# Dynamics of Machinery DOM Interview Question Mechanical Engineering

Dynamics of Machinery DOM

1. What are the conditions for a body to be in static and dynamic equilibrium?

Necessary and sufficient conditions for static and dynamic equilibrium are

1. Vector sum of all forces acting on a body is zero.

2. The vector sum of the moments of all forces acting about any arbitrary point or axis is zero.

The first condition is the sufficient condition for static equilibrium together with the second condition is necessary for dynamic equilibrium.

2. Define static force analysis.

If components of a machine accelerate, inertia is produced due to their masses. However, the magnitudes of these forces are small compared to the externally applied loads. Hence inertia effect due to masses is neglected. Such an analysis is known as static force analysis.

3. Define force and applied force.

Force is a push or pull, which acts on a body changes or tends to change, the state of rest or of uniform motion of the body. A force is completely characterized by its point of application, magnitude, and direction. The external force acting on a system of the body from outside the system is called applied force. The applied forces are classified as an active and reactive force.

5. Which law helps to measure a force quantitatively?

Newton’s second law helps us to measure a force quantitatively.

6. Distinguish between space diagram and free body diagram.

A space diagram is a graphical description of the system. It generally shows the shape and size of the system, the weight, the externally applied loads, the connection, and the supports of the system.
A free-body diagram is a sketch of the isolated or free body which shows all the pertinent weight forces, the externally applied loads, and the reaction from its supports and connections acting upon it by the removed elements.

7. When will the two force members are in equilibrium?

The member under the action of two forces will be in equilibrium if,

1. The two forces are of the same magnitude,

2. The forces act along the same line, and

3. The forces are in the opposite direction

8. Give any three advantages of the free body diagram.

1. Free body diagram assists in seeing and understanding all aspects of the problem.

2. They help in planning the approach to the problem.

3. They make mathematical relations easier for the problem.

9. When will the three force members is in equilibrium.

A body or member will be in equilibrium under the action of three forces if,

1. the resultant of the forces is zero, and

2. the line of action of the forces intersect at a point.

10. Differentiate between static force analysis and dynamic force analysis.

If components of a machine accelerate, inertia forces are produced due to their masses. If the magnitude of these forces is small compared to the externally applied loads, they can be neglected while analyzing the mechanism. Such an analysis is known as static force analysis.

11. What do you mean by inertia?

The property of matter offering resistance to any change of its state of rest or of uniform motion in a straight line is known as inertia.

12. State D’Alembert’s principle.

D’Alembert’s principle states that the inertia forces and torques, and the external forces and torques acting on a  body together result in statical equilibrium.
In other words, the vector sum of all external forces acting upon a system of rigid bodies is zero. The vector sum of all external moments and inertia torques acting upon a system of rigid bodies is also separately zero.

13. How you will reduce a dynamic analysis problem into an equivalent problem of static equilibrium?

By applying D’Alembert’s principle to a dynamic analysis problem, we can reduce it into an equivalent problem of static equilibrium.

14. What do you mean by Equivalent offset inertia force?

Equivalent offset inertia force is the force that can replace both inertia force and inertia torque.

15. State the principle of superposition.

The principle of superposition states that for linear systems the individual responses to several disturbances or driving functions can be superposed on each other to obtain the total response of the system.

16. Give one example each for the linear and non-linear systems.

Linear system: Example: Spring system. Non-linear system: Example: Systems with static or Coulomb friction, backlash.

17. Principle of superposition has the limitation that it cannot be applied for non-linear systems.

18. When the connecting rod is large the piston executes simple harmonic motion.

19. Define Piston’s effort.

Piston effort is defined as the net or effective force applied on the piston, along the line of stroke. It is also known as the effective driving force (or) net load on the gudgeon pin.

20. What do you mean by crank effort or turning moment on the crankshaft?

It is the product of the crank-pin effort (FT) and crankpin radius (r).

21. Define compound pendulum or torsional pendulum.

A rigid body suspended vertically at a point and oscillating with very small amplitude under the action of gravitational force is known as a compound pendulum or torsional pendulum.

