Balance Quality Grade (G-Grade): Definition, Purpose, and Application

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Dynamic balancer “Balanset-1A” OEM

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What is a Balance Quality Grade (G-Grade)?

A Balance Quality Grade, commonly referred to as a “G-Grade,” is a standardized classification defined in ISO 1940-1 and ISO 21940-11 standards that specifies the maximum permissible residual unbalance for a rotor. In other words, the G-grade indicates how precisely a rotor must be balanced. It does not directly measure vibration levels but rather defines an unbalance tolerance based on the rotor’s mass and maximum operating speed.

The number following the letter G (e.g., G6.3, G2.5) corresponds to the maximum vibration velocity of the rotor’s center of mass, expressed in millimeters per second (mm/s). For example, grade G6.3 means that the rotor’s center of mass should not experience vibration exceeding 6.3 mm/s at maximum operating speed, while the stricter grade G2.5 limits this velocity to 2.5 mm/s. The lower the G number, the stricter the balancing requirements: smaller unbalance tolerance and higher balancing precision.

The Purpose of the G-Grade System

The G-grade system was developed to establish a universal standard defining how well a rotor must be balanced. Instead of vague statements like “the rotor must be well balanced,” engineers can specify a precise, verifiable target such as “balance to G6.3.” This standard provides a common language for manufacturers, service engineers, and customers, ensuring that equipment meets the required reliability and safety standards. The main objectives of the G-grade system are:

Limiting vibration from unbalance to acceptable levels. Unbalance causes centrifugal forces and vibrations that can lead to noise, fatigue failures, and accidents. By applying standard balance grades, these vibrations can be controlled within safe limits.

Minimizing dynamic loads on bearings and extending their service life. Continuous vibration acts on bearings like a hammer, accelerating their wear. By limiting unbalance through the required G-grade, the forces acting on bearings are reduced, extending their lifespan.

Ensuring safe rotor operation at maximum design speed. The higher the rotation speed, the stronger the effect of even small unbalance. A strict balance grade guarantees that the rotor will not experience destructive vibrations at its operating speed. This is especially important for high-speed machines (turbines, compressors, etc.), where excessive unbalance can lead to failure.

Providing a clear, measurable acceptance criterion. Having a G-grade standard allows verification during manufacturing and repair of whether the required balance level has been achieved. If the residual unbalance after balancing does not exceed the allowable value for the given G-grade, the rotor is considered to have passed inspection. This approach transforms balancing from an art into a precise science with verifiable criteria.

How are Balance Quality Grades Determined?

ISO standards contain recommendations for selecting G-grades for hundreds of typical rotors and machines. The standard tables (e.g., ISO 1940-1, now superseded by ISO 21940-11) list recommended G-grades for various equipment categories. The selection of a specific grade depends on several factors:

Machine type and purpose. A high-speed turbine or precision spindle requires much more precise balancing (lower G) than a slow-speed agricultural mechanism. Designers consider how sensitive a given machine type is to vibrations and what consequences unbalance may have.

Rotor mass and dimensions. Lighter rotors are generally more sensitive to unbalance and may have stricter requirements. Rotor mass directly enters the permissible unbalance calculation—a heavier rotor can “tolerate” somewhat greater absolute unbalance without increasing vibration compared to a lighter one.

Maximum rotation speed. This is one of the key factors: the higher the speed, the stricter the balance must be. For the same unbalance magnitude, forces increase proportionally to the square of the rotation speed. Therefore, a lower G-grade is selected for high-speed rotors to compensate for the speed effect.

Support structure and mounting conditions. A rotor mounted on flexible (elastic) supports typically requires more careful balancing than one on a rigid foundation, since a flexible system dampens vibrations less effectively. For example, different grades (G16 vs G40) may apply to the same crankshaft depending on whether the engine is mounted on elastic vibration isolators or rigidly.

Examples of Common Balance Quality Grades

Note: The velocity value in mm/s in the grade designation corresponds to the product of specific eccentricity and angular velocity: G = e_per·ω. Thus, the G number indicates the limiting velocity of the displaced center of mass movement during rotor operation. In practice, the grade selection may differ by one level up or down depending on specific requirements and operating conditions.

Calculating Permissible Residual Unbalance

Knowing the required G-grade, you can calculate the maximum permissible residual unbalance—the amount of unbalance that may remain after balancing without exceeding the specified grade. The ISO standard provides the following formula:

U_per (g·mm) = (9549 × G [mm/s] × m [kg]) / n [RPM]

Where:

• U_per — permissible residual unbalance in gram-millimeters (g·mm)
• G — balance quality grade (mm/s)
• m — rotor mass (kg)
• n — maximum operating speed (RPM)

Example: For a rotor with a mass of 100 kg, rotating at a maximum speed of 3000 RPM, that must be balanced to grade G6.3, the permissible residual unbalance is:

U_per = (9549 × 6.3 × 100) / 3000 ≈ 2005 g·mm

This means a total unbalance of approximately 2005 g·mm is permitted for this rotor without exceeding G6.3. In practice, this residual unbalance is distributed between correction planes. For two-plane (dynamic) balancing, the calculated U_per is divided between the planes equally or proportionally to the rotor configuration. Thus, the balancing technician receives a specific numerical target to achieve.

Practical Balancing and Equipment

To achieve the required balance grade in practice, specialized equipment is used. In manufacturing conditions, stationary balancing machines are typically employed, where the rotor is spun and corrected until the residual unbalance drops to the norm for the selected G-grade.

However, in field conditions (e.g., when vibration occurs in an already-installed fan or pump), portable balancing instruments can be used. An example is the Balanset-1A device—a portable two-channel vibrometer-balancer. It enables single-plane or two-plane dynamic balancing directly on equipment in-situ (on-site, without rotor removal).

Fig. 1: Balanset-1A portable vibrometer-balancer connected to a laptop. This compact device includes an electronic measurement module, two vibration sensors, and a laser tachometer, with control and unbalance calculation performed by PC software.

Fig. 1: Balancing tolerance calculation window in Balanset software. The program includes a built-in calculator that automatically computes permissible residual unbalance according to ISO 1940 standards based on rotor mass, operating speed, and selected G-grade.

The device connects to a laptop, measures vibration and unbalance phase using sensors and an optical tachometer, after which the software automatically calculates the required correction weights. Among the Balanset-1A features is automatic calculation of permissible unbalance according to ISO 1940 (G-grades)—the device itself determines to what level vibration must be reduced to achieve, for example, G6.3 or G2.5 grade.

Modern balancing instruments like the Balanset-1A make achieving the required balance grade faster and more reliable. Using standard G-grade terminology and built-in tolerance calculations, engineers and technicians know exactly the criterion for successful balancing. Thus, the standardization of balance quality through G-grades has enabled a common language for describing how “smoothly” a specific rotor should operate and achieving this level of vibration reliability using methods that are understandable and verifiable worldwide.

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