1. High Speed Balancing in the Service Industry – Deformed Rotors
Juan Hidalgo
Manager of Dynamic Analysis & Balancing

2. The Goal of Balancing
The goal of balancing is to obtain good running characteristics of the rotor over the entire speed range, this is particularly important at operating and critical speeds.
Good running characteristics can be defined as
Reduce dynamic loads on bearings
Reduce shaft deflection within operating clearance confines
Reduce transmission of forces to the outside environment
Reduce shaft dynamic stress

3. Generalized Rotor Classifications
Rigid Rotor
1st bending mode critical speed > Operating Speed
Quasi Rigid Rotor
1st bending mode critical speed < Operating Speed
2nd bending mode critical speed > Operating speed
Flexible Rotor
1st bending mode critical speed << Operating Speed

4. Balancing Methods Low Speed Balancing – 2 plane, rigid rotor balance
High Speed Balancing – multi-plane balancing, flexible and quasi-rigid rotors:
Influence Coefficient Method (any rotor type)
Modal Balance (quasi-rigid rotors & flexible rotors)
(N + 2) balance planes
Rigid mode balance
Modal influence coefficient balance
Balance each critical through the speed range

5. Low Speed Balance Best applicable to rigid rotors.
Balancing is carried out in 2 planes.
Addresses translation and couple (rocking) rigid body modes only.
Rigid body forces and overall moments are compensated by balancing.
Does not resolve inner moments that can only be observed at high speed
due to the CF effects of local mass eccentricities.
Severity of local mass eccentricities across a rotor is evaluated from runout
measurements carried out on the rotor.

6. High Speed Balance Applicable to quasi-rigid and flexible rotors.
Balancing is carried out in multiple planes.
Minimizes CF effects at the bearings over the entire speed range.
Resolves inner moments through the use of multiple balance planes to
correct for mass eccentricity - within limits.
When the rotor is modally balanced, the balance condition is retained
regardless of the speed at which the critical modes appear and regardless of
support stiffness conditions.
Affords the verification of mechanical integrity as rotors are typically
run to 110% overspeed.

7. Balancing of Deformed Rotors
These are the type of rotors which are normally omitted from most presentations on balancing.
They are primarily bowed rotors and rotors with high local eccentricities (rotor bow is the most common problem in the industry after misalignment, and perhaps the less recognized)
Bowed rotors can not simply be balanced.
In order to eliminate the effect of the bow, careful evaluation of the rotor eccentricities is needed before the best course of action to remedy the problem can be establish.
Remedies include
Machining to correct journal centerline
Machining to correct local eccentricities
High speed balance
Addition of carefully located balance planes

8. Candidate Rotors for Evaluation and High Speed Balance
Rotors were their mass distribution had been significantly disturb
such as complete or partial rotor rewind
Rotors that have had their journals, couplings or fits machined.
Rotors that required changes of their centers of rotation.
Turbine rotors that have had blades weld repaired or fox hole rivet
repaired – for verification of mechanical integrity.
Rotors that have been balanced multiple times over a long operating
period or that have a history of vibration problems.
Rotors experiencing changed operating vibration resulting from:
Loss of mass due to rubbing or other mechanical means
Permanent bowing or deformation due to rubs or water induction
Cracking
Thermal instability, motorization events

9. Bowed Rotor: IP Rotor Case Study
Runout measurements were carried out as part of the rotor inspection. Measurements revealed a significant bow in the rotor.
Based on the findings, corrective machining was carried out to minimize the mass unbalance resulting from the bow.
In order to assure satisfactory operation, the rotor was high speed balanced and oversped to 110% of operating speed (3960 RPM).

10. Runout Measurements are made at a Series of Points
to Assess Rotor Condition
NOTE: Runout measurements are made by ReGENco as part of any rotor work. Runouts are also routinely
carried out to assess rotor condition prior to high speed balancing of 3rd party rotors.

11. Runout Evaluation
The eccentricity values are used to evaluate the rotor condition and determine corrective moves of the journal centerline to minimize mass unbalance in the case of a bowed rotor.

12. 3D Eccentricity Plot Derived from IP Rotor Runout Data

13. Determination of Corrective Centerline Move
A finite element model of the rotor was developed and the 3D measured eccentricity distribution applied to it.
Based on the above model and using ReGENco’s center correction optimization program a move of the journal centerline was calculated in order to minimize the rotor unbalance distribution and maximize its balance performance.
.006”
Eccentricity centerline

14. IP Rotor Corrective Actions
Reworked TE & EE coupling centers to displace the rotational centerline to minimize mass unbalance
Remachined the TE and EE journals round and true to new centerline
Cleanup machined the shrouds at all blade stages
Cleanup machined the seal areas between the EE blade stages (2-3, 3-4, and 4-5) true and round
Machined the EE coupling rim true and round and machined the coupling face perpendicular to new axis of rotation
Roll / peen the coupling fit and machined it true and round to original fit diameter

15. Reduction in Peak Eccentricity from 6.5 to 3.6 mils
Prior to Machining
After Machining

16. IP Turbine Rotor – High Speed Balance

17. First Run Results – Measurements at Journals

18. Final Balance Results – Measurements at Journals