22. What are the requirements of an equivalent dynamical system?

1. The mass of the rigid body must be equal to the sum of masses of two concentrated masses. i.e. m1 + m2 = m 2. The center of gravity of the two masses must coincide with the center of gravity of the rigid body. i.e. m1 l1 =  m2 l2

3. The sum of the mass moment of inertia of two masses about their center of gravity is equal to the mass moment of inertia of the rigid body. i.e. l1. l2 = (kG)2

23. What are the forces acting on the connecting rod?

1. Inertia force of the reciprocating parts (FI) acting along the line of stroke.

2. The side thrust between the crosshead and the guide bars acting at P and right angles to the line of stroke.

3. Weight of the connecting rod.

4. Inertia force of the connecting rod (FC).

5. The radial force (FR) is parallel to crank, and

6. The tangential force (FT) acting perpendicular to the crank.

24. What is meant by turning moment diagram or crank effort diagram?

It is the graphical representation of the turning moment or crank effort for the various positions of the crank. In the turning moment diagram, the turning moment is taken as the ordinate (Y-axis) and crank angle as abscissa (X-axis)

25. Define Inertia force.

The inertia force is an imaginary force, which when acts upon a rigid body, brings it in an equilibrium position. Inertia force = - Accelerating force = - ma

1. Why balancing dynamic forces is necessary?

If dynamic forces are not balanced, they will cause worse effects such as wear and tear on bearings and excessive vibrations on machines. It is very common in camshafts, steam turbine rotors, engine crankshafts, and  centrifugal pumps, etc.,

2. Write the different types of balancing?

Balancing of rotating masses

1. Static balancing

2. Dynamic balancing

2. Balancing of reciprocating masses.

3. Write the condition for complete balancing?

1. The resultant centrifugal force must be zero and

2. The resultant couple must be zero.

4. Write the equation for balancing a single rotating mass by a single mass?

For balancing a single rotating mass by a single rotating mass, the equation is m1r1 = m2r2.

5. Whether grinding wheels are balanced or not? If so why?

Yes, the grinding wheels are properly balanced by inserting some low-density materials. If not the required surface finish won’t be attained and the vibration will cause much noise.

6. Why complete balancing is not possible in the reciprocating engine?

Balancing of reciprocating masses is done by introducing the balancing mass opposite to the crank. The vertical component of the dynamic force of this balancing mass gives rise to “Hammer blow”. In order to reduce the  Hammer blow, a part of the reciprocating mass is balanced. Hence complete balancing is not possible in reciprocating engines.

7. Differentiate between the unbalanced force due to a reciprocating mass and that due to a revolving masses.

1. Complete balancing of revolving mass can be possible. But the fraction of

reciprocating mass only balanced.

2. The unbalanced force due to reciprocating mass varies in magnitude but is constant in direction. But in the case of revolving masses, the unbalanced force is constant in magnitude but varies in direction.

8. What are the various cases of balancing of revolving masses?

1. Balancing of single rotating mass by a single mass rotating in the same plane.

2. Balancing of single rotating mass by two masses rotating in different planes.

3. Balancing of several rotating masses in a single plane.

4. Balancing of several rotating masses in different planes.

9. What are the effects of an unbalanced primary force along the line of stroke of the two-cylinder locomotive?

1. Variation in tractive force along the line of stroke, and

2. Swaying couple.

10. Define tractive force.

The resultant unbalanced force due to the two cylinders along the line of stroke is known as tractive force.

11. What is a swaying couple?

The unbalanced force acting at a distance between the line of stroke of two cylinders constitutes a couple in the horizontal direction. This couple is known as a swaying couple.

12. What is the effect of hammer blow and what is the cause of it?

The effect of hammer blow is to cause the variation in pressure between the wheel and the rail, such that the vehicle vibrates vigorously. A Hammer blow is caused due to the effect of unbalanced primary force acting perpendicular to the line of stroke.

13. What are in-line engines?

Multi-cylinder engines with the cylinder center lines in the same plane and on the same side of the centerline of the crankshaft are known as an in-line engines.

