This document provides information about balancing high speed rotors. It defines balancing as correcting the mass distribution of a rotor so that vibration forces are within limits. There are four main types of unbalance: static, couple, quasi-static, and dynamic. Unbalance is caused by design errors, material faults, and machining errors. Flexible rotors require balancing at multiple speeds approaching critical speeds to account for deflection. Standards specify limits for vibration amplitudes and bearing forces for acceptance of balancing quality.

1. HIGH SPEED BALANCING
OF ROTORS

2. Importance of BALANCING
It is desirable to have a vibration free rotation in a machine
But
Every rotating system will have vibrations to some extent.
Though all the vibrations are not unwanted , the rotating machinery
vibrations in majority of cases are unwanted vibration which have a
say in the life of the equipment, safety of the personnel working with
these equipment, performance, Noise etc.

3. Importance of BALANCING
Unbalance is one of the factors which causes the
vibrations in a rotating machine.
Balancing increases the Quality
Therefore increases product LIFE and SAFETY

4. What is BALANCING ?
According to ISO 1925,
Balancing is defined as a process by means of
which the mass distribution about its spin axis is
checked, if necessary corrected so that the
bearing forces & the vibrations of the shaft
corresponding to running frequency are within
specified limits.

5. Definition of UNBALANCE
According to ISO 1925,
an unbalance in a rotor is present if, because of
uncompensated centrifugal forces, the bearings are
subjected to vibration forces or displacements.
The component which determines the direction and amount of
centrifugal force is the product of m.r (mass x radius).
In balancing it is called unbalance U and described in the
following way.
U = u.r where
u = unbalance mass
r = radius of unbalance mass

6. Causes of UNBALANCE
Unbalance is always present if the mass
distribution of the rotor relative to its shaft axis
is not symmetrical.
The main causes of unbalance may be divided into
three groups
1. Design or Drawing Errors
2. Material Faults
3. Machining or Assembly Errors.

7. Causes of UNBALANCE
¾ Parts do not have rotational symmetry
¾ Un-machined surfaces on rotor
¾ Run-out and swash errors due to poor fits.
¾ Key shorter than key way
¾ Moving parts not mounted with rotational
symmetry and without ply.
1. Design or Drawing Errors

8. Causes of UNBALANCE
¾ Blow holes in castings.
¾ Unequal material densities.
¾ Unequal material thickness. e.g. in welded
¾ Run-out and clearance in ball bearings.
2. Material Faults

9. Causes of UNBALANCE
¾ Un-machined portions of castings or forgings,
which cannot be made concentric and
symmetrical with respect to the shaft axis
¾ Clamping errors during machining
¾ End faces not perpendicular to shaft axis
¾ Form error produced by welding and casting.
3. Manufacturing or Assembly Errors

10. Causes of UNBALANCE
¾ Permanent deformation caused by manufacturing
process
e.g. a) Release of residual stresses,
b) Distortion during Machining
c) Deformation due to Welding, Soldering, Shrink Fits
¾ Deformation due to unequal tightening of bolts
¾ Variation in assembly components
e.g. a) Different Bolt Lengths,
b) Different types of Washers and Nuts
3. Manufacturing or Assembly Errors Contd…

11. Causes of UNBALANCE
1. Design or Drawing Errors
2. Material Faults
3. Machining or Assembly Errors.
The magnitude of many of these errors can be controlled
but they can never be avoided.
Hence, Balancing becomes necessary.

12. UNBALANCE
The International Standards Organization defines
unbalance as:
Condition which exists in a rotor when vibratory
force or motion is imparted to its bearings as a
result of centrifugal forces.
A more popular definition is:
The uneven distribution of mass about a rotor’s
rotating centerline.

13. Unbalance vs Centrifugal Force
Centrifugal force acts upon the entire
mass of a rotating component,
impelling each particle outward and
away from the axis of rotation in a
radial direction.
If an excess of mass exists on one side of a rotor, the
centrifugal force acting upon this heavy side exceeds the
centrifugal force exerted by the light side and pulls the entire
rotor in the direction of the heavy side.

