JNTU
JNTU B.Tech II MECHANICAL TECHNOLOGY Previous Question Paper (October - 2020 - R15)
Answer any five questions. All questions carry equal marks.
Main
Explain the various types of cam mechanism with a line sketch.
Answer
A cam mechanism consists of a driving member called the cam, a driven member called the follower, and a frame. Cams are classified based on their shape, the motion of the follower, and the path of the follower.
1. Radial or Disc Cam: The follower moves in a direction perpendicular to the cam axis. The cam is a disc with an irregular profile.
2. Cylindrical or Barrel Cam: The follower moves in a direction parallel to the cam axis. The cam is a cylinder with a groove cut into its periphery.
3. Translating Cam: The cam is a plate or block that moves in a reciprocating or oscillating motion, causing the follower to move.
4. Wedge or Flat Cam: A simple wedge-shaped cam that moves back and forth to impart motion to the follower.
Follower Classifications:
- By Surface: Knife-edge, roller, flat-faced, or spherical-faced followers.
- By Motion: Reciprocating (translating) or oscillating (pivoted) followers.
- By Path: Radial (axis passes through the cam center) or offset (axis is displaced from the cam center).
A typical radial cam with a roller follower can be sketched as a circular base profile with an eccentric lobe, where the roller follower is held against the profile by a spring, moving vertically as the cam rotates.
Draw a cam profile to drive an oscillating roller follower to the specifications given below:
(a) Follower to move outwards through an angular displacement of 20° during the first 120° rotation of the cam;
(b) Follower to return to its initial position during next 120° rotation of the cam;
(c) Follower to dwell during the next 120° of cam rotation.
The distance between pivot center and roller center = 120 mm; distance between pivot center and cam axis = 130 mm; minimum radius of cam = 40 mm; radius of roller = 10 mm; inward and outward strokes take place with simple harmonic motion.
Answer
To construct the cam profile for an oscillating roller follower with Simple Harmonic Motion (SHM), follow these steps:
- Layout of Geometry: Draw the cam axis O and the pivot center P at a distance of 130 mm. Draw the base circle of radius 40 mm. The follower arm length is 120 mm.
- Displacement Diagram: Construct a semi-circle with a diameter equal to the maximum angular displacement of 20°. Divide the semi-circle and the angular intervals (120° outstroke, 120° return) into equal parts (e.g., 6 parts). Project the points from the semi-circle onto the displacement diagram to obtain the SHM curve for the 20° oscillation.
- Follower Positions: Draw the arc of the follower arm (radius 120 mm) centered at P. Mark the initial position of the roller center. For each angular increment of the cam, rotate the follower arm by the corresponding displacement angle obtained from the SHM diagram.
- Cam Profile Construction: With the pivot center P as the center, draw arcs representing the path of the roller center for each angular position. Draw the roller circles of radius 10 mm at these centers. The cam profile is the smooth curve tangent to all these roller circles.
- Dwell: During the final 120° rotation, the follower remains stationary at the initial position, resulting in a circular arc on the cam profile.
The final profile is generated by drawing the envelope of the roller positions as the cam rotates relative to the fixed pivot P.
The power is transmitted from a pulley 1 m diameter running at 200 rpm to a pulley 2.25 m diameter by means of a belt. Find the speed lost by the driven pulley as a result of creep, if the stress on the tight and slack side of the belt is 1.4 MPa and 0.5 MPa respectively. The Young’s modulus for the material of the belt is 100 MPa.
Answer
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Determine the width of a 9.75 mm thick leather belt required to transmit 15 kW from a motor running at 900 rpm. The diameter of the driving pulley of the motor is 300 mm. The driven pulley runs at 300 rpm and the distance between the centre of two pulleys is 3 metres. The density of the leather is 1000 kg/m3. The maximum allowable stress in the leather is 2.5 MPa. The coefficient of friction between the leather and pulley is 0.3. Assume open belt drive and neglect the sag and slip of the belt.
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Explain the interference in involute gears.
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Describe the procedure involved in minimizing the number of teeth on the pinion in order to avoid interference.
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Explain the various Stages of SI engine combustion.
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Describe the working principle of centrifugal compressor with a line sketch.
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