1. Mechanical Vibrations
International Islamic University Islamabad.
Abbas Raza
Asst.Prof.

2. What Is Vibration?
Vibration is the physical movement or oscillation of a mechanical part about a reference position.

3. What Is Vibration? Why do we care about vibration?
Vibration is:
Wasted energy
A major cause of premature component failure
Cause of aircraft noise which contributes to crew and passenger discomfort

4. What is Vibration? Scientific Definition Engineering Definition
Any motion that repeats itself after an interval of time
Engineering Definition
Deals with the relationship between forces and oscillatory motion of mechanical systems

5. Why is Vibration Important?
The Tacoma Narrows
Bridge Disaster
November 7, 1940

6. Why is Vibration Important?
Amplitude of oscillation reached 14 feet!

7. What was the cause?

8. …Caused Wind-Induced Vibration (?)
Vortex Shedding
…Caused Wind-Induced Vibration (?)

9. Why is Vibration Important?

10. Why is Vibration Important?

11. Why is Vibration Important?

12. Wing Flutter Failures

13. Basic Concepts Every object has:
Frequencies at which it “likes” to vibrate
Characteristic geometries of vibration

14. Basic Concepts Every object has:
Frequencies at which it “likes” to vibrateç
Characteristic geometries of vibration

15. 1 Modeling Vibration The Ingredients: Realistic Addition:
Inertia (stores kinetic energy)
Elasticity (stores potential energy) 1
Realistic Addition:
Energy Dissipation

16. 1 2 Modeling Vibration The Ingredients: Realistic Addition:
Inertia (stores kinetic energy)
Elasticity (stores potential energy) 1 2
Realistic Addition:
Energy Dissipation

17. 1 2 3 Modeling Vibration The Ingredients: Realistic Addition:
Inertia (stores kinetic energy)
Elasticity (stores potential energy) 1 2
Realistic Addition:
Energy Dissipation 3

18. 1 2 3 Modeling Vibration The Ingredients: Realistic Addition:
Inertia (stores kinetic energy)
Elasticity (stores potential energy) 1 2 3
Realistic Addition:
Energy Dissipation

19. m k c Modeling Vibration The Ingredients: Realistic Addition: Mass, m
Stiffness, k m k c
Realistic Addition:
Damping, c

20. How is this model useful?k c

21. Basic Concepts
Resonance

22. Basic Concepts Resonance A vibration of large amplitude
Occurs when an object is forced near its natural frequency

23. Resonance A vibration of large amplitude
Occurs when an object is forced near its natural frequency
Object

24. Resonance A vibration of large amplitude
Occurs when an object is forced near its natural frequency
Object
Model

25. Vibration Absorbers Used to eliminate vibration of an object Object
(that vibrates too much)

26. Vibration Absorbers Used to eliminate vibration of an object
(‘absorbs’ vibration)
Object
(that vibrates too much)

27. Choose these to eliminate motion of object.
Vibration Absorbers
Used to eliminate vibration of an object
Vibration Absorber
Choose these to eliminate motion of object.
Object
(that vibrates too much)

28. Vibration Absorbers

29. “Characteristic Geometry” of Vibration
Mode Shapes
“Characteristic Geometry” of Vibration
Mode Shapes
Resonance
Natural Frequencies

30. Terminology
Now it is important to understand some common terms used for vibration analysis and their applications.

31. Terminology Amplitude
Amplitude is an indicator of the severity of a vibration. Amplitude can be expressed as one of the following engineering units:
Velocity
Acceleration
Displacement

32. Velocity is the rate of change in position
Terminology Velocity
Velocity is the rate of change in position
Typical velocity units are: IPS (Inches Per Second), mm/sec (millimeters per second)
Velocity is the most accurate measure of vibration because it is not frequency related rpm is the same as rpm.

33. Terminology Acceleration
Acceleration is the rate of change of velocity and is the measurement of the force being produced.
Acceleration is expressed in gravitational forces or “G’s”, (1G = ft/sec/sec)
Acceleration is frequency related, in that rpm is not the same as rpm.

34. Terminology Displacement
Displacement is a measure of the actual distance an object is moving from a reference point.
Displacement is expressed in “mils” 1 mil = .001 inch
Displacement is also frequency related, in that rpm is not the same as rpm.

35. Terminology - Continued
Unit Modifiers:
Since vibration is transmitted as an AC signal, there are four Unit Modifiers that may be used to condition the signal. These modifiers have a direct impact on the measurement value. If the wrong modifier is used, the measurement could be either too high, or too low, thus causing possible maintenance action to be, or not to be, accomplished erroneously.

36. Unit Modifiers:
Peak to Peak - the distance from the top of the positive peak to bottom of the negative peak.
Peak - the measurement from the zero line to the top of the positive peak.
Average (AVG) of peak.
Root Mean Square (RMS) of peak.

37. Terminology Frequencies
The rate of mechanical oscillation in a period of time. Frequency can be expressed in one of the following units:
RPM - Revolutions per Minute
CPM - Cycles per Minute
CPS - Cycles per Second
Hz - Hertz, 1 Hz - 1 Cycle per Second (to convert from Hz to RPM or CPM, apply the following formula: Hz * 60 = RPM.

38. Types of Vibration
Vibration can be classified into one or more of the following categories:
Periodic
Random
Resonant
Harmonic

39. Terminology - Types of Vibration Periodic
Repeats itself once every time period
Result of a mass imbalance in a component or disc.
As the component rotates, it produces a “bump” every rotation which is referred to a the once-per-revolution or “1P” vibration.
This vibration is usually correctable by balancing.

40. Terminology - Types of Vibration Random
Do not repeat themselves
Not related to a fundamental frequency.
An example - the shock that is felt as a result of driving down the road and hitting a pothole

41. Terminology - Types of Vibration Resonant
The natural frequency at which an airframe or mechanical system is inclined to vibrate. All things have one or more resonant frequencies.
Resonant vibrations are the result of a response in a mechanical system to a periodic driving force.

42. Terminology - Types of Vibration Harmonic
Exact multiples of a fundamental frequency
Classified in terms as 1st, 2nd, 3rd…..

43. Terminology Bandwidth
Upper and lower frequency limits of the survey being acquired - either hardware set (with the use of an external band pass filter) or software controlled by the analyzer.
Setting the frequency bandwidth is a way of eliminating vibration data or noise that is of no interest for your particular application.
In the survey above, the frequency bandwidth is 0 CPM to 3000 CPM

44. Terminology Resolution
The resolution of a spectrum is the number of lines or points used to plot the spectrum.
The higher the number of lines, the more data acquired.