Correct Measurement (Detection) of Machine Vibrations

More and more companies recognize that by performing vibration diagnostic condition monitoring and tracking of their rotating machinery, maintenance can be efficiently carried out, maintenance costs can be reduced, and unexpected machine downtime can be less frequent (or even completely eliminated). Suitable instruments for this purpose can be obtained from many companies. However, apart from the differences in capabilities and prices, there is one thing that all portable instruments have in common: machine vibrations must be "detected" with the vibration sensor attached to the instrument. Although this may seem simple, inexperienced operators often make mistakes related to this aspect, which not only make the measurement results inaccurate but can also make them completely questionable.

Where to Measure?

Mechanical vibrations are strongest at their source. The transmission of vibration energy occurs with more or less damping in any material (for example, steel dampens weakly, while rubber strongly absorbs vibrations). The higher the frequency of the vibration, the stronger the damping. Therefore, low-frequency vibrations can be detected at greater distances from the source, but the detection range of high-frequency vibrations (e.g., bearing vibrations) is very limited. In addition to damping, it is also necessary to consider the fact that additional vibration energy loss occurs when vibration is transferred from one body to another (in our case, between machine components). The closer the connection between two elements, the more complete the transfer of vibration energy. Elements not in contact with each other do not follow each other's vibrations. Furthermore, it complicates matters that smaller elements generating high-frequency vibrations (e.g., bearing components) can transfer enough motion energy to excite larger bodies to vibrate. Therefore, as a basic rule, we must measure as close to the vibration source as possible! In the case of rotating machinery, measurements should be taken on the bearing housings, as vibrations originating from the rotating components' faults are transferred here, and vibrations originating from the bearing fault itself (high-frequency) can only be measured here. Never measure on loose casing or separate - non-contact - machine elements if you are interested in vibrations related to the rotating parts of the machine! (Measurements on the latter should only be carried out if there is a suspicion that they may resonate with some excitation of the machine.)

What to Measure With?

It matters what frequency range of vibrations we are interested in. For machine condition monitoring, the frequency range recommended in ISO 10816 is most commonly used, where vibrations are measured between 10 Hz and 1 kHz (scaled in effective velocity). For machines rotating at 3000 revolutions per minute or faster, the frequency range should be set between 10 Hz and 2-3 kHz, while for slow machines (a few hundred revolutions per minute), vibrations should be measured from 2 Hz. Therefore, the measurement tasks and conditions to be considered during measurements vary depending on the machine's speed. Typical industrial machines (1500 or 3000 revolutions per minute) can be accurately measured with most standard sensors supplied with the instruments - mostly ICP type (with built-in charge amplifier) piezoelectric vibration accelerometers, provided that the sensor is properly attached to the measurement object (see later). Electrodynamically sensitive vibration velocity sensors are also equally applicable, and for slower rotating machines, they are often even better than piezoelectric sensors: consider that in piezoelectric sensors, the charge is generated by the force exerted on the piezocrystal by the built-in seismic mass, which is then amplified and considered as a signal proportional to the vibration acceleration. In slow movements - although there may be a large displacement or even velocity - there is hardly any acceleration, so the forces acting on the piezocrystal do not change, and therefore there is no charge or signal. The smallest vibration frequency that can be measured by piezoelectric sensors used for machine vibration measurement is around 1-2 Hz (usually at 0.3 Hz, there is already a 3 dB damping).

For faster rotating machines, however, piezoelectric sensors are almost exclusively suitable for measuring higher frequency vibrations. It is not uncommon for their measurement frequency range to extend up to 15-40 kHz. However, when it comes to the measurability of high-frequency vibrations, the physical fact must also be taken into account that these vibrations can only be well followed by the lightest - highest natural frequency - elements. This naturally applies to the sensor required for the measurement: if it is not screwed on, the lighter the sensor should be for measuring higher frequency vibrations. In the case of magnetic mounting, it should also be as light as possible. (Unfortunately, this also reduces the magnetic holding force, which is particularly disadvantageous.) However, the most significant challenge is the transmission of vibrations from the surface to be measured to the sensor: the nature of the mechanical coupling of the sensor significantly influences the measurement in the high-frequency range.

How to "Attach" the Sensor to the Object to be Measured?

