In order to eliminate the problem of false signals in the shaft vibration of the rear bearing of the Cummins diesel generator, the diesel generator manufacturer combined the opportunity of unit maintenance to modify its vibration measurement device. The shaft measurement probe of the rear bearing of the Cummins diesel generator was changed to a composite probe (contact probe+eddy current sensor), which effectively separated the interference signal and retained the true vibration signal by measuring the shaft vibration through the contact between the measuring rod and the shaft, After replacing the new probe, there was basically no change in vibration before and after grid connection, and the effect was good, basically solving the problem of false vibration in the rotor of the original QFS double water internal cooling generator. However, the disadvantage of this device is that improper installation of the compression spring can also cause measurement interference, and due to its contact measurement, the probe may also wear out after prolonged operation.
The axial vibration caused by insufficient bearing support stiffness is most common on site, as the support stiffness of the rear bearings of multiple Cummins diesel generators is insufficient, resulting in excessive axial vibration of the bearings. Through the external characteristic test of the bearing, the differential vibration between the connecting components of the bearing can be measured, and it can be found that there is significant differential vibration in the vertical or horizontal vibration of the bearing seat. Further inspection can reveal issues such as not tightening the connecting screws of the bearing cover, bearing seat, foundation plate, etc., and insufficient preload. The example analysis is as follows.
Axial resonance of the bearing seat occurs when the natural frequency of the bearing seat in the axial direction is close to or an integer multiple of the rotor operating frequency, causing significant axial vibration. On site, it usually appears in the form of a second harmonic, and the excitation force that causes the second harmonic resonance is mainly the electromagnetic excitation force of the two pole generator of the Cummins diesel generator. It is caused by the uneven vibration at various points on the foundation to form the axial vibration component of the second harmonic, causing the bearing seat to resonate. Secondly, the rigidity asymmetry of the rotor and poor alignment of the rotor cause the unit to exhibit significant second harmonic vibration when there is no excitation current at rated speed.
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