Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "How the ISO 21940-11 Balance Quality Grade Standard Impacts Service Center Balancing" by Gene Vogel, presented at the EASA Convention 2018, discusses the implications of the updated ISO 21940-11 standard on balancing practices in service centers. The ISO standard for balancing tolerances, previously known as ISO 1940-1, has undergone revisions, with the most recent change renaming it to ISO 21940-11. This standard establishes Balance Quality Grades based on rotor mass and maximum operating speed, defining the permissible residual specific unbalance, which, when multiplied by angular velocity, produces the Balance Quality Grade number.
A significant change between the 1986 and 2003 versions of the standard pertains to the location on the rotor where balance tolerances are applied. Initially, tolerances were applied to the correction planes, but the updated standard specifies that residual unbalance tolerances should be evaluated at the bearing planes. This change has raised questions about the validity of balance reports produced by balancing machine instruments and the need for translation calculations to certify that residual unbalance is within tolerance.
Unbalance is measured in units such as oz-in, gram-mm, or gram-inches, and can be described either as an amount of weight at a radius or as a displacement of the rotor's center of gravity from its center axis. The updated standard requires that the reference planes for balance tolerances be at the bearing planes, necessitating translation calculations to determine the equivalent unbalance at these planes. These calculations are complex, as static and couple unbalance do not translate by the same proportions.
For symmetrical rotors with correction planes inboard of the bearing planes, such as many motor rotors, translation calculations may not be necessary. However, for rotors with correction planes outboard of the bearings or overhung rotors, translation calculations are required to comply with the new standard. The paper explains that balancing to well below the required Balance Quality Grade can provide assurance that the final result is within tolerance at the bearing planes.
The paper also highlights practical considerations for balancing machine operations. Hard bearing balancing machines inherently translate forces at the bearing planes into unbalance amounts at the correction planes, while soft bearing machines may require additional instrumentation for translation calculations. Displacement data from soft bearing machines can be converted to ISO 21940-11 Balance Quality Grades using a simple formula, making it easier to ensure compliance with the standard.
In conclusion, the updated ISO 21940-11 standard impacts service center balancing by changing the reference planes for balance tolerances to the bearing planes, requiring translation calculations for certain rotor types. Understanding these changes is crucial for balancing technicians and service center engineers to ensure accurate and compliant balancing practices.
Key Points Covered:
- Overview of ISO 21940-11 Balance Quality Grade Standard
- Changes in the standard from ISO 1940-1 to ISO 21940-11
- Importance of evaluating residual unbalance at bearing planes
- Translation calculations for unbalance at bearing planes
- Practical considerations for balancing machine operations
- Differences between hard and soft bearing balancing machines
- Conversion of displacement data to ISO Balance Quality Grades
Key Takeaways:
- The updated ISO 21940-11 standard specifies balance tolerances at bearing planes.
- Translation calculations are necessary for certain rotor types to ensure compliance.
- Understanding the changes in the standard is essential for accurate balancing.
- Hard bearing machines inherently comply with the new standard, while soft bearing machines may require additional instrumentation.
- Displacement data can be converted to ISO Balance Quality Grades using a simple formula.
Related Reference and Training Materials
Print
