Vibration analysis requires a correct identification of the rotational speed of the machine. How the different frequencies of the spectrum are related to RPM, identifying with high accuracy the order of each frequency, is essential for the analysis process. This is because a relatively small error on the RPM value can cause a significant one on higher harmonics order calculation, as the initial RPM error gets multiplied. This can make the difference, for example, between identifying correctly or not a fault frequency of a roller bearing, and it is also essential to correctly follow the energy of those spectral bands defined in orders.

With portable analyzers, actual speed measurement should be part of the data collection job, even for those (non-synchronous) machines that do not include a VFD. It is typically done by using a strobe light, a hand-held photo/laser tachometer, or by identifying the 1xRPM spectral peak at the vibration spectrum.

In online condition monitoring systems the RPM is typically measured using permanently installed tachometers, or by getting the information via communications with the control system. However, these two options have downsides:

• Tachometers are not always possible to install on some machines, or require a costly installation. Besides it is common to get maintenance problems with this instrumentation, for example due to a bad gap adjustment after a repair.
• Communication with control systems is not always allowed by the plant, or they do not include the RPM information. Sometimes, even if they do, it does not provide the RPM with the necessary accuracy. Other times, only the VFD reference is provided, and this can differ significantly from the real speed due to the slip on induction motors.

Some state-of-the-art CMSs allow the RPM value to be obtained from the vibration itself. This method works well in cases where a clear peak at 1X RPM (or a harmonic) can be observed in the spectrum, which is actually very common. If the system uses interpolation methods to identify spectral peaks, this method can provide excellent accuracy. Therefore, this functionality is highly recommended when there is no tachometer installed, or there is no possibility to obtain the RPM from the control system.

The method is recommended even for those machines that do include a tachometer or communications with the control system, due to the problems discussed above with the high failure rate of the tachometer instrumentation or the lack of accuracy of the RPM value provided by the control system.

Some CMSs allow, in fact, to use a combination of techniques for RPM identification. They do that by measuring the speed using the tachometer installed if that sensor is in good condition and using the vibration method otherwise, and by using this vibration technique to improve the accuracy of the RPM provided by the control system.