Inspection of Machine Structures with Motion Animation

While spectral analysis of machine vibrations during operation is proven to be one of the most effective methods for detecting and proving most faults of machines - such as imbalance, misalignment, bent shaft, bearing fault - the inspection of the machine's structural elements requires different tools. The solution is provided by finite element modeling or motion animation. Due to lower measurement and computational requirements, motion animation is the simplest to apply.

Let's assume that every structural element of the machine (foundation, platform, frame, supports, beams, bearing housings, etc.) moves or deforms during operation due to periodic forces acting on the machine - typically at frequencies of 25 or 50 Hz, or their integer multiples, as most machines operate at 1500-3000 revolutions per minute. These movements are not visible to the naked eye for two reasons. Firstly, our eyes cannot follow changes occurring at 50 Hz and above frequencies (for example, this is why TVs use 50 Hz or faster refresh rates), and secondly, these movements are often on the scale of micrometers (thousandths of a millimeter). How can we make the movements of machine elements visible? The simplest way is to use a stroboscope: illuminate every other (or third, fourth) movement! Since our eyes can only recognize objects at the moment of a strong flash, it appears as if the process is happening at 25 Hz instead of 50 Hz or even 12.5 Hz. Our eyes can follow this, making the appropriately large amplitude - but otherwise invisible - rapid movements visible. (Of course, this is only the case if we do not precisely tune our stroboscope to the frequency under investigation, as otherwise we see a still image.) The method has several disadvantages:

• Unfortunately, we still cannot see small movements in the range of a few tens of micrometers
• Only a limited area can be illuminated and analyzed
• The evaluation of what is seen is quite subjective, and the data is stored solely in memory

We achieve better results than with the stroboscope method by utilizing the capabilities of modern vibration measurement instruments and computer technology, as we also want to observe vibrations here. Our first task is to create a model of the machine structure to be inspected, which includes every structural node. When creating the model, keep in mind to include only as many measurement points as absolutely necessary for detecting the inspected or assumed problem - as measurements need to be taken at these points. (Including an excessive number of points does not improve the inspection results.) The model of a machine element mounted on a plate (such as a base plate and bearing housing) is shown in the diagram below. This is followed by "data collection" tailored to our model. The spatial vibrations (movements) must be measured at each node (in the x, y, and z directions). Since we want to compare (or display together) the movements of multiple points, it is evident that not only the amplitude of the movements but also their relative timing (phase) is important information. The following section reviews the possible measurement methods.

Measurement Methods

Since we want to compare (or display together) the movements of multiple points, it is evident that not only the amplitude of the movements but also their relative timing (phase) is important information. Amplitude-phase measurement with triggering Since we are mainly interested in displacements related to the machine's rotation, we can take the machine's main shaft rotation frequency as the basis for temporal comparison. For this, all measurements must be triggered (synchronized with the rotation) with the speed sensor signal (one pulse per revolution), and then process the detected vibration signal: determine the amplitude and phase angle of the rotation frequency, or some multiple component. As a result of our measurements, we have three (for each spatial direction) vibration amplitude values and phase data at each structural node. Together, these data describe the spatial movement performed by the inspected measurement point (at a given frequency). If we are interested in vibrations at a different frequency (rotation frequency multiple), the measurements must be repeated at each point for that frequency. The advantage of this method is that it does not require a complex instrument for measurements, and they can be carried out relatively quickly. However, the downside is that we obtain data only at one (rotation frequency-dependent) frequency at a time, or two to three distinguished frequencies (rotation frequency multiples) depending on the instrument, and it is essential to have a triggerable signal present on the rotating part. Reference signal (two-sensor) method In this approach, we work with two vibration sensors, considering one as a reference that remains stationary during measurements. The other sensor is successively placed in the predetermined directions of each measurement point. During the measurement, we record the phase and amplitude spectrum of the vibrations. The temporal comparison of the signals is not based on the rotation axis trigger signal but on the vibrations measured by the reference sensor. The advantages of the reference sensor method compared to the trigger pulse measurement are summarized below:

• There is no need for a signal suitable for triggering on the rotating part (in many cases, stopping the machine is necessary to create the trigger signal), and a "visible" rotating part is not required at all
• We obtain animatable data not only at predefined frequencies but also at every frequency present in the spectra where there is coherence with the values measured by the reference sensor

The method's disadvantage is the increased equipment requirement.

