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ENM 314

Mechanical VibrationsLecture 1: Introduction

Dr. Nouby M. Ghazaly

Mechanical Engineering Dept., Faculty of Engineering,

South Valley University,

Qena-83523, Egypt.

Email: [email protected]

3/8/2014 1



Outline of Presentation

1. Introduction

2. Basic concepts of vibration

3. Importance of the study of vibration

4. Sources of vibration

5. Modeling of vibration systems

6. Machinery problems that cause vibration

7. Classification of vibrations3/8/2014 Dr. Nouby M. Ghazaly 2



Basic concepts of vibration

DEFINITION OF VIBRATION?

• Vibration can be defined as simply the cyclic or oscillating

motion of a machine or machine component from its

position of rest.

Any motion that repeats itselfafter an interval of time

•

3/8/2014 Dr. Nouby M. Ghazaly 3



Introduction

Vibration: Any motion that repeats itself after an interval of time.

• Vibratory System

–Storing potential energy (spring)

–Storing kinetic energy (mass) –Energy dissipation (damper)

• Our heart beat, our lungs oscillate, we hear because our ear drum

vibrates…

• The light waves which permit us to see & sound waves throughwhich we hear entail vibration, We move by oscillating our legs.

• Washing Machine, Mechanical Shakers, Mixers, etc.

• Musical Instruments, Clocks, Watches

• Medical Field – Massagers, etc.3/8/2014 Dr. Nouby M. Ghazaly 4



Goals and Study Areas

• Goals of Vibration Engineering:

• Suppress/eliminate bad vibrations

• Generate desired forms and levels of good vibration

• Areas of Study in Vibration:

1.Modeling and Analysis

2.Design and Modification3.Instrumentation

4.Monitoring and Experimentation

5.Control3/8/2014 Dr. Nouby M. Ghazaly 5



Applicable Engineering Fields

• Aerospace Engineering: Design and development of aircraft and

space craft (analyze and avoid excitation of resonances by engine,

atmospheric, control surface excitations, etc.). Proper vibration

isolation and control (active and passive methods)

• Civil Engineering: Design of bridges and buildings (modal analysis);

Dynamic stability (avoid catastrophic self-excited vibration)

• Electrical Engineering: Hardware degrade/malfunction faster (e.g.,

components in PCs and control devices)

• Analysis and testing during design and development stages, for

proper vibration performance

• Vibration testing during production (for quality control)




Applicable Engineering Fields

• Manufacturing Engineering: Machine tool vibration Degrades

product quality; increases rate of wear and tear, frequency of

malfunction, tool breakage and repair (increased frequency/cost of

maintenance); creates noise and operator discomfort

• Proper design of machine tools and components

• Incorporation of vibration control, isolation, and proper mounting

practices

• Mechanical Engineering: Reduced vehicle vibration Improved

performance, ride quality and comfort; reduced maintenance and

repair Suspension design, engine balancing, active and passive

control, body design 3/8/2014 Dr. Nouby M. Ghazaly 7



Importance of studying

vibration

• RESONANCE like Tacoma Narrows Bridge

• Similar problems in machine tools, vehicles,

turbines, pumps, compressors, buildings,

aircraft & spacecraft systems• Excessive vibration leads to loosening on

parts, noise & eventual failure

• Effects of vibration on human body:

Discomfort, noisy.• Fatigue, Loss of Efficiency , Sound quality in

products




IMPORTANCE OF STUDYING

VIBRATION

Benefits of considering vibration analysis

1. Higher Speeds

2. Greater Flexibility

3. Lighter Weight

4. Larger Excitation Forces

• Machine speeds doubled during the past fifty years Vibration-excitation forces

due to unbalances, non-symmetries, misalignments, etc. would increase

without proper measures

• Material optimization, energy efficiency, and compact modern designs of

machinery (light-weight) and structures: Thin members; high flexibility;

complex modes of vibration; high amplitudes.