14. What is the condition to be satisfied for the complete balance of the in-line engine?

1. The algebraic sum of the primary and secondary forces must be zero, and

2. The algebraic sum of the couples due to primary and secondary forces must be zero.

15. Why radial engines are preferred?

In radial engines, the connecting rods are connected to a common crank and hence the plane of rotation of the various cranks is the same, therefore there are no unbalanced primary or secondary couples. Hence radial engines are preferred.

UNIT-III (FREE VIBRATION)

1. What are the causes of vibration?

The causes of vibration are unbalanced forces, elastic nature of the system, self excitations, winds, and earthquakes.

2. Define Period and cycle of vibration.

The period is the time interval after which the motion is repeating itself. The cycle is defined as the motion completed during one time period.

3. Define the frequency of vibration.

It is the number of cycles described in one second. Its unit is Hz.

4. How will you classify vibration?

1. Free vibrations

a) Longitudinal vibration,
b) Transverse vibration and
c) Torsional vibration.
2. Forced vibrations, and

3. Damped vibration.

5. What is meant by free vibration and forced vibrations?

When no external force acts on the body, after giving it an initial displacement, then the body is said to be under free or natural vibration.
When the body vibrates under the influence of external force, then the body is said to be under forced vibrations.

6. What do you mean by damping and damped vibration?

The resistance against the vibration is called damping.
When there is a reduction in amplitude over every cycle of vibration, then the motion is said to be damped vibration.

7. Define resonance.

When the frequency of external force is equal to the natural frequency of a vibrating body, the amplitude of vibration becomes excessively large. This phenomenon is known as resonance.

8. What do you mean by a degree of freedom or movability?

The number of independent coordinates required to completely define the motion of a system is known as the degree of freedom of the system.

9. A cantilever beam has an infinite number of degrees of freedom.

10. Define steady-state and transient vibrations.

In ideal systems, the free vibration continues indefinitely as there is no damping. Such vibration is termed steady-state vibrations.
In real systems, the amplitude of vibration decays continuously because of natural damping and vanishes finally.  Such vibration is a real system is called transient vibration.

11. What is equivalent spring stiffness?

Equivalent spring stiffness is the measure of the overall spring stiffness of any system having more than one spring connected in series or parallel.

12. List out the various methods of finding the natural frequency of free longitudinal vibrations.

1. Energy method,

2. Equilibrium method and

3. Rayleigh’s method.

13. What is the principle of Rayleigh’s method of finding natural frequency of vibrations?

The principle of Rayleigh’s method is “the maximum kinetic energy at the mean position is equal to the maximum potential energy or (strain energy) at the extreme position.

14. A shaft supported in the long bearing is assumed to have both ends fixed for solving transverse vibration problems.

15. The damping force per unit velocity is known as the damping coefficient.

16. Distinguish between critical damping and large damping.

If the system is critically damped, the mass moves back very quickly to its equilibrium position within no time.  Whereas in large damping, the mass moves slowly to the equilibrium position.

17. When do you say a vibrating system is underdamped?

The equation of motion of a free damped vibration is given by d x + c dx + s = 0
dt2m dtm
If (s/m) > (c/2m)2 , then radical becomes negative. The two roots k1 and k2 are known as complex conjugates.  Then the vibrating system is known as under damping.

18. Define the critical or whirling or whipping speed of a shaft.

The speed at which resonance occurs is called the critical speed of the shaft. In other words, the speed at which the shaft runs so that the additional deflection of the shaft from the axis of rotation becomes infinite is known as critical speed.

19. What are the factors that affect the critical speed of a shaft?

The critical speed essentially depends on

a) the eccentricity of the C.G. of the rotating masses from the axis of rotation of the shaft,

b) diameter of the disc,

c) span of the shaft, and

d) type of supports connections at its ends.

20. What are the causes of critical speed?

1. Eccentric mountings,

2. Bending due to self-weight, and

3. Non-uniform distribution of rotor material.

21. Define damping ratio.

It is defined as the ratio of the actual damping coefficient to the critical damping coefficient.

22. Define logarithmic decrement.

Logarithmic decrement is defined as the natural logarithm of the amplitude reduction factor. The amplitude reduction factor is the ratio of any two successive amplitudes on the same side of the mean position.

