6 steps for on-site fault vibration analysis

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I. Understand the vibration history: When machine vibration increases suddenly or gradually, determine whether the vibration remains stable during that period; if not, identify which parameters it is related to, etc. For example, a sudden increase in vibration may be due to a sudden imbalance of the rotor or a sudden decrease in its stiffness ； Is amplitude instability related to load adjustment, temperature changes, starting and stopping, etc.? II. Understand the maintenance history: Determine whether any maintenance was carried out before or after the onset of vibration. If the vibration level was low prior to the maintenance, then the details of that maintenance and the components that were replaced must be clearly known. In such cases, the cause of the fault can often be identified during the maintenance process itself. If the vibration was high before the maintenance and showed no significant change after it, this can also rule out the issues that have been addressed through maintenance. III. External observation and testing of the equipment: Understanding the basic parameters of the equipment, such as its temperature, noise level, pressure, speed, load, oil temperature, and vibration, allows for a quick assessment of its overall operating condition and helps to determine what type of fault may be present. For some common faults, an accurate diagnosis can even be made in this way; whereas relying solely on signal analysis tends to complicate simple issues. IV. Determining the location of the fault: Usually, the fault is located at the position with the maximum amplitude. As the distance from the source of the fault increases, the excitation force gradually diminishes. Of course, there are exceptions to this rule as well; for example, in vertical equipment, due to the relationship between height and stiffness, a failure in the pulled portion can cause the motor at the upper part to vibrate the most. V. Simple method for determining the cause of faults: It is possible to accurately identify certain causes of faults by examining the relationship between various parameters and vibration, as well as the direction of the vibration. For example, by comparing the vibration amplitudes in the horizontal and vertical directions, it is possible to determine some of the causes of faults; however, it is first necessary to be aware of the installation structure of the equipment, that is, to have a general understanding of the relative stiffness in these two directions. The equipment is installed on a solid or rigid support, which is different from being installed on an elastic support; here only equipment with a rigid foundation is discussed. Assuming the fan is installed on a rigid concrete foundation, the vertical stiffness of the equipment will be greater than its horizontal stiffness. At this time, for common faults such as mass imbalance, the horizontal amplitude is generally greater than the vertical amplitude. If the vertical vibration is greater than the horizontal vibration, it indicates that the vertical stiffness is lower than the horizontal stiffness, which may be caused by looseness or excessive clearance. If the vertical vibration of the wheel-side bearing is high, it may also be caused by an up-and-down misalignment ； Horizontal vibration near the impeller side generally indicates rotor imbalance, while horizontal vibration near the coupling side is often due to misalignment of the coupling. For axial vibration, it is generally necessary to consider whether misalignment or shaft bending has occurred. Of course, the axial amplitude could also be caused by rotor imbalance at the extended end. It should be noted that disturbances in the fluid often cause axial vibration of the rotor; therefore, the load can be adjusted for observation. VI. Combination and comparison of vibration data: When equipped with instrumentation, the results should be combined with measurements taken by those instruments. If there is existing vibration data, it is essential to compare it to identify which frequencies have experienced significant changes. Faults that cause abnormal vibrations all generate vibrations with certain frequency components, which may be a single frequency, a set of frequencies, or a certain frequency band. Based on the frequency composition of the vibration signals, the time-domain waveform, and the phase relationships between different components, a number of faults that are impossible to occur can be ruled out quickly. This allows attention to be focused on a few possible causes of failure, which can then be examined one by one taking all the characteristics into account.