• Increased power levels of modern machinery Stronger vibration excitations, if

no measures taken




Good & Bad Vibrations

• Good Vibrations (Serve a Useful Purpose):

1. Shakers in parts of machines.

2. Vibration medical aid

3. Jack hammer vibration necessary for the task

4. Musical instrument vibration

5. Polishers, finishers, and mixers.

• Bad Vibrations (Unpleasant or Harmful):

1. Seismic destruction

2. Wind induced vibration

3. Vibration in machine tools

4. Vehicle vibrations (due to road disturbances, engine, etc.)

5. Noise-generating vibrations

6. Vibration in electrical components (e.g., relays). 3/8/2014 Dr. Nouby M. Ghazaly 10



SOURCES OF VIBRATION

Vibrations can be introduced into a product through awide variety of sources such as:

1. Impact loads due to shipping and handling or due to

accidental drops

2. Sinusoidal loads due to rotating equipment

3. Random loads from transportation over rough roads in

trucks or aerodynamic loads in aircraft/spacecraft or rail

loads in rail transportation

4. Seismic loads during earthquakes




Effect of force on machine

Forces generated within the machine cause vibration. These forces may:

1. Change in direction with time, such as the force generated by a rotating

unbalance.

2. Change in amplitude or intensity with time, such as the unbalanced magnetic

forces generated in an induction motor due to unequal air gap between themotor armature and stator (field).

3. Result in friction between rotating and stationary machine components in much

the same way that friction from a rosined bow causes a violin string to vibrate.

4. Cause impacts, such as gear tooth contacts or the impacts generated by the

rolling elements of a bearing passing over flaws in the bearing raceways.5. Cause randomly generated forces such as flow turbulence in fluid-handling

devices such as fans, blowers and pumps; or combustion turbulence in gas

turbines or boilers.




Machinery problems that cause

vibration

• The most common machinery problems that cause vibration:

1. Misalignment of couplings, bearings and gears

2. Unbalance of rotating components

3. Looseness4. impairment of rolling-element bearings

5. Gear wear

6. Rubbing

7. Aerodynamic/hydraulic problems in fans, blowers and pumps

8. Electrical problems (unbalance magnetic forces) in motors

9. Resonance

10. Eccentricity of rotating components such as "V" belt pulleys orgears 3/8/2014 Dr. Nouby M. Ghazaly 13



Modeling of vibration systems

3/8/2014 Dr. Nouby M. Ghazaly 1. 14



Modeling example




msxs

ks cs

xumu

kt u

Quarter bus/truck/car model

u = road profile input

kt = tire spring constant

mu = unsprung mass

xu = displacement of unsprung mass

ks = suspension spring constant

cs = suspension damping constant

ms = sprung mass

xs = displacement of sprung mass




Resonance Frequency Ranges of

Human body sections• Eyeball, Intraocular Structure (20-90 Hz)

• Head (axial mode) (20-30 Hz)

• Shoulder Girdle (4-5 Hz)

• Chest wall (50-100 Hz)

• Arm (5-10 Hz)

• Hand (30-50 Hz)

• Abdominal Mass (4-8 Hz)

• Spinal column (axial mode) (10-12 Hz)

It may be noted that the abdominal mass mode(around 5 Hz)

makes us nauseating and is avoided in automotivedesign.

Top gun pilots had problem with a particularmaneuver when the eyeball

socket went into resonance.




Resonance

A vibration of large amplitude

Occurs when an object is forced near its natural

frequency

m

c k

x

t

M

e

Object Model3/8/2014 Dr. Nouby M. Ghazaly 18



Classification of Vibrations • Free Vibrations: No forcing excitation (representative of natural

dynamics; needs two forms of energy storage and interchange)

• Forced Vibrations: Forced by a repetitive excitation (generated

internally or transmitted from an external source).

• When the frictional dissipation of energy is neglected, the motion

is said to be undamped . Actually, all vibrations are damped to

some degree.




Questions?

3/8/2014 Dr Nouby M Ghazaly 20

mechanical vibrations -lecture1

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