od screw is fitted in the hub of a gear wheel having 80 teeth which meshes with a pinion of 20 teeth. The mechanical efficiency of pinion and gear wheel drive is 90%. The axial thrust on screw in taken by a collar bearing 250mm outside diameter and 100mm inside diameter. Assuming uniform pressure conditions find the diameter of pinion shaft and height of nut when friction coefficient for vertical screw and nut is 0.15 and that of collar bearing is 0.2. Take τ=50MPa and Pb=1.4MPa(10 marks)