Understanding Balancing Tolerance

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Definition: What is Balancing Tolerance?

Balancing tolerance is the maximum permissible amount of residual unbalance that can remain in a rotor after balancing is completed. It represents the acceptance criterion that defines whether a rotor is adequately balanced for its intended service. Balancing tolerance is expressed either as a specific unbalance mass at a given radius (in gram-millimeters or ounce-inches) or as vibration amplitude (in mm/s or mils).

Tolerances are defined by international standards, primarily the ISO 21940 series, which specify balance quality grades based on rotor type, service speed, and application. These standards ensure consistent, safe, and effective balancing across industries and equipment types.

Why Balancing Tolerance Matters

Establishing appropriate balancing tolerances is crucial for several reasons:

• Safety: Excessive residual unbalance can lead to machine failure, creating safety hazards for personnel and surrounding equipment.
• Equipment Longevity: Operating within tolerance minimizes vibration-induced wear on bearings, seals, and structural components, extending service life.
• Quality Assurance: Tolerances provide objective acceptance criteria for balancing work, ensuring consistent quality.
• Economic Balance: Tolerances represent a practical compromise between the cost of achieving perfect balance (which is impossible) and acceptable operational performance.
• Industry Standards Compliance: Meeting recognized tolerances demonstrates compliance with industry best practices and may be required by regulations or warranties.

ISO 21940-11: The Primary Standard

ISO 21940-11 (formerly ISO 1940-1) is the internationally recognized standard for balance quality requirements. It defines a series of balance quality grades denoted as G-grades, where G stands for “balance quality grade” and the numerical value represents the specific unbalance eccentricity in millimeters per second.

Common Balance Quality Grades (G-Grades)

The standard defines G-grades ranging from G 0.4 (highest precision) to G 4000 (lowest precision). Common grades include:

• G 0.4: Precision grinding machine spindles, gyroscopes (highest precision)
• G 1.0: High-precision machine tool spindles, turbochargers
• G 2.5: Gas and steam turbines, rigid turbo-generator rotors, compressors, machine tool drives
• G 6.3: Most general machinery, electric motor rotors (2-pole), centrifuges, fans, pumps
• G 16: Agricultural machinery, crushers, multi-cylinder diesel engines
• G 40: Slow-running equipment, rigidly mounted four-cylinder diesel engines

Lower G-numbers indicate tighter tolerances (less permissible unbalance), while higher G-numbers allow more residual unbalance.

Calculating Balancing Tolerance

The permissible residual unbalance depends on three factors: the rotor’s mass, its service speed, and the selected balance quality grade. The calculation follows this relationship:

Online Tolerance Calculator

For quick and accurate calculation of permissible residual unbalance, use our Residual Unbalance Tolerance Calculator. The calculator automatically computes tolerance values based on ISO 1940/21940 standards for various machine types, rotor mass, and operating speed, with options for single-plane or two-plane balancing.

Formula for Permissible Residual Unbalance

U_per = (G × M) / (ω / 1000)

Where:

• U_per = Permissible residual unbalance (gram-millimeters or g·mm)
• G = Balance quality grade (e.g., 6.3 for G 6.3)
• M = Rotor mass (kilograms)
• ω = Angular velocity (radians per second) = (2π × RPM) / 60

Simplified Formula Using RPM

For practical use, the formula can be simplified to:

U_per (g·mm) = (9549 × G × M) / RPM

Where:

• M = Rotor mass in kilograms
• RPM = Service speed in revolutions per minute
• G = Balance quality grade number

Example Calculation

Consider a motor rotor with the following specifications:

• Mass: 50 kg
• Operating speed: 3000 RPM
• Required balance quality: G 6.3

U_per = (9549 × 6.3 × 50) / 3000 = 100.4 g·mm

This means the maximum permissible residual unbalance for this rotor is approximately 100 g·mm. If the correction plane radius is 100 mm, this equates to 1.0 gram of residual unbalance at that radius.

You can verify this calculation or compute tolerances for different machine types using our online calculator.

Single-Plane vs. Two-Plane Tolerances

The calculated tolerance applies to the total unbalance in a single plane for single-plane balancing. For two-plane (dynamic) balancing, ISO 21940-11 provides guidelines for distributing the total tolerance between the two correction planes, typically allocating the tolerance to each plane based on the distance between planes and the rotor’s geometry.

Vibration-Based Tolerance

While ISO 21940-11 specifies unbalance mass limits, field balancing often uses vibration amplitude as the acceptance criterion because it is directly measured. Vibration-based tolerances are typically defined by:

ISO 20816 Series

These standards specify acceptable vibration limits for various machine types based on RMS velocity (mm/s or in/s). Common zones include:

• Zone A: Newly commissioned machines (very low vibration)
• Zone B: Acceptable for long-term operation
• Zone C: Acceptable for limited periods, corrective action should be planned
• Zone D: Unacceptable, immediate corrective action required

Practical Field Criteria

Many balancing technicians use these rules of thumb:

• Vibration reduced to less than 25% of initial level = successful balance
• Absolute vibration below 2.8 mm/s (0.11 in/s) = generally acceptable for most industrial equipment
• Residual vibration below 1.0 mm/s (0.04 in/s) = excellent balance

Factors Affecting Achievable Tolerance

The ability to meet balancing tolerance depends on several practical factors:

1. Equipment Capabilities

• Measurement precision of balancing instruments
• Sensitivity of vibration sensors
• Resolution of weight placement (how accurately weights can be positioned)

2. Rotor and Machine Characteristics

• Mechanical condition (looseness, bearing wear, foundation issues can prevent achieving tight tolerances)
• Operating at or near critical speeds makes precise balancing more difficult
• Non-linearity in the system response

3. Practical Constraints

• Accessibility of correction planes
• Available weight increments (can only add weights in discrete amounts)
• Angular resolution of mounting holes or attachment points

Tolerance vs. Balancing Capability

It’s important to distinguish between:

• Specified Tolerance: The maximum permissible residual unbalance as defined by standards or specifications
• Achievable Balance: The actual level of balance that can be practically achieved given equipment capabilities and constraints
• Economic Balance: The point beyond which further improvement is not cost-effective

For most industrial field balancing, achieving unbalance levels 2-3 times better than the required tolerance represents excellent work and ensures a margin for measurement uncertainties and operational variations.

Documentation and Acceptance

Proper documentation of balancing tolerance includes:

• Specified G-grade or tolerance value
• Calculated permissible residual unbalance (U_per)
• Measured residual unbalance after balancing
• Comparison showing compliance: Measured ≤ Permitted
• Acceptance signature or notation

This documentation provides objective evidence that the balancing work meets specifications and serves as a baseline for future maintenance evaluations.

When to Use Tighter or Looser Tolerances

Tighter Tolerances Justified When:

• High-speed operation (critical for safety and bearing life)
• Precision equipment requiring minimal vibration
• Lightweight or flexible structures sensitive to vibration
• Equipment located near vibration-sensitive processes or instruments

Looser Tolerances Acceptable When:

• Low-speed, heavy-duty equipment
• Robust construction with high vibration tolerance
• Short-run or infrequent-use equipment
• Economic considerations outweigh incremental performance gains

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