Machine Design 1 - May 2013

Mechanical Engineering (Semester 6)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) Explain design considerations of castings and forgings.(10 marks) 1 (b) Explain different thread profiles used in power screws.(5 marks) 1 (c) Define static and fatigue stress concentration factors and establish relation between them.(5 marks) 2 (a) Design a knuckle joint for a static load of 30kN. Draw the failure diagrams.(15 marks) 2 (b) Explain different types of keys.(5 marks) 3 (a) A bracket subjected to a vertical force of 10kN is shown in figure 1. It is fastened to a vertical plate by means of four identical bolts. Determine the size of bolts by using maximum shear stress theory if maximum permissible shear stress in any bolt is limited to 50N/mm².(14 marks) 3 (b) A steel plate, 80mm wide and 10mm thick, is joined to another steel plate by means of a single transverse and double parallel fillet welds as shown in figure 2. The strength of the welded joint should be equal to the strength of the plates to be joined. The permissible tensile and shear stresses for the weld material and the plates are 100 and 70N/mm² respectively. Find the length of each parallel fillet weld assuming tensile force passes through centre of gravity of three welds.(6 marks) 4 A solid shaft is supported on bearings C and D, 1000mm apart between centres. A pulley having diameter 1250mm is mounted at 250mm to the left of bearing C and a spur gear having 750mm pitch circle diameter is mounted 350mm to the right of the bearing D. The gear is driven by a pinion located horizontally behind, while pulley transmits power to another pulley of same diameter located vertically below. The coefficient of friction between the pulley and belt is 0.3. The shaft rotates in clockwise direction looking from right. The pulley and gear weigh 2700N and 900N respectively. Design the shaft, if the power transmitted is 20kW at 150rpm. Assume the load suddenly applied with minor shock.(20 marks) 5 (a) A machine component is subjected to fluctuating stress that varies from 40 to 100N/mm². The corrected endurance limit for the component is 270N/mm². The ultimate tensile strength and yield strength of the material are 600 and 450N/mm² respectively. Find the factor of safety using:
i) Soderberg line.
ii) Goodman line.
iii) Gerber theory.
Also find the factor of safety against static failure for maximum stress.(10 marks) 5 (b) A semi elliptic leaf spring used for suspension of truck has 12 leaves, two of which are extra full-length leaves. The centre-to-centre distance between the spring eyes is 1.2m and the central band is 80mm. The leaves are made of alloy steel having yield strength 1200N/mm² and modulus of elasticity 2?10⁵N/mm² and factor of safety is 3. The spring is to be designed for a maximum load of 10kN. The leaves are pre-stressed so as to equalise stresses in all leaves. Determine the width and thickness of leaves if the ratio of total depth to the width of spring is 2.5. Also find the deflection at the end of the spring.(10 marks) 6 (a) Design a closed coiled helical spring used for boiler safety valve which is required to blow steam at the pressure of 1.5N/mm². The diameter at valve is 50mm. The initial compression of the spring is 40mm and the lift is limited to 20mm. The number of cycles of load are expected to be about 10⁵. The material of the spring is chrome vanadium steel for which the strength values are:
and G=80x10³N/mm²
Use static approach. Also check for solid stress.(15 marks) 6 (b) What are the different stresses induced in a flat belt, derive equations for the same.(5 marks) 7 (a) A chain drive is to be used to transmit 5kW power from an electric motor running at 1000rpm to a machine running at 500rpm. The service conditions involve light shock. Select a standard roller chain and specify the corrent centre distance between the axes of sprockets. Also calculate actual factor of safety.(10 marks) 7 (b) A 50kW power is to be supplied to and air compressor rotating at 380rpm through a V-belt. The drive speed is 960rpm. The centre distance is limited to 1.5m. Determine the size and number of belts.(10 marks)