23. What is meant by dynamic magnifier or magnification factor?

It is the ratio of maximum displacement of the forced vibration to the deflection due to the static force.

24. What is meant by transmissibility?

When a machine is supported by a spring, the spring transmits the force applied on the machine to the fixed support or foundation. This is called transmissibility.

25. Define transmissibility ratio or isolation factor.

The ratio of force transmitted to the force applied is known as the transmissibility ratio.

26. Briefly explain elastic suspension.

When machine components are suspended from elastic members, the vibration force produced by the machine components will not be transmitted to the foundation. This is called as elastic suspension.

27. Specify any two industrial applications where the transmissibility effects of vibration are important.

UNIT-IV (FORCED VIBRATION)

1. Specify the importance of vibration isolation?

When an unbalanced machine is installed on the foundation, it produces vibration in the foundation. So, in order to prevent these vibrations or to minimize the transmission of forces to the foundation, vibration isolation is important.

2. What are the methods of isolating the vibration?

1. High-speed engines/machines mounted on foundation and supports cause vibrations of excessive amplitude because of the unbalanced forces. It can be minimized by providing spring-damper, Etc.

2. The materials used for vibration isolation are rubber, felt cork, etc. These are placed between the foundation and vibrating body.

3. Define torsional vibration.

When the particles of a shaft or disc move in a circle about the axis of the shaft, then the vibrations are known as torsional vibrations.

4. Differentiate between transverse and torsional vibration.

1. In transverse vibrations, the particles of the shaft move approximately perpendicular to the axis of the shaft.  But in torsional vibrations, the particles of the shaft move in a circle about the axis of the shaft.

2. Due to transverse vibrations, tensile and compressive stresses are induced. Due to torsional vibrations,  torsional shear stresses are induced in the shaft.

5. Define node in torsional vibration.

Node is the point or the section of the shaft at which the amplitude of the torsional vibration is zero. At nodes, the shaft remains unaffected by the vibration.

6. Define torsional equivalent shaft.

A shaft having variable diameter for different lengths can be theoretically replaced by an equivalent shaft of  uniform diameter such that they have the same total angle of twist when equal opposing torques are applied at  their ends. Such a theoretically replaced shaft is known as torsionally equivalent shaft.

7. What are the conditions to be satisfied for an equivalent system to that of the geared system in torsional vibrations?

1. The kinetic energy of the equivalent system must be equal to the kinetic energy of the original system.

2. The strain energy of the equivalent system must be equal to the strain energy of the original system.

Q1. What's noise?

A: It's any sound that interferes with our activities, conversations, or rest. A given sound maybe music or amusement for one person and noise for another one.

Q2. Does a sound have to be very loud to be considered as noise?

A: Not necessarily. Sometimes a very weak noise, such as a leaking tap, may distract us terribly. But louder noises are much more hazardous.

Q3. What's the noise made of?

A: As it happens with any sound, noise is the result of air vibration.

Q4. Does noise affect only humans?

A: Not at all. Noise does affect also animals and plants. The noise from helicopters flying over the Catarata Falls  (at the North East of Argentina) for touristic reasons, for instance, has focused several animal species to migrate,  altering severely the ecological balance of a wide area.

Q5. Can noise cause damage to buildings?

A: Perhaps, if it is extremely loud. However, most likely such damage will be the result of strong vibrations caused by the same device which produces the noise. For instance, heavy machinery operating at a factory might cause cracks on the neighboring walls.

Q6. May I go deaf because of noise?

A: Extremely loud noises, such as an explosion close to your ears, may injury your ears forever. But even noises that aren't that loud, if listened to for several hours a day, may cause deafness after several years.

Q7. What noises are the most hazardous ones for my ears?

A: High-pitched noises are more dangerous than low-pitched ones. Noises that are short but very loud, such as those from impacts, blows with a hammer or explosions are particularly hazardous.

Q8. How loud must be noise in order to be hazardous?

A: Any noise which makes it impossible to talk with a person who is close to you is dangerous, and so is a noise-causing your ears to hurt or ring.

