VI Balance Grades - Vibration - Vibration Institute

Balance Grades:
Electric Motor Experience
Vibration Institute
Piedmont Chapter 14
12/01/2006
Clay Boyd, PE
CBM Analyst
704-382-3608
chboyd@duke-energy.com
References
Schenck Trebel Corporation
535 Acorn Street
Deer Park, NY 11729
Toll Free: 1-800-873-2352
http://www.schenck-usa.com
http://www.mpta.org/MPTABalancingPrimer.pdf
http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf
Motor Repairs
 Motor Repair Specification
 Balance Grades
 Initial Balance with Shop Balance Machine
 Final Balance with Motor Assembled
Motor Repair Specification
 Work to be performed
 Acceptable Repair Practices
 Required Reporting
 Acceptance Criteria, including BALANCE GRADE.
Balance Grades
 Balance Grades are used to specify the allowable residual
imbalance for rotating machinery.
 The ISO 1940 standard defines balance grades for
different classes of machinery. (Rigid Rotors Only*)
 Example: Balance Grade “G2.5” is recommended for
Steam Turbines, Machine Tools and Small Electric
Armatures.
* ISO 11342 defines the balance quality requirements for rotors in a flexible state
Permissible Residual unbalance per unit rotor
weight (g*mm/g*1000) or Permissible cg
displacement (mm*1000)
Balance Grades: Sample Chart
Service Speed (rpm)
http://www.plant-maintenance.com/articles/balancingqualitylimits.pdf
Balance Grades
 A rotor balanced to G2.5 will vibrate at 2.5 mm/sec
(Velocity) if freely suspended while rotating at service
speed. (2.5 mm/sec = 0.10 IPS)
V(mm/sec) = 2pe(mm)*RPM
60sec/min
e(mm) = Residual Imbalance (g-mm)
Rotor weight(g)
Balance Grades
 Balance Grade is a function of

Rotor Mass (lbm, Kg, g)

Service speed (rpm)

Residual imbalance (g-in, oz-in, lb-in, g-mm…)
Initial Balance with Shop Balance Machine
It is difficult if not impossible to achieve Balance
Grades G1.0 or G0.4 in a balance machine only
Balance Grades G1.0 and G0.4 have special requirements.
 G1.0

The rotor must be mounted in its own service bearings

No end drive (for balance machines)
 G0.4

The rotor must be mounted in its own housing and bearings

Must be run under normal service conditions: Load,
Temperature..

Self driven
Initial Balance with Shop Balance Machine : Roller Size
“…the roller diameter should differ from the journal
diameter by at least 10%, and the roller speed should
never differ less than 60 rpm from the journal speed…. “
p.54 Fundamentals of Balancing, 2nd Ed. 03/1983; Schenk Trebel
Final Balance with Motor Assembled
 Motor Setup
 Flexible or Rigid Rotor – Large 2-pole motors
 Effect of startup heating
 Interference of 60 Hz electrical vibration on unloaded 2pole Motor phase readings
Final Balance with Motor Assembled
Motor Setup
 Motor on Solid Base
 Elevate on shims at base bolt locations
 Shim to eliminate soft foot
 Secure with bolts or clamps
 Lubricate bearings
 Monitor vibration from the first start
Flexible or Rigid Rotor – Large 2-pole motors
In test runs at service speed, moving two test
masses from the end planes to the center plane,
results in a vibration change of less than 20%.
pp.21-22 Fundamentals of Balancing, 2nd Ed. 03/1983; Schenk Trebel
Graphic: http://www.schenck-usa.com/lib_101_types_unbal.asp
Effect of startup heating
60 Hz
Trial 1 (1:25 PM)
Trial 2, 3, 4, 5 … 6 (9:00 PM)
1X Vibration BC 2C CBPM
mils pk-pk
5.
4.
3.
2.
1.
5.
4.
3.
2.
1.
1.
1.
2.
3.
4.
5.
2.
3.
4.
5.
Run1(ODE)
Run2(ODE)
Run3(ODE)
Run4(ODE)
Run5(ODE)
Run6(ODE)
Run1(DE)
Run2(DE)
Run3(DE)
Run4(DE)
Run5(DE)
Run6(DE)
As low as reasonably
achievable.
Before and After MOV Spectrum
2 Year Overall Trend
Questions?
Balance Grades:
Electric Motor Experience
Clay Boyd, PE
CBM Analyst