Freshers Mechanical Engineers Interview Questions & Answers
76. State D’ Alembert’s principle and write down its importance.
Ans: D’ Alembert’s principle enables us to replace a given system by a massless rigid body so that forces acting on it are equivalent to those on real body and then enables to determine the forces transmitted to other paired rigid body. This way the problem of kinetics gets reduced to equivalent problem on statics.
77. What is the effect of inertia of reciprocating parts on the engine frame ?
Ans: Inertia of reciprocating parts subjects engine frame to the force required to accelerate the reciprocating mass and thus subject them to primary disturbing force and secondary disturbing force. Secondary disturbing force comes into play due to obliquity of connecting rod and has twice the frequency of the primary force.
78. How the reciprocating masses of engine are balanced ?
Ans:
Four conditions need to be ensured for balancing reciprocating masses.
(i) Primary forces must balance
(ii) Primary couples must balance
(iii) Secondary forces must balance
(iv) Secondary couples must balance.
79. Why only partial primary balance is achieved and not full primary balance in case of reciprocating masses ?
Ans: Balancing mass of same amount as reciprocating mass may balance primary force but introduce another unabalanced vertical force. Thus compromise is effected by balancing only a portion of primary forces.
80. What is hammer blow in locomotives ?
Ans: Vertical unbalanced force introduced due to balancing weight for balancing primary force at high speeds may be sufficient enough to lift the wheels from the rails and this condition is known as hammer blow.
81. How may types of bonded strain gauges used ?
Ans:
(a) Fine wire strain gauge
(b) Metal foil strain gauge
(c) Piezo resistive gauge.
82. In a spring mass system, mass is doubled and spring stiffness halved. The natural frequency of longitudinal vibration will be … ?
Ans: Halved.
83. What is PID controller ?
Ans: A suitable combination of three basic modes — proportional integral and derivative (PID) used to improve all aspects of system performance is called PID controller.
The characteristics of three modes of control are :
Proportional controller — Stabilises gain but produces a steady state error.
Integral controller — Eliminates steady state error
Derivative controller — Reduces rate of change of error
84. In forced vibrations, how the spring force, damping force and inertia force behave in relation to direction of displacement ?
Ans: Spring force is always opposite to displacement, damping force lags by 90°, and inertia force is in phase with displacement.
85. On what factors depends the critical damping coefficient ?
Ans: Critical damping coefficient depends on mass and natural frequency.
86. Name various functional elements of an instrumental system ?
Ans:
(1) Primary sensing element
(2) Variable conversion (or) transducer element
(3) Variable manipulation element
(4) Data transmission element
(5) Data processing element
(6) Data presentation element
(7) Data storage and playback element.
87. What is the use of a hot-wire anemometer?
Ans: It is used to measure the flow rate and used in study of varying flow conditions.
88. Differential between main function of flywheel and governor.
Ans: The function of flywheel is to reduce fluctuation of speed during a cycle and the function of governor is to control the mean speed over a period for output load variations. Flywheel has no influence on mean speed of prime mover and governor has no influence over cyclic speed fluctuation.
89. On what factor depends the natural frequency of a system ?
Ans: It depends on the mass and stiffness of system.
90. State the condition for the stability of a governor. Whey is an isochronous governor not stable ?
Ans: For a governor to be stable, the ratio of controlling force and radius of governor balls (r) must increase with increase in radius. Further controlling force must be zero for a certain value of r and be negative below it.
In isochronous governor, ratio of controlling force and radius of governor is constant for all values of radius and as such it is not able to attain a definite position for particular speed.
91. State Grashof s law for fourbar linkage mechanism.
Ans: According to it, the sum of the shortest and longest link length can’t be greater than the sum of the remaining two link lengths, if there is a continuous relative motion between two members.
92. State the conditions for a four-bar linkage to be called a Grashof s linkage and also when this linkage would become a double crank mechanism ?
Ans: For Grashof s linkage, shortest link is fixed and sum of shortest and longest link is less than the sum of other two links.
For double crank mechanism, two alternate links are of same length and one of larger link is fixed.
93. What are inversions ?
Ans: Inversions are different mechanisms obtained by fixing different links in a knematic chain but keeping relative motions of links unchanged with respect to one another.
94. How many inversions are possible in a kinematic chain having N links.
Ans: N inversions.
95. Name the three stages of a generalised measuring system.
Ans: (i) Sensing and converting the input to a convenient and practicable form.
(ii) Processing/manipulating the measured variable.
(iii) Presenting the processed measured variable in quantitative form.
96. What is the function of an intermediate modifying system ?
Ans: Various functions of an intermediate modifying system are
(1) Amplification
(2) Modification (modulation)
(3) Data Processing
(4) Data transmission.
97. What is static pressure ? How is it measured.
Ans: The pressure caused on the walls of a pipe due to a fluid at rest inside the pipe or due to the flow of a fluid parallel to walls of the pipe is called static pressure. This static pressure is measured by inserting a pressure measuring tube into the pipe carrying the fluid so that the tube is at right angle to the fluid flow path.
98. State when the meshing surfaces can be called conjugate ?
Ans: Meshing surfaces are called conjugate when they follow the law of gearing, i.e. the common normal at the point of contact always passes through a pitch point on the line joining the centres of rotation.
99. Give Kelin’s construction and its use.
Ans: Triangle representing line joining piston centre and crank rotation point, crank and connecting rod is drawn. Connecting rod line is produced to intersect a perpendicular through crank rotation point on line joining piston centre and crank rotation point. Draw a circle with radius equal to projected length and centre as common point on connecting rod and crank. Another circle is drawn with piston centre as centre and radius of length of connecting rod. Draw a line joining points cutting two circles and meeting connecting rod at same point. Polygon joining lines of crank, part of connecting rod, line along chord to central axis and line from last point to crank rotation centre is acceleration polygon.
100. When a tensile member fail in shear ?
Ans: If its shear stength is less than half of its tensile strength.