Condition monitoring and monitoring of rotating machinery with vibration measurement

The physical basics

Every solid body is capable of performing vibrations in multiple directions at different frequencies. The largest displacements can be experienced at the body-specific natural frequency, as at this frequency (resonates) the body vibrates in the given direction (hence the concept of resonance frequency). Naturally, no body starts vibrating on its own, but excitation - thus external force - is required. The greater this force, and - in the case of alternating forces - the more the rate of change matches the body's natural frequency, the greater the vibrations the solid body performs in the direction determined by the force. In the case of rotating machinery, the sources of vibrations are the unavoidable alternating forces that occur during machine operation. These forces can never be completely eliminated, as they arise, among other things, from the normal alternating operation of the machine (e.g., reciprocating machines), the residual unbalance of rotating components, and the periodic forces originating from the drive (e.g., network harmonics). The effect of the forces present during normal operation on individual machine components should be imagined as if each machine component were part of a spring-mass oscillating system. The rotating machinery consists of numerous such oscillating systems, which are almost without exception interconnected and excite each other. Due to the resonance properties of solid bodies, each machine element tends to follow the effect of the alternating force at its own frequency. This applies not only to the moving components of the machine but also to all supporting elements. The frequency of the vibrations measurable on the machine and the corresponding amplitude depend on the stiffness and mass of the mechanical elements. The smaller the machine element, the higher the frequency but lower the amplitude of the vibrations it performs.

Selection of measurement location

Every mechanical vibration is strongest at its point of origin. The transfer of vibration energy through any material occurs with more or less strong damping. The higher the frequency of the vibration, the stronger the damping effect. As a result, low-frequency vibrations can be detected at greater distances from the source, whereas the detection range of high-frequency vibrations (e.g., bearing vibrations) is very limited. Furthermore, only the lightweight elements with high natural frequencies can effectively follow high-frequency vibrations, not the heavy bodies. The energy content of high-frequency vibrations that can be transferred by lightweight elements is too small to provide sufficient excitation for a larger body to perform vibrations. In addition to the mentioned damping, further vibration energy loss occurs when vibration is transferred from one body to another (in our case, between machine components). The closer the connection between the two elements, the easier it is to transfer the vibration energy. Elements not in contact with each other do not follow each other's vibrations. Therefore, the measurement should be taken as close as possible to the vibration source. In the case of rotating machinery, it is advisable to measure on the bearing housings, as the vibrations resulting from faults in rotating components are transmitted here, and vibrations originating from bearing faults (high frequency) can only be measured here. Do not measure on loose casings or separate - not in close contact - machine elements if you are interested in vibrations related to the rotating components of the machine! (Measurements on these mentioned elements should only be carried out if there is a suspicion that they resonate with some excitation of the machine.)

Figure: Typical measurement points on rotating machinery (examples)

Vibration velocity measurement (or broadband vibration measurement or vibration level measurement) When a measurement method providing a machine condition characteristic that is easy to handle and interpret is required, then broadband vibration measurement or vibration level measurement is the appropriate method. Handheld instruments designed for this purpose measure the effective value of vibration velocity (RMS, i.e., the square root of the sum of the squares of the vibration components). Example: Let's assume that the vibration originates from several components, unbalance (4 mm/s), misalignment (2 mm/s), and gear mesh (5 mm/s). In this case, the resultant vibration - i.e., the effective value measured by the device - will be 3.9 mm/s.

Vibration effective value =√((42 + 22 + 52)/3)^¬ = 3.9 mm/s

The usual frequency range of the measurement is from 10 to 1000 Hz (according to ISO 10816-3) or even up to 10 to 3200 Hz. These ranges cover the most common frequencies typical of mechanical problems in rotating machinery. For example, unbalance, mechanical looseness, resonance, as well as misalignments of shafts and gears are excellently noticeable. However, it does not provide information on which one dominates. The use of handheld instruments for broadband vibration measurement - in accordance with various vibration evaluation standards - is recommended for measurements on the bearings (or their housings) of rotating machinery. Users without experience are advised to base the evaluation of measurement results on the ISO 10816-3 standard (which has replaced the old ISO 2372 and ISO 3945). However, there are technologies that require stricter requirements than the standard, as well as cases allowing higher vibration levels than the standard.

