How to Perform Dynamic Balancing on a Grinding Mill Rotor

This procedure applies to grinding mill rotors (including main mill rotors, fan impellers, classifier rotors, and drive shafts) and focuses on field dynamic balancing (the most practical method for large grinding mills, without full rotor disassembly).

Core Principle

Dynamic balancing eliminates mass unbalance that causes vibration, noise, bearing wear, and structural damage by adding/removing correction weights at precise angles and positions to align the rotor’s center of mass with its rotational axis.

1. Safety Preparation (Mandatory)

1. Implement Lockout-Tagout (LOTO) for the mill motor, drive system, and control panel to prevent accidental startup.
2. Allow the rotor to cool to <50°C; remove accumulated graphite/powder dust to avoid explosion risks (especially for combustible graphite dust).
3. Wear PPE: safety goggles, anti-static gloves, respirator, and anti-slip work shoes.
4. Clear the surrounding area; ensure no personnel stand in the rotor rotation plane during testing.
5. For mills with dust hazards: prohibit open flames, use spark-free tools, and confirm good ventilation.

2. Pre-Balancing Inspection

First eliminate non-unbalance vibration causes:

• Check for heavy dust buildup on rotor blades/hubs (the #1 cause of unbalance in grinding mills).
• Inspect rotor wear, cracks, bending, or loose blades/liners.
• Verify bearing clearance, shaft alignment, and loose foundation bolts.
• Confirm the rotor runs freely without rubbing or mechanical interference.
• Clean the rotor surface thoroughly to ensure accurate balancing.

3. Required Tools

• Portable field dynamic balancer (with vibration sensor & photoelectric tachometer)
• Reflective tape (for rotational speed/phase detection)
• Correction weights (bolts, steel clamps, welding weights – spark-free for dust mills)
• Wrenches, calipers, marker, and balance weight calculation software
• Magnetic vibration sensor (mounted on bearing housing)

4. Step-by-Step Field Dynamic Balancing Procedure

Step 1: Mark the Rotor & Mount Sensors

1. Stick reflective tape on the rotor shaft or hub as a phase reference mark (0° position).
2. Attach the vibration sensor to the bearing housing (horizontal/vertical direction) and connect the tachometer probe to the reflective tape.
3. Set the balancer to the rotor’s operating speed (RPM) and select single-plane or two-plane balancing
   • Small fans/impellers: single-plane balancing
   • Large mill main rotors/classifiers: two-plane balancing (front and rear ends)

Step 2: Measure Initial Unbalance

1. Start the mill and run the rotor at normal operating speed (no-load).
2. The balancer records:
   • Vibration amplitude (velocity/displacement)
   • Phase angle of unbalance
3. Stop the rotor; save the initial unbalance data (baseline value).

Step 3: Add a Trial Weight

1. Calculate a suitable trial weight (based on rotor weight, diameter, and RPM) and attach it temporarily at the 0° reference position.
2. Restart the rotor and record the new vibration and phase data.
3. Stop the rotor and remove the trial weight.

Step 4: Calculate Correction Weight & Position

The dynamic balancer automatically computes:

• Exact mass of correction weight
• Precise angular position (degree) on the rotor
• Installation plane (front/rear for two-plane balancing)

Step 5: Install Permanent Correction Weights

• For grinding mill rotors: Use bolt-on weights or steel clamps (preferred for dust-explosive environments; avoid welding unless permitted).
• Install weights firmly at the calculated angle; ensure no loose parts.
• For heavy unbalance: Split the weight into multiple small weights to avoid stress concentration.

Step 6: Re-Test & Verify Balance

1. Restart the rotor and measure residual vibration.
2. If vibration still exceeds the standard: repeat the correction process (fine-tune weight/position).
3. Once qualified, secure all weights and clean the rotor.

5. Static vs Dynamic Balancing

6. Acceptance Standards (ISO 1940)

For grinding mill rotors, the balance quality grade is typically G6.3–G2.5

Residual vibration limits (normal operating temperature):

• Vibration velocity: ≤2.8 mm/s RMS (excellent)
• Vibration velocity: ≤4.5 mm/s RMS (acceptable)
• Axial/radial displacement: ≤0.05 mm

7. Post-Balancing Maintenance

1. Tighten all weight fasteners and check for looseness after 24 hours of operation.
2. Record balancing data: weight mass, position, initial/final vibration values.
3. Schedule regular cleaning: dust accumulation on rotors will re-cause unbalance within 1–3 months.
4. Inspect bearings monthly; unbalance is the leading cause of premature bearing failure.

8. Common Troubleshooting

• Vibration unchanged after balancing: Incorrect sensor position, rotor rubbing, or bearing damage.
• Unbalance recurs quickly: Severe dust buildup, loose rotor parts, or material erosion.
• Phase shift unstable: Weak reflective tape signal, unstable speed, or foundation looseness.
• Weight falls off: Inadequate fastening; use lock washers or welding (spark-free) for fixation.