Q9. Besides the effects of noise on hearing, are there any other effects?

A: Yes: it raises blood pressure and causes heart diseases, it stimulates violence, causes stress, and lowers concentration. In the case of children, it affects growth and interferes with learning.

Q10. What part of the ear is affected the worst by noise?

A: First let's say that the ear is actually made up of three parts: the outer ear (the pinna and the ear canal), the middle ear (the eardrum and three tiny bones called hammer, anvil, and stapes), and the inner ear. The latter is a  snail-shaped cavity in the bone containing a lot of amazingly small cells called hair cells (one millimeter contains about 500 such cells). These cells are a sort of tiny microphones, and without them, a hearing would be impossible. It is their small size that makes them so vulnerable. Repeated exposure to loud noises gradually destroys them.

Q11. Is it possible to recover from noise-induced hearing loss?

A: Most, unfortunately, the answer is no. The hair cells inside the inner ear, once destroyed, won't regenerate.  That's why you should always take care of them.

Q12. Then, how come we recover from temporary deafness caused by a cold?

A: That's because then the middle ear gets filled with mucus, preventing the sound passage through it towards the inner ear. Once you recover from the cold, the mucus gets out from there and you can hear well again. As you can see, there is no destruction of hair cells.

Q13. Is it possible to become noise-addicted?

A: Many specialists say yes. That's another reason to avoid exposure to an excess of noise.

Q14. What are the main causes of urban noise?

A: One of the prime causes of noise is traffic flow: cars, trucks, buses, motorcycles (particularly those with defective silencers, or with no silencer at all). In the cities with an airport close to them or inside their limits,  aircraft noise is quite a serious problem. Other causes are the shops and factories which fail to comply with noise regulations, as well as discotheques, stadiums, shows outdoors, etc.

Q15. What can be done to abate noise?

A: It depends on the kind of noise. If it is a noise whose production is banned by any regulation (like a loud noise from your neighbor’s house), you might call the police and ask that the law is enforced (most frequently it is not as simple as it sounds). Other kinds of noise may only be reduced by resorting to preventative action. For instance, with public campaigns in order to educate drivers in driving quietly, avoiding the unnecessary use of the horn, and accelerating only in order to speed up and not for calling the attention of other drivers.

Q16. What can we, children, do to improve overall quietness?

A: The best thing you can do is to learn why the noise is not a good trade, and to add to your habits those of  "sound hygiene". When you become grown-ups and be in charge of Planet Earth, you won't make the same mistakes as we make today.

Q17. What is "sound hygiene"?

A: It is a series of individual and social attitudes which everyone should have to avoid the hazards of noise and,  at the same time, to care for the sound environment making our Planet a more friendly place to live in.

Q18. What do you mean by sound environment?

A: It's all the sounds arriving at our ears at a given location and time.

Q19. Which are the regulations on noise?

A: Quite a question! (or "quiet" a question?) Depending on where you live, you may find federal, state (or provincial), and local regulations. Federal regulations tend to be more general, stating principles and overall policies, and local ordinances, on the other hand, are more specific, often dealing with situations that have proven to be problematic. You may get information at the offices of those governmental agencies dealing with the environment or with health care.

Q20. I've been told that in order to insulate noise coming through a wall I should cover it with carpet. Is it true?

A: No. In order to insulate sound you need heavy materials, such as a thick brick wall, or else double gypsum board walls with the air space in between filled up with glass fiber. This subject is quite technical, but I  wished at least to warn you that carpets, styropor, or other light materials aren't good sound insulators.

What is noise?

Noise is defined as an unwanted sound. A sound might be unwanted because it is loud, distracting, or annoying.

How is noise measured?

Literally speaking, noise can't be measured directly, since there is no instrument for objectively detecting how  "unwanted" something is. What can be measured is the sound level, quantification of a sound's pressure or intensity, and relation to its loudness. Sound level is measured in decibels (dB), by a device called a sound level meter.

What are typical decibel levels of some common sounds?

