Mechanical Vibration 2 Marks questions with answer

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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

1.      Mass of the body

2.      Elasticity of available spring.

3.      Dash – pot which is for domping

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? 

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

2.      This method can also account for f\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.

K =      F = N/m
                        S
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
C =    F  = N/m/sec
          v

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’s 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 in 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 know but in random magnitude of excitation is not known

42.              What methods are available foe 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 parallel.

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.