Understanding Balance Quality Grades (G-Grades)

Definition: What is a Balance Quality Grade?

A Balance Quality Grade, commonly referred to as a “G-Grade,” is a standardized classification defined in the ISO 1940-1 and ISO 21940-11 standards that specifies the maximum permissible residual unbalance for a rotor. It is not a measure of vibration itself, but rather a tolerance for unbalance based on the rotor’s mass and maximum service speed. The G-grade represents a constant peripheral velocity of the rotor’s center of mass, expressed in millimeters per second (mm/s). A lower G-number signifies a stricter tolerance and a higher-precision balance requirement.

The Purpose of the G-Grade System

The G-grade system was developed to create a universal, standardized method for specifying how well a rotor needs to be balanced. Instead of vaguely stating “the rotor must be well balanced,” engineers can specify a precise, verifiable target like “Balance to G6.3.” This system provides a common language for manufacturers, maintenance teams, and clients, ensuring that equipment meets the required operational standards for reliability and safety. The primary goals are to:

• Limit vibration caused by unbalance to acceptable levels.
• Minimize dynamic forces on bearings, extending their lifespan.
• Ensure the rotor can operate safely up to its maximum design speed.
• Provide a clear, measurable acceptance criterion for new and repaired equipment.

How are Balance Quality Grades Determined?

The ISO standards provide a comprehensive table that recommends G-grades for hundreds of different types of rotating components. The selection of a specific grade depends on factors like:

• Machine Type: A high-speed turbine requires a much better balance (lower G-grade) than a slow-speed agricultural machine.
• Rotor Mass: Lighter rotors are often more sensitive to unbalance.
• Operating Speed: The higher the speed, the greater the centrifugal force from a given unbalance, thus requiring a better balance.
• Support Structure: Rotors on flexible supports may need a better balance than those on rigid foundations.

Examples of Common Balance Quality Grades (from ISO 1940-1)

The following list illustrates the wide range of applications and their corresponding G-grades, from lowest to highest precision:

• G 40: Car wheels, crankshaft drives for slow engines.
• G 16: Agricultural machinery parts, drive shafts.
• G 6.3: Standard grade for many industrial components like electric motor armatures, pump impellers, fans, and process plant machinery. This is one of the most commonly specified grades.
• G 2.5: High-speed and high-precision components, such as gas and steam turbines, turbo-compressors, machine-tool drives, and turbo-generator rotors.
• G 1.0: Grinding machine drives, very small and high-speed armatures.
• G 0.4: The highest level of precision for components like gyroscopes, precision grinding spindles, and equipment for the semiconductor industry.

Calculating Permissible Residual Unbalance

The G-grade is used in a formula to calculate the maximum permissible residual unbalance (U_per) that can remain in the rotor after balancing.

U_per (in g·mm) = (9550 × Rotor Mass [kg] × G-Grade [mm/s]) / Max. Service Speed [RPM]

This formula tells the balancing technician the specific target they must achieve. For example, a 100 kg rotor with a max speed of 3000 RPM, balanced to G6.3, would have a permissible residual unbalance of (9550 * 100 * 6.3) / 3000 ≈ 2005.5 g·mm. This total unbalance is then typically distributed between the two correction planes.

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