ArticleIs The Machine Good or Bad
by Rene Archambault
This paper examines the process of machine condition monitoring and detecting faults using vibration analysis. It explains the need to efficiently identify low vibration level faults using as little apriori knowledge of the machine as possible. The limitations of ISO 10816 severity level guidelines are explained with recommendations of additional analysis methods to detect bearing faults, gear faults, motor rotor bar faults and other faults that produce low-level amplitude signals. CPB Spectrum, MaxPeak and Kurtosis analysis techniques used to detect these low-level signals are clearly explained in this paper with a description of an enhanced fault detection algorithm based on CPB spectral data. Charts are also embedded in the paper with examples that show the results of these fault detection methods. This paper shows how these detection methods can help answer the question: Is the machine good or bad!
One of the basic tasks of machine condition monitoring vibration monitoring is fault detection, i.e. the process of detecting if the machine is running properly or if abnormal operating conditions or deteriorated mechanical components are present. Although this appears easy enough to accomplish with modern technology and know-how, it is not always the case as many faults do not necessarily result in higher vibration levels and sometimes, information is required about the machine configuration before the proper analysis technique can be used and the fault identified. When this information is not available, it is very difficult to assess the machine condition, hence the need for fault detection methods which require as little a-priori knowledge about the machine as possible. The remainder of this article will focus on such techniques, and deals with vibration data derived from accelerometer signals.
OVERALL VELOCITY LEVELS FOR GENERAL MACHINERY CONDITION MONITORING
There seems to be a good consensus around the world that the RMS velocity level is the one of the best indicator of overall machinery condition and if exceeded by too much, can create dangerous situations for a given machine. The new ISO 10816-3, which replaces the old ISO 2372 from which it is derived, covers a large range of machinery from 120 RPM to 15000 RPM (large steam, gas and hydro-electric turbines, as well as reciprocating machines, are excluded from this part of the standard). The standard gives recommended values for newly commissioned machines, and unacceptable levels for four categories of machines, based on the power rating and the type of mounting of the machine, as shown on Figure 1. (refer to the standard for more details). Although it provides baseline values that can serve as a guide when no other data is available for a specific machine, the user is warned to also look for level changes during the life of the machine, as these changes could reveal a significant change in the condition of the machine, even though the absolute levels have not exceeded the published values. Typically, a change of 6 dB (or a factor of 2) is considered significant, 10 dB a very important change (factor of 3) and 20 dB is unacceptable (factor of 10). For some specific machinery, a weighting curve is suggested for measuring the overall level (see figure 2, which relates to reciprocating machinery — part 6 of the standard), as a constant velocity level all across the frequency range could lead to unacceptable displacements at very low frequency and dangerous acceleration levels at high frequency.”
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