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Problem Identification:
The Resonance Institute technical team visited the factory where the equipment listed below is located in order to resolve a vibration issue:
Cold rolling mill motor-gearbox set Variable motor speed in the 900–1500 RPM range, which is critical for production. When the motor is operating at 1500 RPM, high-amplitude 1X and 2X peaks are observed at the gearbox output.
Initial estimates indicate that the problem stems from misalignment of the cardan shaft. However, subsequent cardan shaft alignment control measurements have shown that the misalignment level is within acceptable limits. An acceleration test was performed on the reducer to enable detailed analysis. Following this test, it was understood that the drive frequencies of the cardan shaft 1X and 2X entered the natural frequency range, thus creating a resonance situation as the motor approached 1500 RPM.
Additional Measurements:
The existence of resonance has been proven, but it has been understood that the machine’s operating speeds cannot be altered in a way that would prevent resonance.
Generally, resonance occurs due to weak equipment bases. However, this equipment is mounted on a highly rigid reinforced concrete base.
Since the cardan shaft length is ~5 metres, a question arose in the analyst’s mind: ‘Is this observed resonance caused by the critical speeds of the shaft?’
The analysis expert, acting on his suspicion, conducted impact tests on the lower and upper shafts. The tests proved the existence of bending natural frequencies in regions close to the frequencies observed in the acceleration tests.
The minor differences between the acceleration test and the impact tests are thought to stem from the forces generated on the shaft during operation (the forces generated during the shaft acceleration phase may increase the shaft’s rigidity).
Additional Measurements:
Design Change: A change in the shaft design can shift the shaft critical speeds to a safer frequency range (away from 1X and harmonics). This can be achieved by changing the mass and stiffness ratios. This action will resolve the root cause.
Installation of Cardan Shaft Support Structure: A cardan shaft support structure can be implemented to shift resonance regions. This application can also reduce the load on the gearbox and roll bearings.
Elimination of Forced Vibrations: Small forces caused by imbalance and/or misalignment can cause high-amplitude vibrations when they are effective in the resonance zone. To eliminate these forces, balancing (correcting the shaft if it is bent) and alignment work can be performed on the shaft. This work can help reduce vibration amplitudes.
Correction Methods:
- Due to high vibration levels, the rolling mill was not operating at full speed/load.
- Following the preparation of the vibration analysis report, the company that designed and manufactured the cardan shaft sent a supervisor. On the supervisor's recommendation, changes were made to the cardan shaft tolerances, which resolved the vibration problem. As a result of this tolerance change, it is believed that the shaft critical speeds have been shifted to a safer frequency range.
- Following this change, the customer stated that the vibration levels were below the ‘acceptable’ limits. The line can be brought up to full speed/load without any problems when required.
