Balancing Grades are a quality characteristic used to compare rotors of different weights and rotating speeds in terms of how well they must be balanced.
The balancing tolerance requirements for rotors with rigid behavior are specified in the ISO 21940-11. Part 11 of the standard includes, besides necessary numbers of correction planes and methods to verify the residual unbalance, a definition of the Balancing Tolerances and Balancing Grades.
This article will look into what Balancing Grades are and how to determine the balancing tolerance for the correction planes of a rotor.
Based on experience, the ISO Standard recommends certain Balancing Grades for individual rotor types. These recommendations can be used by the manufacturer of components and machines to determine the required balancing tolerance in order to achieve a satisfactory operation of the rotor in service.
The Balance Quality Grade (abbr. G) is expressed in millimeters per second (mm/s) and defines the maximum allowable vibration velocity of a rotating work piece gravity center. It is a product of the
G = eper x Ω
• eper = permissible eccentricity of the gravity center in mm
• Ω = Max. Operational Angular Velocity in rad/s (Ω = RPM x π/30)
Example: G 2.5 = max allowable vibration velocity of c.o.g. is 2.5 mm/s
All Rotors have an initial unbalance that causes vibration while rotating. The complete elimination of unbalance in a rotor is technically and practically impossible. Instead, technical requirements determine how precisely a rotor must be balanced for satisfactory vibration.
Once the Balancing Quality Grade “G” is determined by the application, the permitted residual unbalance Uper can be calculated by multiply the eccentricity eper times the rotor Mass m:
Uper = m x eper = m x G/ω
Instead of calculating the Residual Unbalance the Graph shown later can be used.
In addition to the technical requirements, economic considerations are also necessary when determining the balancing tolerance. The lower or tighter the rotor tolerance, the more time-consuming and thus cost-intensive it is to achieve. This is why a rotor should only be balanced to the necessary tolerance, rather than balancing as low as possible.
Three steps are required to use balancing grades to determine the permitted balancing tolerance of a rotor:
Select a balancing grade from the Table, based on the Rotor type and application.
|Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently unbalanced
|Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently balanced
|Crankshaft drives, inherently unbalanced, elastically mounted
|Crankshaft drives, inherently unbalanced, rigidly mounted
|Complete reciprocating engines for cars, trucks and locomotives
Cars: wheels, wheel rims, wheel sets, drive shafts
Crankshaft drives, inherently balanced, elastically mounted
Crankshaft drives, inherently balanced, rigidly mounted
Drive shafts (cardan shafts, propeller shafts)
Aircraft gas turbines
Centrifuges (separators, decanters)
Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds up to 950 r/min
Electric motors of shaft heights smaller than 80 mm
Process plant machines
Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds above 950 r/min
Gas turbines and steam turbines
Audio and video drives
Grinding machine drive
Spindles and drives of high-precision systems
Table for guidance for balance quality grades for rotors with rigid behavior per ISO 21940-11
Use the Chart below to determine the permissible residual unbalance eper first and then multiply with the Rotor Mass m to obtain the permissible residual unbalance Uper.
Or as an alternative, you can use the following formula:
Uper = m x eper = m x G/ω
The determined permissible residual unbalance Uper applies for the complete Rotor. Depending on Rotor dimensions and configuration the tolerance needs to be split to the correction planes.
Describing the exact way on how to allocate the total permissible residual unbalance to the correction planes would go beyond the scope of this article. But in general, the following statements can be used for symmetrical Rotors as a rule of thumb: