## Vibration Analysis Interview Questions & Answers

1. Question 1. What Is Meant By Vibrations?

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

Vibration is occasionally desirable. For example the motion of a tuning fork, the reed in a woodwind instrument or harmonica, or the cone of a loudspeaker is desirable vibration, necessary for the correct functioning of the various devices.

2. Question 2. Define Force Vibration?

Forced vibration is when an alternating force or motion is applied to a mechanical system. Examples of this type of vibration include a shaking washing machining due to an imbalance, transportation vibration (caused by truck engine, springs, road, etc), or the vibration of a building an earthquake.

In forced vibration the frequency of the vibration is dependent on the frequency content of the force or motion applied, but the magnitude of the vibration is strongly dependent on the behaviour of the mechanical system.

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4. Question 3. What Is Meant By Logarithmic Decrement?

Logarithmic decrement method is used to measure damping in time domain. In this method, the free vibration displacement amplitude history of a system to an impulse is measured and recorded. Logarithmic decrement is the natural logarithmic value of the ratio of two adjacent peak values of displacement in free decay vibration.

5. Question 4. Define Transmissibility?

Transmissibility is a term that is a term that is used to describe the response of a vibration isolation system. Literally, transmissibility is the ratio of displacement of an isolated system to the input displacement. It is used to describe the effectiveness of a vibration isolation system. Transmissibility varies with frequency.

6. Question 5. What Is Dry Friction Damper?

The dry-friction damper consists of a shock-absorbing mass with a flexible link with the frame, dry friction shoes coupled to the mass, and an expansion spring to provide the necessary amount of dry friction. The damper is designed to reduce normal pressure on the contact surfaces when there is a change in direction of the absorbing mass by incorporating an inertia mass which has a flexible link with the shoes.

During oscillation in a system, inertia mass undergoes various accelerations and the greater the acceleration on the inertia mass the smaller is the effort with which shoes are pressed against the friction surfaces. With a sufficiently rigid link the acceleration of the inertia mass is virtually equal to the acceleration of the absorbing mass which means that with maximum acceleration of the absorbing mass the dry friction force will be the least.

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8. Question 6. Mention The Uses Of Vibration?

In the branch of engineering vibration is useful in the analysis, design, construction, operation and maintenance of complex structures.

9. Question 7. What Is Rayleigh’s Method, Write Its Applications?

It is a method used for calculating approximate natural frequencies for a vibrating system assuming a deflected shape and balancing kinetic and strain energies.

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11. Question 8. What Is The Critical Speed Of Shaft?

The angular speed at which a rotating shaft becomes dynamically unstable with large lateral amplitudes, due to resonance with the natural frequencies of lateral vibration of the shaft is called as the critical speed of shaft.

12. Question 9. Define Continuous Beam?

A beam having more than two supports is called as continuous beam.

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14. Question 10. What Is Meant By Natural Vibration?

Natural vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

15. Question 11. Define Resonance?

Resonance is the tendency of a system to oscillate at maximum amplitude at a certain frequency. This frequency is known as the system’s natural frequency of vibration, resonant frequency, or eigenfrequency.

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17. Question 12. Mention Important Types Of Free Vibrations?

Type of free vibration are, pulling a child back on a swing and then letting go or hitting a tuning fork and letting it ring.

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19. Question 13. What Is Meant By Viscous Damping?

A method of converting mechanical vibrational energy of a body into heat energy, in which a piston is attached to a support, is called viscous damping.

20. Question 14. Define Vibration Isolation?

Vibration isolation, in structures, of those vibrations or motions that are classified as mechanical vibration; involves the control of the supporting structure, the placement and arrangement of isolators, and control of the internal construction of the equipment to be protected.

21. Question 15. What Is An Accelerometer And What Is Its Use?

An accelerometer is a device for measuring acceleration. An accelerometer inherently measures its own motion (locomotion), in contrast to a device based on remote sensing. One application for accelerometer is specifically configured for use in gravimetry.

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23. Question 16. Define Influence Coefficients?

It is defined as action required for or due to unit acceleration. It is used for deriving the equations of motion for a vibrating system. There are two types of influence coefficient; stiffness influence coefficient and the flexibility influence coefficient.

24. Question 17. What Is Continuous System?

A continuous system has infinite degree of freedom hence infinite number of natural frequencies. These systems have their inertia and stiffness properties distributed in a continuous way.

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26. Question 18. What Are Three Elementary Part Of A Vibrating System?

1. Mass of the body
2. Elasticity of available spring.
3. Dashpot which is for dumping.
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28. Question 19. What Is Logarithmic Decrement?

Logarithmic decrement is the “logarithmic ratio of any two consecutive amplitudes on the same side of the main position” it is a measure of decay of amplitude of the vibrating system it is denoted.

29. Question 20. Define The Term Magnification Factor?

Magnification factor or magnifier is defined as the ratio of amplitude of vibration to the amplitude of zero frequency deflection.

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31. Question 21. How Can We Make A System To Vibrate In One Of Its Natural Made?

The motion where every point the system executes harmonic motion with one of is natural frequencies of the system, is called the principal mode of vibration, the amplitude for one of the masses is taken as unity the principal mode is said to be normal mode of vibration.

32. Question 22. What Is Basic Assumption Is Deriving Dunker Lay’s Formula?

1. Dunker lay’s formula is applicable to a uniform diameter shaft carrying several loads.
2. This method can also account for self weight of the shift.
33. Question 23. How Does A Continuous System Differ From A Discrete System In The Nature Of Its Equation Of Motion?

Continuous system is equivalent to an infinite elements of masses concentrated at different points. The equation of the continuous systems is derived on the assumption that the bodies are homogeneous and isotropic & that they obey Hooke’s law within the elastic limit.

