The causes of vibration in a rotating machine usually fall into four categories: imbalance, resonance, misalignment, and mechanical failure.
1. Rotor imbalance
It is a common cause of vibration. Factors such as poor rotor manufacturing, accumulation of foreign objects on rotor blades, and unbalanced motor rotors can all contribute to this. Another reason for significant vibration due to imbalance is poor stiffness of the equipment base or resonance.
Keyways and keys are also causes of unbalanced vibration.
Thermal bending of the shaft is another phenomenon that causes rotor imbalance. Generally, the vibration caused by thermal bending changes slightly with load. However, if the equipment foundation resonates with its rotation, there is a high possibility of severe vibration. Therefore, the key to prevention is that the material of the shaft must meet the requirements; and the base of the machine must be solid and reliable.
2. Resonance
The resonant frequency of the system depends on the number of its degrees of freedom; the resonant frequency is determined by mass, stiffness, and attenuation coefficients. The resonant frequency of the machine support should be far from any excitation frequency. For new installations, the required foundation stiffness can be consulted with the manufacturer to achieve this goal. For existing devices with resonant frequencies equal to the rotational speed, there are two options – reducing the excitation force to the rated limit or changing the resonant frequency. The latter can be achieved by increasing the system stiffness and mass. When dealing with resonance issues, it is better to change the resonant frequency.
Resonance can also be caused by misalignment between the rotor and stator system components or mechanical and electrical failures.
Resonance frequencies at operating speeds can also cause faults. They can also be triggered by misalignment or mechanical and electrical failures. However, compared to problems at the same frequency, these resonances cause less common issues.
3. Misalignment
It can cause radial and axial excitation forces at operating speed and twice the operating speed. However, it is not possible to conclude the absence of misalignment problems simply because one or two of the above phenomena do not occur. At the same time, the thermal expansion of the unit should be considered, and a 1.5-3mm gap should be left between a pair of couplings.
4. Mechanical failure
Low-quality couplings, poor bearings, and inadequate lubrication, as well as unstable supports, are all reasons for generating excitation forces of different frequencies and amplitudes.
(1) Poor-quality couplings are mainly manifested in poor casting quality, skewed connection holes, burrs, and rubber gaskets that quickly deteriorate, causing the coupling to change from a soft connection to a hard connection, resulting in vibration and wear.
(2) Replacement of radial bearings is generally a simple replacement. To avoid vibration when changing new bearings for vibration reduction, contact coloring inspection of the outer ring of the bearing should be performed, and the bearing seat should be treated if necessary.
(3) Axial fluctuation is the cause of another vibration problem of the rotating machine, including couplings, bearings, and the rotating machine itself. Generally, the axial thrust of the rotating machine is constrained by thrust bearings. However, if the axial alignment is poor and the rotor rubs axially, severe axial vibration may occur.
(4) Weak supports, that is, when the four feet of the rotating machine are not on the same plane, when the rotating machine is fastened by bolts at these four points, if the bearings are not aligned, it will inevitably cause severe vibration. Therefore, during the installation of the rotating machine, it is necessary to first symmetrically tighten several fastening points with an appropriate torque. Then, loosen one fastening point at a time and measure the vertical deformation at that point with a dial indicator. If the vertical deformation is greater than 0.05mm, a shim should be added under this support, with a thickness equal to the deformation. Repeat this process until the vertical deformation at each point is less than 0.05mm when loosening.
(5) Problems with the rotating machine’s base and foundation may be the direct cause of excessive vibration. Therefore, the anchor bolts must have sufficient strength, the concrete foundation must be solid without cavities, and the anchor bolts should be checked frequently during operation for loosening and fracture, and timely elimination should be carried out. At the same time, check whether the supporting devices of the rotating machine are firmly attached.
In addition, vibration caused by cavitation in pumps is also a relatively common phenomenon. Radial rubbing between the rotor and stator can also easily cause vibration, which is induced by the excitation frequency. Therefore, during the assembly of the rotating machine, it is necessary to pay attention to the clearance between the moving and stationary parts. In addition, large axial-flow fans may experience surge, while steam turbines may have bearing oil film oscillations. In summary, after familiarizing oneself with the common causes of turbine vibration faults, various vibration measuring instruments can be used for handling and preventing vibrations.
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Post time: Apr-29-2026