JNTU B.Tech III Examinations, Aug/Sep 2008
DESIGN OF MACHINE MEMBERS-II
( Common to Mechanical Engineering and Production Engineering)
1. (a) Specify the types of lubricant with examples.
(b) A 50 mm diameter journal bearing rotates at 1500 rpm. L/D=1, radial clearance 0.05mm, minimum film thickness = 0.02 mm. Calculate the maximum radial load that the journal bearing can carry and still operate under hydrodynamic condition. For this load calculate power load in friction and the oil temperature assuming the Hg and Hd. Absolute viscosity = 20 × 10?3 PaS
(1 centipoise = 10?3 PaS). Specific gravity of oil 0.88, specific heat of oil 2.1kJ/kgoC
2. (a) What is equivalent static load.
(b) The radial reaction on a bearing is 9000 N. It also carries a thrust of 5000N. The speed of the shaft is 1000 rpm. The outer ring stationary. Expected average life of bearing is about 25,000 hours. The load on the bearing is smooth, the service is 8 hours/day. Select,
i. Suitable roller bearing
ii. What is the rated 90% life of selected bearing?
iii. Compute the probability of the selected bearing surviving 25,000 hours
3. The following data refer to a 4-stroke single cylinder vertical engine
Piston diameter = 125mm
Speed of the engine=1200 r.p.m
Weight of the reciprocating parts = 45N
Design stress for the material used = 80N/sq.mm
Design bearing stress=10N/sq.mn
Design stress for bolts=35N/sq.mm
Design a suitable connecting rod and check for stresses due to inertia.
4. What are the stresses to which an over hanging shaft is subjected to? How would you proceed to design such a crank shaft.
5. (a) Discuss the phenomenon of surge in spring, and how do you eliminate surge in spring?
(b) With the aid of a diagram explain the modified soderberg line method for various stresses in a spring.
6. (a) Derive an expression for the length of open flat belt.
(b) Design a cast iron pulley transmitting 20 kW at 300 r.p.m. The diameter of pulley is 550 mm and has four straight arms of elliptical cross-section in which the major axis is twice the minor axis.
7. (a) Explain the methods of improving the wear strength of a gear drive.
(b) A compressor running 300 rpm is driven by a 15kW, 1200 rpm motor through a 20o full depth involute gears. The centre distance is 375 mm. Choosing suitable materials for the pinion and the gear, design the drive. Assume medium shock conditions.
8. (a) Why are square threads preferable to V threads for power transmission? Explain.
(b) The lead screw of a lathe has square threads of 24 mm outside diameter and 5 mm pitch. In order to drive the tool carriage, the screw exerts an axial pressure of 2.5 kN. Find the efficiency of the screw and the power required to drive the screw, if it is to rotate at 30 rpm. Neglect bearing friction. Assume coefficient of friction of screw threads as 0.12.