Successful rebalancing of the KM03/KM020 coal mill at HKM

Precision down to the tonne, courtesy of the Assembly Team at TS Technologie + Service GmbH.

A technical milestone in maintenance: using a highly sophisticated mathematical method, the rotor of the KM03 coal mill was balanced with such precision that the residual imbalance of the entire system – which weighs several tonnes – is less than 4 grams. A resounding success for the plant’s operational reliability and service life.

The challenge: 144 impact tools in dynamic equilibrium.

Replacing the impact tools on industrial mills is one of the most demanding tasks in heavy maintenance. On the KM03/KM020 coal mill, a total of 144 hammer sets (each consisting of a support arm and a hammer head) had to be refitted.

With an average set weight of over 36 kg, the total rotating weight runs into the tonnes. As no two components are identical down to the gram, a purely random assembly would inevitably lead to significant imbalances. This would result in vibrations of more than 4.5 mm/s – which would cause extreme wear on the main bearings, cracks in the weld seams and, in the worst case, unplanned downtime of the plant.

The solution: pre-balancing through longitudinal symmetry and cross-pairing.

To eliminate vibrations from the outset, a complex digital assembly plan was developed prior to physical installation. This was based on the precise weighing of all 144 individual components.

The optimisation strategy was based on three core physical principles:

1. Centring of masses (Z-axis symmetry): The heaviest sets of beaters were deliberately positioned in the solid inner planes of the rotor (planes 6 and 7). The weight of the sets decreases systematically towards the outer planes (planes 1 and 12).
2. Cross-pairing per plane: Within each of the 12 rotor planes, the weights were arranged so that heavier and lighter components are positioned exactly opposite one another (180° offset) and their centrifugal force vectors cancel each other out.
3. Spiral phase shift to counter wobble: To prevent momentary imbalance (axial wobble of the rotor), the remaining weights were arranged in a staggered pattern along the length of the shaft, like a zip-shaped spiral.

The result: Mathematical perfection in reality.

The subsequent mathematical vector analysis of all 12 planes confirmed the project’s success in black and white:

• Total static residual imbalance: a mere 3.55 grams! Calculated against the total mass of the rotor, this is symbolically equivalent to the weight of a single sugar cube.
• Internal bending moments minimised: virtually all individual planes achieved an internal residual imbalance of less than 1.5 grams (many even less than 0.5 grams)

Conclusion

Following the precise and meticulous installation carried out by the maintenance team, the KM03 coal mill is now running absolutely flawlessly, quietly and smoothly. The vibration readings are well within the ‘deep green’ range of the ISO standard. Thanks to this data-driven, precision work, mechanical stresses have been reduced to a minimum, which drastically increases the service life of the bearings and ensures long-term production reliability.

A big thank you goes to the whole team for their meticulous preparation and the excellent craftsmanship demonstrated during the installation!

Glückauf!

Paul David Carstens
Abteilungsleiter Montage HKM Kokerei