JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year MECHANICAL TECHNOLOGY Previous Question Paper (March - 2021 - R15 / R13)
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Main
A cam profile consists of two circular arcs of radii 30 mm and 25 mm, which are joined by circular arcs. The follower is a roller of 15 mm radius, and its line of action is a straight line passing through the cam axis. The lift of the follower is 30 mm, and the total angle of action is 140°. The cam rotates at 600 rpm. Find the maximum velocity of the follower while it is in contact with the straight flank of the cam.
Answer
To find the maximum velocity of the follower while in contact with the straight flank, we first identify the geometry of the cam. The cam profile consists of a base circle of radius rb = 25 mm and a nose radius rn = 30 mm. The lift L = 30 mm. The roller radius rr = 15 mm. The effective radius of the base circle is Rb = rb + rr = 25 + 15 = 40 mm, and the effective radius of the nose is Rn = rn + rr = 30 + 15 = 45 mm.
The straight flank is tangent to the base circle. For a radial roller follower, the velocity v of the follower during the straight flank motion is given by the formula:
where ω is the angular velocity of the cam and θ is the angle of rotation. Given N = 600 rpm, the angular velocity is:
The maximum displacement on the straight flank occurs at the point of transition to the circular arc. The length of the straight flank can be determined by the geometry of the lift. Since the follower moves along a straight line passing through the cam axis, the velocity on the straight flank is constant if the flank is radial, but here it is defined by the geometry of the circular arcs. For a straight flank inclined at an angle, the velocity is v = ω × rb × tan(φ), where φ is the pressure angle. At the maximum extent of the straight flank, the velocity is calculated as:
Given the constraints and the geometry of the circular arcs, the velocity is dominated by the cam speed and the effective radius. Substituting the values, the maximum velocity of the follower is approximately 2.51 m/s.
Sketch a V-belt drive, and describe its features. Derive an expression for the ratio of belt tensions on the tight side and slack side (i.e., T1 and T2) for this drive.
Answer
A V-belt drive consists of a belt with a trapezoidal cross-section running in V-shaped grooves on pulleys. The wedge action increases the frictional grip between the belt and the pulley, allowing for higher power transmission with lower initial tension compared to flat belts.
Features:
- Compact design suitable for short center distances.
- Quiet and smooth operation with minimal vibration.
- High efficiency due to the wedging action.
- Capable of absorbing shock loads.
- Easy installation and replacement.
Derivation of Tension Ratio:
Consider a small element of the V-belt subtending an angle dθ at the center of the pulley. Let T be the tension on one side, T+dT on the other, Rn be the normal reaction, and μ be the coefficient of friction. The groove angle is 2β.
Resolving forces radially:
Resolving forces tangentially:
Substituting Rn into the tangential equation:
Integrating between T1 and T2 over the angle of contact θ:
Thus, the ratio of tensions is:
Draw a neat sketch and explain the working of differential gear in an automobile in obtaining different speeds for the inner and outer rear wheels of the automobile, while moving along a curved path.
Answer
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Two equal spur gear wheels of involute teeth with 20° pressure angle have a module pitch of 3. If the number of teeth is 54, and the arc of contact is 1.75 times the circular pitch, find the addendum of the wheel.
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Describe the use of Coal-water slurry as alternate fuels in I C engines.
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Explain the following strokes in an I C engine: Suction, Compression, Expansion, Exhaust.
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Write a brief classification of I C engines.
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A single cylinder C I engine with a brake thermal efficiency of 30% uses high speed diesel oil having a calorific value of 42,000 kJ/kg. If its mechanical efficiency is 80%, find the indicated specific fuel consumption in kg/kW h.
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How is the performance of a two-stage reciprocating air compressor improved by intercooling? Explain.
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A single stage reciprocating air compressor takes in 8 m3/min of air at 1 bar and 30°C and delivers it at 6 bar. The clearance is 5% of the stroke. The expansion and compression are polytropic with the value of n=1.3. Calculate the power of the compressor.
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What is multistage air compression? What are its advantages?
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Explain the effect of clearance on work of compression in a reciprocating air compressor, and derive the expression of work done for compression with clearance volume.
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