14. Unbalance vs Centrifugal Force Contd…
A rotating element having unbalance,
will vibrate due to the excess
centrifugal force exerted during
rotation by the heavier side of the
rotor. When at rest, the excess mass
exerts no centrifugal force and,
therefore, causes no vibration. Yet the
actual unbalance is still present.
Unbalance is independent of rotational speed and remains the
same, whether the part is at rest or is rotating
(provided the part does not deform during rotation )
Centrifugal force increases with the square of the speed : F = m.r.ω²

15. Definitions
• Rotor : according to ISO definition , a rotating body with
journals supported in bearings is a rotor.
• Shaft Axis : It is the line connecting the middle points of shaft
journals
• Central principal inertia axis : the central principal
inertia axis is that body axis about which the mass of the rotor is
distributed symmetrically so that no free centrifugal forces occur
during rotation.
• Critical speed : characteristic speed at which resonance of
a system is excited. i.e., it is the speed at which it equals the
natural frequency of shaft giving rise to increase in vibration
amplitudes.

16. Types of unbalance
Types of unbalance
The four different types of unbalance which can
exist on the rotor are :
1. Static unbalance
2. Couple unbalance
3. Quasi-static unbalance
4. Dynamic unbalance

17. Static unbalance
• Static unbalance exists
when the principal axis of
inertia is displaced parallel
to the shaft axis.
• This type of unbalance is
found primarily in narrow,
disk-shaped parts such as
flywheels and turbine
wheels.
• It can be corrected by a single mass correction placed
opposite the center-of-gravity in a plane perpendicular to
the shaft axis, and intersecting the CG.

18. Couple unbalance
• Couple unbalance is that
condition for which the principal
axis of inertia intersects the shaft
axis at the center of gravity. This
condition arises when two equal
unbalances are positioned at an
axial distance on a rotor and
spaced 180º from each other.
Couple unbalance tries to tilt the rotor axis.This couple unbalance can
only be removed by another couple and it must be corrected in at least
two planes.
The correcting couple unbalance must be of the same magnitude as the
original couple unbalance. If the distance of seperation L is very large
then correction masses can be correspondingly small.

19. Quasi-static unbalance
Quasi-static unbalance is that condition of unbalance for which
the central principal axis of inertia intersects the shaft axis at a
point other than the center of gravity. It represents the specific
combination of static and couple unbalance where the angular
position of one couple component coincides with the angular
position of the static unbalance. This is a special case of dynamic
unbalance.

20. Dynamic unbalance
Dynamic unbalance, is that condition in which the central principal
axis of inertia is neither parallel to, nor intersects with the shaft
axis. It is the most frequently occurring type of unbalance and can
only be corrected (as is the case with couple unbalance) by mass
correction in at least two planes perpendicular to the shaft axis.
Dynamic unbalance is a combination of static unbalance and
couple unbalance, where the angular position of the static
unbalance relative to the couple unbalance is neither 0º nor 180º.

21. Balancing Tunnel

22. BALANCING FACILITIES
BHEL- Hyderabad unit has two balancing facilities.
1. High speed Balancing Tunnel - OBT
2. Vacuum Balancing Tunnel - NBT
OBT was commissioned in 1976
NBT was commissioned in 1983 with DH9 Pedestals.
DH70 pedestals are added in 2004.
Types of Rotors being Balanced
1. Turbo-generator rotors upto 130MW
2. Steam turbine rotors: HP,IP,LP & drive turbines
3. Gas turbine rotors (Low speed Balancing Only)
4. Exciter armatures
except for GT rotors, all other rotors are balanced at their operating speeds.

23. Pedestals used
1. DH4
2. DH6
3. DH70
4. DH9
All these Pedestals are Designed and supplied by
M/s. Schenck - Germany

24. DH4 Pedestals
• Weight of rotor 60-1200 kg
• Max. rotational speed 20000 rpm
• Max. journal diameter 110mm
• Stiffness per pedestal 250 N/µm

25. • Weight of rotor 400-8000kg
• Max. rotational speed 15000rpm
• Stiffness per pedestal 600 N/µm
DH6 Pedestals

26. • Weight of work piece 1000- 20000 kg
• Max. rotor diameter 2250 mm
• Total stiffness per pedestal 1000 N/µm
• Max. speed for Balancing 10000 rpm
DH70 Pedestals