This is one of the most delicate topics. The sensing direction of the sensors almost always coincides with their central axis. To ensure accurate measurement, it is essential that the sensor closely follows the motion of the measuring point (i.e., the surface of the machine element) in the desired direction. Depending on the quality of tracking the vibrations, we may need to expect different measurement results. What is the reason for this? Of course, the fact that losses occur during vibration transmission also applies to the sensor - the higher the frequency of the vibration, the more difficult it is for the sensor to follow it (due to the inertia of its own mass). Therefore, the detectability of high-frequency vibrations primarily depends on the connection established between the sensor and the measuring surface. The relationships are shown in the diagram on the right.

Frequency Response Attachment Dependency

Many instruments come with a sensor probe that allows measurements to be taken at hard-to-reach points or points unsuitable for magnetic attachment for various reasons. However, high-frequency vibrations cannot be accurately or at all measured with this method. The sensor only follows vibrations that are transmitted through manual probe pressure. The frequency of these vibrations can never exceed 2-3 kHz. It is a misconception to try to measure vibrations of 5 kHz or higher frequency with a manual probe or even with a sensor mechanically integrated into the instrument. These vibrations do not reach the sensor; however, the sensor's own frequency, the resonating probe, and the components of the instrument will influence our measurements. To verify the accuracy of measurements taken with the probe, temporarily increase the probe pressure. If the readable value changes, we need to check the contact created with the measurement point (presence of paint, loose element, etc.). If this does not help, always use a holding magnet or mount the sensor directly onto the machine with a threaded stud.

In practice, the use of magnetic mounting bases (holding magnets) has become a common solution, as they provide consistent measurement conditions for repeated measurements, are quick to work with, and do not require the person conducting the measurement to hold the sensor at the measurement point during data collection. The magnets supplied by sensor and instrument manufacturers - assuming correct handling - can maintain the appropriate holding force for years to ensure a strong connection between the sensor and the measuring surface in industrial (non-sterile laboratory) conditions. However, it is important to remember that for the transmission of high frequencies, all contaminants and thick paint layers must be removed! These act as mechanical filters regardless of the strength of the applied magnet.

It is not surprising that even when using a holding magnet, measurements can only be taken in a very limited frequency range, as the magnet's force alone ensures the transmission of vibrations. The disadvantage is that to achieve greater adhesion, a larger magnet would need to be used, but the magnet and the sensor act as one body, which becomes less willing to track high-frequency vibrations as its weight increases (and its resonant frequency decreases). In general, it can be said that with the use of a proportional size magnet and a suitable measuring surface (flat, clean, unpainted), measurements can be taken up to 8-10 kHz.

Extending the Frequency Range

If we want to measure vibrations beyond this frequency limit, we are forced to use completely different mounting technologies. By gluing a washer to the surface to be measured and screwing the sensor onto it, we can measure up to 20 kHz with the right adhesive. The measurable frequency range expands to about 30 kHz if we opt for direct adhesive mounting of the sensor. (Of course, this assumes the use of a sensor with an appropriate frequency range.) And if we want to measure up to 40 kHz, only one solution is possible: the sensor must be mounted directly onto the measuring surface (flat, clean, unpainted), and we must aid the vibration transmission by placing wax between the sensor and the measuring surface.

It should be noted that in some cases, we may face additional problems. On one hand, the weight of the sensor (and the probe or magnet) can affect the measured value because it influences the vibration of the object being measured. As a rule of thumb, we should be cautious about accepting measurement results obtained on machine elements lighter than ten times the total weight of the sensor (including the magnet or probe). On the other hand - as shown in the first figure - the so-called coupling resonance occurs at the upper frequency limit of every mounting technology. This can lead to the resonance amplification of vibrations at around 3 kHz in measurements with a probe and around 10 kHz in magnetic mounting. The consequence of this is reflected in completely unrealistic measurement results.

If we encounter "strange" values during vibration measurements, the reason may be that we did not choose the sensor coupling correctly. It is worth implementing the coupling with the appropriate care, as our measurement depends much more on this than on the capabilities of the sensor or the instrument. For example, even if an instrument can measure up to 20 or even 40 kHz, it is futile if vibrations are detected with a magnet or - in the worst case - a probe.

Rahne Eric (PIM Ltd.) pim-ltd.com, machineryexpert.com  

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