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Balance Grades:
Electric Motor Experience
Vibration InstitutePiedmont Chapter 1412/01/2006
Clay Boyd, PECBM Analyst
References
Schenck Trebel Corporation535 Acorn StreetDeer Park, NY 11729Toll Free: 1-800-873-2352http://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 residualimbalance for rotating machinery.
The ISO 1940 standard defines balance grades fordifferent classes of machinery. (Rigid Rotors Only*)
Example: Balance Grade “G2.5” is recommended forSteam Turbines, Machine Tools and Small ElectricArmatures.
* ISO 11342 defines the balance quality requirements for rotors in a flexible state
Balance Grades: Sample ChartPe
rmiss
ible
Resid
ualu
nbal
ance
peru
nitr
otor
weig
ht(g
*mm
/g*1
000)
orPe
rmiss
ible
cgdi
spla
cem
ent(
mm
*100
0)
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 servicespeed. (2.5 mm/sec = 0.10 IPS)
V(mm/sec) = 2pe(mm)*RPM60sec/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 BalanceGrades 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 journaldiameter by at least 10%, and the roller speed shouldnever 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 2-pole Motor phase readings
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
Final Balance with Motor Assembled
Flexible or Rigid Rotor – Large 2-pole motors
In test runs at service speed, moving two testmasses 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)
1X Vibration BC 2C CBPMmils pk-pk
5.
4.
3.
2.
1.
1.
2.
3.
4.
5.
5. 4. 3. 2. 1. 1. 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)
Trial 2, 3, 4, 5 … 6 (9:00 PM)
As low as reasonablyachievable.
Before and After MOV Spectrum
2 Year Overall Trend
Balance Grades:
Electric Motor Experience
Clay Boyd, PECBM Analyst
Questions?