27. • Weight of rotor 2500-50,000 kg
• Max. rotor diameter 3300 mm
• Max. length of rotor 10,500 mm
• Max. Bearing journal diameter 475mm
• Total stiffness per pedestal 1800 N/µm
• Max.speed for balancing 6000 rpm
• vacuum achievable 1 Torr
DH9 Pedestals

28. CONSTITUENTS OF BALANCING FACILITY
CONSTITUENTS OF BALANCING FACILITY
• Drive system
- drive motors
- gear box
- intermediate shaft
• pedestals
• Measuring system
• Control panels
• Lube oil system
- atmospheric oil
system
- vacuum oil system
• Vacuum system
• Jacking oil system
• Pedestal stiffening
device

29. CLASSIFICATION OF ROTORS
From balancing angle, rotors can be broadly
categorized as under.
• Rigid rotors :
A rotor is considered rigid when it can be corrected
in any two (arbitrarily selected) planes and after
these corrections , its unbalance does not
significantly exceed the balancing tolerance at any
speed up to max. service speed.
• Flexible rotors : rotors not covered as per above
definition due to their elastic deflections. Such
rotors need special attention and procedure for
balancing depending upon no. of critical speeds.
e.g. all drive turbine rotors and generator rotors.

30. BALANCING PROCEDURE
BALANCING PROCEDURE
• Rotors are balanced according to ISO 11342. The following
procedure is adopted in general.
1. Low speed balancing
2. High speed balancing
3. Performance of over-speed test
4. Final balancing for customer acceptance
Balancing is done either by adding the material/weights or
removing the material, of which former is generally adopted.
There will be necessary planes designated for making the
corrections along length of rotor radially or axially.
• The number of balancing planes depend on balancing
procedure adopted and possible critical speeds i.e. Modes.
Generally, if the operating speed of rotor exceeds the nth
critical speed at least 2n+1 correction planes are needed
along the rotor length.

31. Balancing of flexible rotors
A rotor which does not satisfy the definition of rigid rotor and
undergo significant change / bending deflection at different
speeds is called a flexible rotor.
Ex: Almost all compressor drives , Many TG drives.
Balancing of flexible rotors consist of following steps.
1. Low speed ( Rigid rotor ) balancing
2. High speed ( Flexible rotor ) balancing

32. Balancing of flexible rotors
1. Low speed ( Rigid rotor ) balancing
• First the unbalance is compensated in rigid mode
by selecting end correction planes ( preferably ).
This may be particularly advantageous for the
rotors significantly affected by only the first
flexural critical speed.

33. Balancing of flexible rotors
2. High speed ( Flexible rotor ) balancing
• First Mode :The rotor is run to some safe speed approaching
the first flexural critical speed. This will be termed as first flexural
balancing speed and vibration values are recorded.
• If the low speed balancing has been omitted ,the trial mass set
will usually consists of only one mass, which for rotors which are
essentially symmetrical about mid-span will be placed towards
the middle of the rotor . ( the plane of C.G.)
• If the low speed balancing has been performed, then the trial
mass set will usually consist of masses located at three distinct
correction planes.( 3-point or V- correction). In this case , the
masses are proportioned so that the low speed balancing is not
upset.

34. Balancing of flexible rotors
• For each of the above corrections, vectorial changes are calculated
by plotting the initial and final vector after correction. From this the
magnitude and angle of correction to be applied are computed. This
way the correction is refined to cancel the effects of unbalance at
the first flexural balancing speed.
• Now the rotor should run at any speed upto first flexural critical
speed with out any significant amplification of vibration (or
force ).
• Second Mode : The rotor is run to some safe speed approaching
the second flexural critical speed and vibration values are recorded.
A trial mass set, usually couple correction set up i.e. two weights in
end planes opposing each other, with each one in one end plane is
selected. With this correction refinement, the vibration values at
second flexural critical speed are reduced, in the manner discussed
above.

35. Balancing of flexible rotors
• The above operations are continued for balancing speeds, close to
each flexural critical speed in turn within the permissible speed
range. Each new set of trial masses should be chosen so that they
have significant effect on the appropriate mode, but do not
significantly affect the balance which has already been achieved at
lower speeds.
• In this way, the rotor is balanced to required grades of balancing
tolerances as specified in balancing standards.
• At our works, for compensation of unbalance and balancing the
combination of :
Modal balancing method and
Influence coefficient method is employed to arrive at the
required permissible limits.

