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Measuring low vibration levels in low-speed applications


When analyzing possible failures in low-speed application (rotational speed between 0.5 and 600 revolutions per minute (RPM)), there are many variables to consider. As these applications typically produce low levels of vibration, proper data can be hard to obtain. Below we present the most important monitoring methods you need to know.

There are multiple tools and processes, both electronic and manual, that can aid in diagnosing a low-speed problem. By analyzing the vibration in rotating machinery, it is possible to detect possible faults such as unbalance, bent shaft, cracked shaft, misalignment, looseness, gear faults, motor faults and impellor/blade defects. However, as these faults mainly occur in high speed machinery (>600 RPM), additional techniques or tools for proper monitoring can be necessary.

Measurement techniques: Velocity vs. acceleration enveloping
Mathematically speaking, velocity equals displacement divided by time (V=d/t). Measuring low-speed applications in velocity is not without issues, as the response (displacement) from the bearing is difficult to measure. Consider for example a paper mill slewing bearing where the bearing elements do not have large mass relative to the machine. Here, the velocity generated is relatively small and therefore the vibration probe cannot recognize it.

Therefore, other means of measurement are needed if there is to be repeatable success in measuring these low-speed applications. An effective method for this is acceleration enveloping – or to put it another way, the use of filtered acceleration signals.

Using this technique, repetitive signals (produced as a rolling element) are enhanced as they pass over a damaged area in the bearing. On the other hand, non-repetitive signals are degraded. Once the energy has been captured, it is processed and filtered through one of several filters. Based upon the filter selection, the analyst will then have the best possible result of the data.

It is very important that the analyst collects and stores the time domain spectrum when analyzing low-speed applications. In the frequency spectrum, it is sometimes impossible to see the defect frequencies. Therefore, the time domain must also be analyzed.

The importance of mounted accelerometers for effective analysis
When analyzing low-speed applications – using mounted accelerometers is highly recommended. Why? Because permanently mounted accelerometers assure that data is repeatable and consistent. Also, these permanent sensors allow for the greatest response to the sensor when working with low-speed applications.

If a decision not to use a permanently mounted accelerometer has been made – it is of high importance to select a flat surface to press the accelerometer against. To ensure consistency in the data, measurements should be taken at the same precise location for comparison (moving the accelerometer only a few inches can produce drastically different vibration readings). Mark the measurement point with a permanent-ink marker to ensure that the measurements are taken at the exact location every time.

Final Remarks: What to have in mind when analyzing low-speed applications
While some manufacturers have created low-speed vibration monitoring devices that include specific guidelines for analyzing low-speed applications, others only provide a recommended setup for the analysis.

If you are looking for additional insights regarding monitoring and analysis of low-speed applications, please download our dedicated White Paper: Vibration Monitoring of Low-Rotational-Speed Applications using Accelerometers [embed link] or consult one of our SKF engineering experts [link].