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referred to as a G-Grade, is a classification system defined by ISO standards (specifically ISO 21940-11, which superseded the older ISO 1940-1) to specify the acceptable limit of residual unbalance for a rotor. It provides a standardized, internationally recognized method for engineers, manufacturers, and maintenance personnel to define how precisely a rotor needs to be balanced for its specific application.

The G-Grade itself is a number, such as G6.3 or G2.5, that 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 higher level of precision and a tighter balance tolerance.

How Do G-Grades Work?

The G-Grade is not the final tolerance itself but is the key parameter used to calculate it. The core idea is that for a given level of operational smoothness, a faster-spinning rotor must be balanced more precisely than a slower-spinning one. The G-grade system accounts for this relationship.

The permissible residual specific unbalance (e_per, in g·mm/kg or µm) is calculated by dividing the G-grade by the maximum service angular velocity (Ω, in rad/s). The final permissible residual unbalance (U_per, in g·mm) is then found by multiplying this by the rotor’s mass (M, in kg).

A simplified and more common formula is:

U_per (g·mm) = (9550 * G * M (kg)) / n (RPM)

Where:

• U_per is the maximum allowable residual unbalance.
• G is the Balance Quality Grade.
• M is the mass of the rotor.
• n is the maximum service speed in revolutions per minute.

ISO Balance Quality Grade Table (Examples)

The ISO standard provides a comprehensive table that recommends G-grades for hundreds of different types of rotors. This is the most practical part of the standard, as it gives clear, application-specific guidance. Some common examples include:

• G 40: Crankshaft drives for large, slow marine diesel engines.
• G 16: Crankshaft drives for large trucks and locomotives; parts of agricultural machinery.
• G 6.3: Flywheels; pump impellers; fans; parts of process plant machinery. This is a very common grade for general industrial machinery.
• G 2.5: Gas and steam turbines; turbo-generator rotors; machine-tool drives; medium and large electric motor armatures.
• G 1.0: Grinding machine drives; small electric armatures; computer storage devices.
• G 0.4: Spindles of precision grinders; gyroscopes.

Why Are G-Grades Important?

• Standardization: They provide a clear and unambiguous way to specify balancing requirements. A manufacturer can state that a pump impeller must be “balanced to G6.3,” and any balancing shop in the world will understand the required precision.
• Prevents Over-Balancing: Balancing a rotor to a tighter tolerance than necessary is expensive and time-consuming. G-grades help select an appropriate and economical level of precision for the application.
• Ensures Reliability: Selecting the correct G-grade ensures that the machine will operate with acceptable vibration levels, reducing wear on bearings, seals, and structures and preventing premature failure.

← Back to Main Index