34. Question 24. What Ate Various Methods Available For Vibration Control?

1. Removing the Causes of vibration.
2. Putting the screen if noise is the objection.
3. Placing the machinery on proper type of isolators.
4. Shock absorbers.
5. Dynamic vibration absorbers.
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36. Question 25. What Are Vibrometer?

A vibrometer is an instrument to measure the displacement of a vibrating machine part generally; the instrument natural frequency is designed twice as slow as the slowest vibration recorded.

37. Question 26. What Is Common Type Of Damping?

1. Viscous damping.
2. Dry friction damping.
3. Structural damping.
4. Slip or interfacial damping.
38. Question 27. Why Is It Important To Find The Natural Frequency Of A Vibrating System?

When the frequency of externally excited system equal to natural frequency of vibration system it get failure due to resonance. So to avoid the resonance at vibrating system natural frequency must be known.

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40. Question 28. What Happens To The Response Of An Undamped System At Resonance?

In damped vibrating system; the system get vibrate till it’s frequency reaches to the natural frequency. So it likely cause to failure of body. So if system is having undamped vibration it leads to failure of body or system.

41. Question 29. What Are Principal Coordinates?

Principal coordinates: The three directions in space i.e. x, y, z direction are known as the basic or principal coordinates these are very important in designing of robots as it decide the degree of freedom for every action.

42. Question 30. Define The Flexibility And Stiffness Influence Coefficients?

Flexibility: It is defines as the design that can adapt any change when any external change occurs.

Stiffness influence coefficients:  It is defined as when the system is unconstrained the stiffness matrix is positive semi definite hence a constant is used to show the stiffness of system is knows as stiffness influence coefficient denoted as ‘K’.

43. Question 31. What Is Rayleigh’s Principle?

Rayleigh principle: It is stated that the distribution of the potential and kinetic energies of conservation, elastic system in the fundamental mode of vibration is such that the frequency is minimum.

44. Question 32. How Many Natural Frequencies Does A Continuous System Have?

A continuous system which is under a vibration has only one natural frequency which creates the resonance if the frequency of system matches with natural frequency.

45. Question 33. What Is The Difference Between A Vibration Absorber And A Vibration Isolator?

Difference between a vibration absorber and a vibration isolator:

A vibration absorber is a device that can absorb the vibration and make its intensity low while an isolator is device that can keep apart the vibration between two surface or system in contact in which one is vibrate continuously.

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47. Question 34. What Is An Accelerometer?

A accelerometer is device or a transducer that sense the acceleration of system and convert in into a useful signal are known as accelerometer.

48. Question 35. What Are The Causes Of Vibration?

• Unbalanced centrifugal forces in the system.
• Elastic nature of the system.
• External excitation applied on the system.
• Winds may cause vibrations of certain systems such as electricity lines, telephones lines etc.
49. Question 36. Give Two Examples Each Of The Bad And Good Effects Of Vibration?

1. Proper readings of the instrument cannot be taken.
2. Many building, structures and bridges may fall.

Good effects:

1. Useful for the propagation of sound.
2. Vibratory conveyors.
3. Musical instruments.
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51. Question 37. Define Degree Of Freedom Of A Vibrating System?

The minimum number of independent coordinates required to specify the motion of a system at any instant is known as degrees of freedom of the system.

52. Question 38. In Vibration Analysis, Can We Always Disregard Damping?

No.

53. Question 39. Can We Identify A Nonlinear Vibration Problem By Looking At Its Governing Differential Equation?

Yes.

54. Question 40. What Is The Difference Between Deterministic And Random Vibration?

In deterministic the magnitude of excitation force is know but in random magnitude of excitation is not known.

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56. Question 41. What Methods Are Available For Solving The Governing Equations Of A Vibration Problem?

Rayleigh method, energy method, equilibrium method.

57. Question 42. How Do You Connect Several Springs To Increase The Overall Stiffness?

By connect springs in parallel.

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59. Question 43. What Is The Difference Between Harmonic Motion And Periodic Motion?

The motion which repeat itself after an equal interval of time while harmonic motion is one form of the periodic motion. All the harmonic motions are periodic in nature while the vice-versa is not always true.

60. Question 44. Define Vibration?

When a particle goes on one side from mean position and returns back and then it goes to other side and again returns back, then it is known as one vibration. In other words, to and fro motion of a particle about a fixed point is known as vibration.

61. Question 45. Name Different Types Of Vibrations?

There are three important types of vibrations from subject point of view:

• Free or natural vibrations.
• Damped vibrations.
• Forced vibrations.
62. Question 46. Define Free Vibrations?

If the vibrations of a particle after giving it an initial displacement remain continued, then the vibrations are called free or natural vibrations. No external force acts on the particle. In other words, the vibrations of the particle with fundamental frequency under the influence of the restoring force are called free vibrations.

63. Question 47. Define Damped Vibrations?

The vibrations of a body whose amplitude goes on reducing over every cycle of vibrations are known as damped vibrations. This is due to the fact that a certain amount of energy possessed by the vibrating body is always dissipated in overcoming frictional resistance to the motion.

64. Question 48. Define Forced Vibrations?

When the body vibrates under the influence of external periodic force, then the vibrations are known as forced vibrations.

The body does not vibrate with its natural frequency, but it vibrates with the frequency of the driver.

65. Question 49. Define Time Period Related To Vibratory Motion?

The time interval after which the motion is repeated itself is called time period. It is usually expressed in seconds.

66. Question 50. Define Time Cycle Related To Vibratory Motion?

The motion completed during one time period is called cycle.

67. Question 51. Define Frequency Related To Vibratory Motion?