1. Unbalance vibration occurs when the center of mass of a rotating object is not aligned with its center of rotation, causing a wobbling motion. 2. There are three main types of unbalance: static, couple, and dynamic. Static unbalance can be corrected by adding or removing weight in one plane, while couple and dynamic require weights added in two or more planes. 3. Unbalance vibration produces a single frequency vibration at the object's rotational speed and can cause damage, noise, and reduced machine life if not addressed.

1. Unbalance Vibration
Agung Setio (
Ezra Febrinanda (
Kevan Muhammad Rizki (
Nanda Dhiyan Maulana (
Jurusan Teknik mesin
Departemen Teknik Mesin Dan Industri
Fakultas Teknik
Universitas Gadjah Mada

2. What’s Unbalance Vibration?
The uneven distribution of mass around an axis of rotation. A rotating mass, or rotor, is said to
be out of balance when its center of mass (inertia axis) is out of alignment with the center of rotation
(geometric axis). Unbalance causes a moment which gives the rotor a wobbling movement
characteristic of vibration of rotating structures.

3. Type of Unbalance Vibration
Static Unbalance Couple Unbalance Dynamics Unbalance

4. 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.
Static Unbalanced is very dangerous in the system. It
can be removed by some treatment like :
A. Adding or removing weight in only one correction plane.
B. Correction weights in-line at opposite ends of the rotor

5. Correction of Static Unbalance (1)
In Scenario A, we can see static unbalance can be corrected by
adding or removing weight in only one correction plane. In the figure above
, the balance correction weight in scenario A is added as one singular weight
addition in the same plane as the unbalance. This will result in a well
balanced rotor.
In scenario B, the static unbalance is corrected by placing the
correction weights in-line at opposite ends of the rotor. This method is
typically used when it is not possible to add a single correction weight at
the center portion of the rotor. This results in a statically balanced rotor;
however, during faster rotations, there is an increased chance of bending
moments.

6. Scenario C shows an unacceptable attempt of balancing a rotor. The correction
weight was added in a different plane than the one containing the rotor center of
gravity. The rotor may be considered statically balanced, due to the fact that no
heavy spot would swing to the bottom if the rotor were suspended and allowed to
spin freely; however, when the rotor is rotated, the original heavy spot and
correction weight, being located in different planes, produce moments of inertia
which cause the central principal axis to intersect the rotating centerline, thus
creating another type of unbalance.
Correction of Static Unbalance (2)

7. 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. Couple unbalance
can only be detected when the part is rotating and can be identified by
comparing the bearing or shaft vibration amplitude and phase readings at
each end of the rotor.
Readings from a rotor experiencing couple unbalance will reveal
equal amplitudes of vibration with phase readings which differ by 180
degrees. Again, this method of detecting the type of unbalance does not
apply to overhung rotors.

8. Correction of Couple Unbalance
Couple Unbalanced can corrected by
placing the correction weights minimal in the 2
side of the system. In the example picture
beside, we must add a correction weight in the
right and left side of the system.

9. Dynamics 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.
Generally, a condition of dynamic unbalance will reveal
comparative phase readings which are neither the same nor directly
opposite one another. This type of unbalance can only be solved by
making weight corrections in a minimum of two planes.

10. Correction of Dynamics Unbalance
To Correct Dynamic Unbalance, it is
necessary to make vibration measurements
while the machine is running and to add
balancing masses in two planes.

11. Cause of Unbalance Vibration
Unbalance can be caused by a number of factors. Several examples include:
• Improper component manufacturing
• Uneven build up of debris on the rotors, vanes or blades
• The addition of shaft fittings without an appropriate counter balancing procedure
• Vane/Blade erosion or thrown balance weights

12. Characteristic of vibration caused by Unbalaced
Key characteristics of vibration caused by unbalance:
• It is a single frequency vibration whose amplitude is the same in all-radial directions
• It is sinusoidal, occurring at a frequency of once per revolution (1x)
• The Spectrum generally not contain harmonics of 1x running speed, unless the unbalance
is severe
• Amplitude increases with speed

13. Effects of Unbalaced Vibration
• Vibration
• Noise
• Decreased life of bearings
• Unsafe work conditions
• Reduced machine life
• Increased maintenance

14. Spectrum Analysis of Unbalance Vibration (1)
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.

15. • 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
Spectrum Analysis of Unbalance Vibration (2)