Mechanical Vibration

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. 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. 

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. 

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. 

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. 

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. 

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. 

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. 

9.   Define continuous beam.

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

9. 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. 

10. 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. 

11. 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. 

12. 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. 

13. 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. 

14. 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. 

15. 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. 

16. 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. 

18. What are three elementary part of a vibrating system?

      Mass of the body

      Elasticity of available spring.

      Dash – pot which is for damping

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 by  

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. 

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.

22.              What is basic assumption is deriving Dunkerlay’s formula? 

      Dunkerlay’s formula is applicable to a uniform diameter shaft carrying several loads.

     This method can also account for self-weight of the shift.

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 are derived on the assumption that

the bodies are homogeneous & isotropic & that they obey Hooke’s law within the elastic

limit. 

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.

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.

26.              What are common type of damping? 

1)      Viscous damping            2)      dry friction damping

3)      structural damping      4)      slip or interfacial damping

27.              Define spring stiffness and damping constant. 

Spring stiffness (K) : It is the force required to produce unit displacement in the

direction of applied force it is expressed in N/m.

Damping coefficient (C) : It is the damping force or resistance force developed per unit

velocity of viscouse fluid it is expressed in N-sec/m

28.              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. 

29.              What happens to the response of an undamped system at resonance? 

In undamped 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. 

30.              What are Principal coordinates? 

Principal coordinates: The three directions in space i.e. x, y, z direction are known as

the basic or principle co-ordinates these are very important in designing of robots as it decide

the degree of freedom for every action. 

31.              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’. 

32.              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. 

33.              How many natural frequencies does a continuous system have? 

A continuous system which is under a vibration have only one natural frequency

which create the resonance if the frequency of system matches with natural frequency. 

34.              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 it’s 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.

35.              What is an Accelerometer? 

A accelerometer is device or a transducer that sense the acceleration of system and

convert into a useful signal are known as accelerometer. 

36.              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.

37.              Give two examples each of the bad and good effects of vibration

Bad effects

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

38.              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

39.              In vibration analysis, can we always disregard damping?

No

40.              Can we identify a nonlinear vibration problem by looking at its governing

differential equation?

Yes

41.              What is the difference between deterministic and random vibration? In

deterministic the magnitude of excitation force is known but in random magnitude of

excitation is not known

42.              What methods are available for solving the governing equations of a

vibration problem?

Rayleigh method, energy method, equilibrium method

43.              How do you connect several springs to increase the overall stiffness?

By connect springs in series

44.              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.

45.              Define the terms: cycle, amplitude, phase angle, frequency, period and

natural frequency.