37. Balancing norms followed at BHEL
Balancing norms followed at BHEL -
-Hyderabad
Hyderabad
• Generators :
Generators are balanced as per product standard
TG 45228 Rev : 03.
Vibration displacement( pk-pk) measured on
bearing pedestals forms the basis for acceptance
criteria of balancing quality. The acceptance limits
are
At operating speed : 10µ (pk-pk)
At all other speeds : 16µ (pk-pk)

38. Balancing norms followed at BHEL
Balancing norms followed at BHEL -
-Hyderabad
Hyderabad
• Turbines : (Utility & Industrial )
Turbine rotors are balanced as per Product standard
TC 51357 Rev: 03.
The acceptance criteria for industrial drive turbines
is “ the forces on each bearing pedestals should be
limited to 7% of the journal static loading at MCS &
10% at critical speeds.’’
The acceptance criteria for utility turbines is “ the
vibration amplitude( pk-pk) at each bearing pedestal
should be limited to 4 microns at operating speed &
6 microns at all other speeds.’’

40. • INTERNATIONAL STANDARDS•
• ISO 1925:2001 Mechanical vibration -- Balancing -- Vocabulary•
• ISO 1940-1:1986 Mechanical vibration -- Balance quality
requirements of rigid rotors -- Part 1: Determination of permissible
residual unbalance•
• ISO 1940-2:1997 Mechanical vibration -- Balance quality
requirements of rigid rotors -- Part 2: Balance errors•
• ISO 2041:1990 Vibration and shock -- Vocabulary•
• ISO 2953:1999 Mechanical vibration -- Balancing machines --
Description and evaluation
(available in English only)•
• ISO 2954:1975 Mechanical vibration of rotating and reciprocating
machinery -- Requirements for instruments for measuring vibration
severity•
• ISO 3719:1994 Mechanical vibration -- Symbols for balancing
machines and associated instrumentation•
• ISO 4866:1990 Mechanical vibration and shock -- Vibration of
buildings -- Guidelines for the measurement of vibrations and
evaluation of their effects on buildings•
• ISO 5344:1980 Electrodynamic test equipment for generating
vibration -- Methods of describing
equipment characteristics•
• ISO 5348:1998 Mechanical vibration and shock Mechanical

41. LIST OF BALANCING AND VIBRATION STANDARDS
S.Nr Standard Nr. Description of the standard
API- 616, IV Edition 1998 Gas Turbines for the Petroleum, Chemical
and GasIndustry Services.
VDI-2059, Part-I Nov1981 Shaft Vibrations for Turbo sets; Principles
for measurement and Evaluation.
VDI-2059,Part-II,Jun1990 Shaft Vibrations of Steam Turbo-sets for
Power Stations ; Measurement and
Evaluation.
VDI-2059,Part-III,Oct1985 Shaft Vibrations of Industrial Turbo-sets ;
Measurement and Evaluation.
ISO-10816-1, I Edition 1995 Mechanical Vibration-Evaluation of machine
vibration by measurements on non-rotating
parts; Part-I, General guidelines.
ISO-10816-4,1 Edition 1998 Mechanical Vibration-Evaluation of machine
vibration by measurements on non-rotating
parts; Part-4; Gas Turbine driven sets
excluding air craft derivatives.
ISO-7919-1, II Edition 1996 Mechanical Vibration of non-reciprocating
machines- Measurements on rotating
shafts and evaluation criteria;Part-1:
General guidelines.

42. 12 CEI/IEC 34-14,II Edition 96 Rotating Electrical Machines-Part –14 ;
Mechanical vibration of certain machines
with shaft heights 56 mm and higher-
Measurement, evaluation and limits of
vibration.
13 ISO-1940 , Edition 1973 Balance quality of rotating rigid bodies.
14 ISO 11342, I Edition 1994 Mechanical vibration-Methods and
criteria for the mechanical balancing
of Flexible rotors.
15 API-612,Edition June1995 Special purpose Steam turbines for
Petroleum, Chemical , and Gas Industry services.