A whisper is 30 dB, conversational speech is 60 dB, and someone shouting at you from an arm's length away is  85 dB. Noise levels of home appliances range from 50 dB (a refrigerator) to 95 dB (a food processor). Lawn equipment and power tools have noise levels of 80–120 dB.

How many decibels can the human ear handle?

Immediate and irreversible nerve damage can be caused by sounds at 140 dB or higher (120 dB in young children). However, damage also occurs at lower sound levels, and this harm accumulates over time. Any sound above 85 dB can cause wear and tear on your ears that reduces your hearing acuity over time. Read more: Safe noise exposure limits

What is the loudest sound possible?

Sound is normally carried in the air as a pressure wave. When the pressure of a sound wave becomes as high as the air pressure itself, the sound becomes a shock wave. Normal air pressure at sea level is 14.7 pounds per square inch (psi), or 101,325 pascals (Pa), which is equivalent to 194 decibels (dB). So 194 dB is the loudest sound possible in air at sea level; beyond that point, it becomes a shock wave. (Sound waves that are transmitted  through water or other substances would have different limits.)

What are the effects of noise on human health?

Noise has direct physiological effects such as hearing damage (including hearing loss and tinnitus or ringing in the ears), as well as cardiovascular and hormonal disturbances. Indirect effects include sleep loss, interference with concentration and learning, mood changes and aggression, and social isolation.

How does noise affect babies and children?

Because the ear canal of a young child is smaller than an adult's, sound pressure is up to 20 dB greater than that in an adult ear. In addition to the threat to a child's hearing, noise causes physiological and mental stress, and significantly impacts learning and cognitive development. Background noise also interferes with speech perception and language acquisition.

What is "white noise"?

White noise is a sound similar to radio static, or the sound a fan makes, that is often used to mask unpleasant sounds. Some people find it helpful for sleeping, and it can be a soothing sound for babies.

What is noise pollution?

Noise pollution is manmade sound in the environment that may be harmful to humans or animals.

What are the most common sources of noise pollution?

Worldwide, the most common sources of noise pollution are cars, trucks, and other motor vehicles. Planes and trains also contribute to noise pollution. Other sources include factory machinery, power tools, and construction equipment.

What problems does noise pollution cause for people?

The World Health Organization (WHO) cites seven categories for the ways noise adversely affects human  health:

Noise-induced hearing impairment
Interference with speech communication
Sleep disturbances
Cardiovascular and physiological effects
Mental health effects
Effects on the performance of tasks
Annoyance and effects on behavior

What problems does noise pollution cause for animals?

Wild animals rely on their hearing for detecting predators, finding mates, establishing territory, and recognizing warning alerts. Unnaturally high levels of noise can damage their hearing and can also mask more subtle sounds that they need to hear in order to survive and reproduce. They may also react with a fight-or-flight response to  artificial sounds such as aircraft noise, thereby using up valuable energy reserves to flee from a non-existent

predator. If noise in an area becomes too intrusive, animals may shift to a new territory or alter their migration patterns, which can create new complications for their mating and survival.

What are the laws regarding noise pollution?

Occupational noise is treated as a health and safety issue and is regulated at the state or national level in many countries. Community noise is typically regarded as a nuisance issue rather than a matter of health and is normally regulated at local levels of government. The regulations and levels of enforcement vary widely across different communities, and worldwide. Noise-generating products such as automobiles and aircraft may be controlled by industry regulations, and building codes may set requirements for reducing sound transmission in new building construction projects.

What can I do personally to reduce my own noise pollution?

Avoid using high-noise yard tools such as leaf blowers and power hedge trimmers.
Keep your motor vehicle's muffler in good condition.
Only honk your horn in an emergency.
Train your dog not to bark inappropriately.
Put your cell phone on "vibrate" mode, and excuse yourself to a private area to conduct a phone conversation. Turn off the TV if no one is watching it.
If you want to enjoy loud music, use headphones.

How can I join with others to reduce noise pollution?

If you have a concern about a specific noise issue in your community, the most effective way to make a  difference is to join a local group that shares your concern. If there is no existing group, you may want to organize one yourself. NoiseOFF is a broad-based coalition that can provide you with information, contacts, and tools to help.