Mechanical Vibrations -Lecture1
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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
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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
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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
•
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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
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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
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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)
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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
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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
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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
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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
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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
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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.
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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
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Modeling of vibration systems
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Modeling example
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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
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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.
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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
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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.
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