Assignment of Mechanical Vibration

1. UNBALANCED VIBRATION
MECHANICAL VIBRATION

2. Group :
1. Ahmad Arju Robbani NIM : 15/385201/TK/43863
2. Ahmad Abrar NIM : 15/385200/TK/43862
3. Arkaizen NIM : 15/385207/TK/43869
4. Begawan Diazafa Izan NIM : 15/385209/TK/43871

3. WHAT IS ?
Unbalance is defined as an unequal distribution of mass causing
the mass axis to differ from the bearing axis. During rotation, the
unequal mass along with the radial acceleration due to rotation create
a centrifugal force. This results in force on the bearings and/or
vibration of the bearings.

4. Characteristic of Unbalance Vibration
Static Unbalance
• Static unbalance is a condition of unbalance where the central principal axis is displaced parallel
to the rotating centerline. It can be detected by placing the rotor at its point of rotation on each
end. The heavy side of the rotor will swing to the bottom. A part is considered statically balanced
when it does not rotate regardless of the position in which it is placed.
• When testing a rotor that is symmetrically supported between identical bearings, identical
vibration amplitude and phase readings will be measured at the bearings / end of shaft if the
unbalance is truly static. This does not apply for rotors which are mounted in an overhung
configuration.

5. Characteristic of Unbalance Vibration
STATIC UNBALANCE

6. Characteristic of Unbalance Vibration
Couple Unbalance
• Couple unbalance exists when two unbalances exist 180 degrees apart, but in different
planes. This condition of unbalance has a central principal mass axis intersecting the rotating
centerline. Unlike static unbalance, couple unbalance cannot be detected by allowing the rotor to
spin freely.

7. Characteristic of Unbalance Vibration
• Couple Unbalance

8. Characteristic of Unbalance Vibration
Dynamic Unbalance
• Dynamic unbalance is the most common type of unbalance and is defined simply as unbalance
where the central principal axis and the rotating centerline do no coincide or touch. This type of
unbalance exists whenever static and couple unbalance are present, but where the static
unbalance is not in direct line with either couple component. As a result, the central principal
axis is both tilted and displaced from the rotating centerline.

9. Cause of Unbalance Vibration
Dynamic Unbalance

10. Cause of Unbalance Vibration
• Blow Holes in Castings
Cast rotors such as pump impellers or large sheaves have blow holes or
sand traps which result from the casting process. While undetectable
through normal visual inspection, blow holes may be present within
the material and create a significant source of unbalance.

11. Cause of Unbalance Vibration
• Eccentricity
Eccentricity exists when the geometric centerline of a part does not
coincide with its rotating centerline. The rotor itself may be perfectly
round; however, for one reason or another, the center of rotation is off
centered.

12. Cause of Unbalance Vibration
• Addition of Keys and Keyways
A manufacturer may balance their product with a full key, a half key, or
no key at all. Thus, if both a pulley and a motor manufacturer were to
balance their components without keys, they would be unbalanced
when assembled together with the added weight of a key.

13. Cause of Unbalance Vibration
• Clearance Tolerances
The most common source of unbalance is the accumulation of
tolerances in the assembly process of a machine. An example of when
this occurs is when the bore in a pulley is larger than the shaft
diameter. A key or setscrew would be required to fill the gap, thus
pushing the shaft to one side of the shaft rotating centerline.

14. Cause of Unbalance Vibration
• Corrosion or Wear
Many rotors, particularly fan, blower, compressor, pump rotors, or any
other rotors involved in the material handling processes, are subject to
corrosion, abrasion, or wear. If the corrosion or wear does not occur
uniformly, unbalance will result.

15. Cause of Unbalance Vibration
• Deposit Build-Up
Rotors used in material handling may become unbalanced due to the
unequal build-up of deposits (dirt, lime, etc.) on the rotor. The
resultant gradual increase in unbalance can quickly become a serious
problem when portions of the deposits begin to break away. As small
deposits break off, the vibration increases, which in turn, breaks off
even more deposits, thus, creating a serious unbalance.

16. SPECTRUM ANALYSIS
Diagnoses
The use of overall vibration, FFT (Fast Fourier Transform) spectra and
phase measurements aids in diagnosing unbalance problems

17. SPECTRUM ANALYSIS
FFT Spectrum Analysis
Vibration caused by pure unbalance is a once per revolution sinusoidal
waveform. On an FFT spectrum, this appears as a higher than normal
1x amplitude. While other faults can produce high 1x amplitude, they
usually also produce harmonics. In general, if the signal has harmonics
above once per revolution, the fault is not unbalance. However,
harmonics can occur as unbalance increases or when horizontal and
vertical support stiffness differs by a large amount.

18. SPECTRUM ANALYSIS
• Phase Analysis
The use of phase measurements aids in the diagnosis of unbalance
problems.
• Note: All phase readings are ±30° due to mechanical variance.
The sensor shows a 90° phase shift between the horizontal and
vertical positions when readings are compared. When the system
involves static unbalance, there is usually no phase shift across the
machine or coupling in the same measurement position.

19. SPECTRUM ANALYSIS
Analysis Spectrum
• High amplitude at 1 x rpm
• Low amplitude at 1 x rpm in axial geared
• Amplitude ratio between horizontal and vertical is big, except in cases
with asymmetrical stiffness

20. Reference :
• Rao, Singiresu S., 2011, Mechanical Vibrations Fifth Edition, Prentice
Hall, Upper Saddle River
• http://www.irdbalancing.com/unbalance---cause-of-vibration.html,
accessed : December 12th, 2017

21. TERIMA KASIH