Manufacturing Trend 2008/09, Technical Diagnostics Section

"Instead of firefighting and major repairs"

Condition-based maintenance with vibration diagnostics (IV.)

The condition assessment and monitoring of rotating machinery can be achieved using vibration measuring and analyzing tools within the framework of modern condition-based maintenance. In connection with this, in our series, we review the physical basics of measurements, the selection criteria for measuring locations and measuring instruments, as well as the correct application of vibration sensors, which represents the most sensitive point in this topic. Every solid body has the ability to perform vibrations of different frequencies in multiple directions. The largest displacements can be experienced at the body-specific natural frequency, as the body "resonates" at this frequency in the given direction (hence the concept of resonance frequency.) Of course, no body starts vibrating on its own. Excitation - thus external forces - are required. The greater this force, and in the case of alternating forces, the closer its frequency is to the body's natural frequency, the greater vibrations the solid body performs in the direction induced by the force.

Physical basics

In the case of our rotating machinery, the source of vibrations is the naturally occurring alternating forces during machine operation. These forces can never be completely eliminated and arise, among other things, from the machine's normal alternating operation (e.g., reciprocating machines), residual unbalance of rotating components, or periodic forces originating from the drive (e.g., gear, characteristics of electric motors, network harmonics). Therefore, the forces are present during normal operation. The effect on individual machine components should be imagined as each machine component being part of a spring-mass oscillating system. Our rotating machine consists of many such individual oscillating systems, which are almost all interconnected and excite each other. Due to the mentioned resonance properties of solid bodies, each machine element tends to follow the effect of the alternating force at its own frequency. This applies equally to every moving and supporting element of the machine. The frequencies of the vibrations measurable on the machine and their corresponding amplitudes depend on the stiffness and mass of the mechanical elements. The smaller the machine element, the higher the frequency but the smaller the amplitude of the vibration it performs.

Selection of measuring location

Naturally, mechanical vibrations are strongest where they originate. The transmission of vibration energy in any material occurs with more or less strong damping. The higher the frequency of the vibration, the stronger the damping. As a result, 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. Regarding the measurability of high-frequency vibrations, it is important to note that only small-weight elements with high natural frequencies are capable of accurately tracking them, while heavy bodies are not. In addition to the mentioned damping, it should be considered that further vibration energy loss occurs when vibrations are transmitted from one body to another (in our case, between machine components). The closer the connection between two elements, the more strongly the vibration energy is transferred. Elements not in contact with each other do not follow each other's vibrations. Furthermore, it complicates matters that small-weight elements performing high-frequency vibrations (e.g., bearing components) transfer motion energy that is simply too small to excite larger bodies to perform vibrations. Therefore, measurements should be taken as close as possible to the source of vibration. In the case of rotating machinery, it is advisable to do this on the bearing housings, as vibrations originating from the rotating components' defects spread here, and the vibrations originating from the bearing defects themselves (high frequency) can only be measured here. Do not measure on loose casing or separate - non-closely connected - machine elements if you are interested in vibrations related to the rotating components of the machine! Measurements on the mentioned elements are only worthwhile if there is a suspicion that they are prone to resonating with one of the machine's excitations.

Selection of measuring instrument

It matters what frequency range of vibrations we are interested in. For machine condition assessment, the frequency range recommended in ISO 10816 is most commonly applied, where vibrations are measured between 10 Hz and 1 kHz (scaled in velocity effective value). 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 task and the conditions to be considered for the measurement vary based on the machine's speed. Typical industrial machines (1500 or 3000 revolutions per minute) can be accurately measured with most instruments equipped with "standard" sensors - mostly ICP type (with built-in charge amplifier) piezoelectric vibration acceleration sensors, provided that the sensor is properly attached to the measurement object. Electrodynamically operated vibration velocity sensors are equally suitable, and in the case of slower rotating machines, they are often even better than piezoelectric sensors: consider that in piezoelectric sensors, the charge is generated by the force acting on the piezocrystal due to the seismic mass built into it, which is then amplified, and can be considered as a signal proportional to the vibration acceleration. In slow movements - although the displacement or even the velocity can be high - 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 (small-sized) piezoelectric sensors used for machine vibration measurement is around 1-2 Hz (usually at 0.3 Hz, there is already a 3 dB damping). However, for faster rotating machines, piezoelectric sensors are almost exclusively suitable for measuring higher frequency vibrations. It is not uncommon for their measurement frequency range to extend up to 15–30 kHz. Regarding the measurability of high-frequency vibrations, it is important to consider the physical fact that only small-weight elements with high natural frequencies can accurately track them.This naturally applies to the sensor required for measurement as well. If the installation is not done screwedly, for measuring higher frequency vibrations, a sensor with the smallest possible weight should be used. When using a holding magnet, it should also be as light as possible. (Unfortunately, this also reduces the holding force of the magnet, which is specifically disadvantageous.) However, the biggest issue 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. Correct application of vibration sensors This is truly the most delicate topic. The sensing direction of the sensors almost always coincides with their central axis. To ensure accurate measurement, it must be ensured that the sensor closely follows the motion of the measuring point (i.e., the surface of the machine element) in this direction. Depending on the quality of vibration tracking, different measurement results may need to be expected. What is the reason for this? Naturally, 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. Therefore, the detectability of high-frequency vibrations primarily depends on the relationship established between the sensor and the measuring surface. The relationships are shown in the following graph. Frequency response fixation dependency Many instruments come with a sensor touch probe, which allows measurements to be taken at hard-to-reach - unsuitable for magnetic fixation - measuring points. However, high-frequency vibrations cannot be accurately measured with this method. The sensor only follows the vibrations that are transmitted through manual touch pressure. This can never be more than 2-3 kHz. It is completely wrong to try to measure vibrations with a manual touch probe or even a sensor mechanically integrated with the instrument at 5 kHz or higher frequencies. These vibrations will not reach the sensor; however, the sensor, the touch probe vibrating with it, and the natural frequency of the instrument components will affect our measurement. To check the accuracy of measurements with the touch probe, temporarily increase the touch pressure. If the readable value changes, we need to check the contact created with the measuring point (paint, loose element, etc.). If this does not help, always use a holding magnet or mount the sensor directly on the machine with a threaded stud. As a general solution in practice, the use of magnetic mounting bases (holding magnets) has proven effective, as they provide consistent measurement conditions for repeated measurements, are easy to work with, and the operator does not have to hold the sensor on the measuring point during data collection. The magnets supplied by sensor and instrument manufacturers - assuming proper 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, do not forget that in order to transmit high frequencies, all contaminants and thick paint layers must be removed. These act as mechanical filters regardless of the strength of the applied magnet. Rahne Eric (PIM Ltd.) pim-ltd.com, machineryexpert.com

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