Figure: Typical digital vibration level meter (VMI Viber-A+)

Standards are generally based on vibration velocity measurements expressed in mm/s effective value (RMS). The readability of the measurement result is enhanced by interpreting the read value as the average speed of back-and-forth motion. The effective value of vibration velocity best reflects the extent of unwanted phenomena and adverse energy effects. These always cause wear and material fatigue in the machine structure. ISO 10816-3 classifies machines into classes and distinguishes between flexibly and rigidly mounted machines. (The latter corresponds to the classification based on the resonance frequencies and the base speed of the machines. For example, a machine mounted with rubber pads or springs - thus flexibly - often shows resonances at low speeds, with the machine exhibiting large oscillations even at very low speeds. If the speed exceeds the critical resonance frequencies, the vibration level decreases. In the case of rigidly mounted machines, such a phenomenon does not occur.) Modern machines operate at high speeds and have relatively flexible bearings, peripherals, and foundations. Therefore, these can be treated as flexibly mounted even if they are not fixed with rubber pads or springs. In these cases, ISO 10816-3 allows slightly higher vibration levels compared to rigid mounting.

Figure: Recommended vibration level limits according to ISO 10816-3 (excerpt) Machine classes specified in ISO 10816-3 standard

Machine Class	General Description of Machines
I.	Group of small machines, including electric motors with a power of less than 15 kW.
II.	Group of medium-sized machines, including stable machines that perform only rotary motion, pumps, fans, machines fixed to separate foundations up to 300 kW transmitted power, and electric motors with a power of 15-75 kW.
III.	Group of large, heavy machines that perform only rotary motion, as well as power machines with heavy or massive foundations.
IV.	Group of power and work machines placed on a flexible base performing rotary motion, rotating at high speeds with their large mass (turbines, turbo-generators).

By using standards, it is very easy to decide whether certain machines can be operated further or not. As a basic rule, it is acceptable that if a machine shows vibrations greater than 3 mm/s effective value (including the most common types of machines such as electric motors, pumps, fans, generators), the cause of the vibration must be identified. Do not continue to operate a machine that vibrates more than 7 mm/s without being sure that the machine's endurance allows long-term operation under such conditions.

The following list contains experiential threshold values for flexible machines similar to the mentioned standard(s) and a simplified explanation of vibration levels. This list can be used as an initial approximation when assessing a machine that has been newly commissioned or used for a short period. The values are given in mm/s units. Vibration level threshold values cannot be applied to reciprocating machines (such as compressors, internal combustion engines) and machines operating with constant mechanical "friction" or "impact" (such as grinders).

Experiential Vibration Level Evaluation Table [source: VMI]

0 … 3	Small vibrations. No or very little bearing load. Mostly low noise level.
3 … 7	Perceptible vibration levels often concentrate on a specific component or machine direction. Noticeable bearing load. Sealing problems. Increased noise level. It is recommended to identify the cause at the next scheduled shutdown. Until then, it is advisable to operate the machine under supervision and measure the vibration level again at shorter intervals to detect deterioration as soon as possible.
7 … 18	Large vibrations. Bearings are hot. Bearing overload results in frequent replacements. Seals are bad, various leaks are possible. Shafts and foundations break. High noise level. Immediate intervention should be planned, and everything should be done to identify the cause. The equipment wears out very quickly.
18 …	Very strong vibrations and very loud noise that cannot be reconciled with the safe operation of the machine. The machine should be shut down if the costs of stopping operation can be justified technically or economically. Otherwise, the operating time should be minimized. There is no known machine that could withstand this vibration level without significant internal or external damage.

Rahne Eric (PIM Kft.) pim-kft.hu, gepszakerto.hu

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