It is common practice to conduct our measurements while the machine under investigation is in operation, as we are usually interested in the root causes of significant machine and structural vibrations. Therefore, regardless of the measurement method used, the physical limitations of the procedure must be taken into account. Perhaps the most important limitation is the number of channels (measurement points) that can be measured simultaneously. In most cases, measurements are performed with one or two sensors (on one or two channels simultaneously), therefore we can only obtain comparable data if constant vibrations are present over time, at least during the measurement period. This is a serious limitation because with several hundred measurement points, the required time can be measured in hours, thus long-term stability (consistent operating conditions) is necessary. If this cannot be ensured, it is easily conceivable that variable amplitude and phase relationships may lead to incorrect results. (From this perspective, the reference sensor method is less sensitive, as changes in phase relationships mostly uniformly affect the entire system, including the signal recorded by the reference sensor. Due to the relative measurement method, the change occurring in both signals is eliminated, thus not affecting the evaluability.) If possible or necessary, the operational vibrations of the machine under investigation can be replaced by an exciter machine, which continuously generates vibrations with a force and frequency determined by us - usually within certain limits. In this case, we do not need to worry that the amplitude and phase of the vibrations are not constant over time.

Modeling, Evaluation

After collecting the measurement data, the evaluation can begin. By persistently analyzing the results (comparisons, coherence checks, etc.), any potential faults in the structure under investigation can be identified, but the possibility of error - especially with large datasets - is quite high, as the fault may not manifest prominently. To expedite the analysis and minimize errors, computer-based motion animation - based on graphical representation - is used. During the animation, the motion of the machine structure at a particular frequency is depicted with exaggerated amplitudes - but correctly phased - and greatly slowed down. With a well-crafted 3D model (three-dimensional or spatial) - and of course flawless measurements - a plethora of machine and structural faults can be detected. The easiest to detect are the clearances between individual machine elements. The lack of coordinated movement can lead to serious vibrations and other consequences if the machine elements should rigidly connect to each other. The most frequently checked connections are the mounting points of the bearing-bearing housing-base plate-base system, as well as the entire interconnected support and building structures. The clearance is very easily noticeable in the animated diagram, as there is a significant amplitude and phase difference in the movement of adjacent (connected) machine elements. The animation below shows the phases of the movement of a machine element improperly fixed to the base plate.

Diagrams: movement of an unrestrained rigid cube on a flat surface [source: PIM]

In the structure under examination, cracks and fractures manifest in a similar manner to clearances, but their detection is much more complicated. While in the search for clearances, when creating our model and selecting measurement points, we clearly know where to look for potential faults (connection of machine elements), for the - less visible - cracks and fractures, measurements within a specific machine element are necessary for detection. In this case, the success of the examination depends on modeling, as incorrect conclusions may be drawn from measurements taken too infrequently or in inappropriate planes. Therefore, it is worth noting that if during the analysis and animation of the measurements the slightest suspicion arises regarding any fault, it is advisable to conduct further - more detailed - examinations in the vicinity of the specific points (if feasible).

Diagrams: movement of a clamped sheet bending and twisting at resonance frequency [source: Energopenta]

Furthermore, the animation method is effective in demonstrating the resonance behavior of individual structural elements. The examination has a single - but very significant - requirement: we need to excite the structure with a force at the frequency corresponding to its natural frequencies (if this would not occur naturally during machine operation due to some constructional or assembly error).

More complex models than those shown on the previous page can also be created - the detail (and complexity) of the model primarily depends on the task. However, the number of points in the model sets a limit on the increasing measurement work and the graphical comprehensibility of the results.

Diagram: machine group with shaft coupling fault [source: PIM]

Diagram: compressor structural frame with base fracture [source: VMI]

The model depicting the pulsation of a pressurized tank is shown in the animation below. Collecting data from numerous measurement points is no small task, but the visual representation is stunning... (Our latter two diagrams depict models created with the VMI AB product, using the VibShape hardware- (instrument-) independent motion animation PC software.)

Figure: pulsation (deformation) of a tank under pressure [source: VMI]

In summary, we can say that motion animation is a great complementary tool in the hands of the diagnostician. It makes amplitude-phase data visible, easily and quickly analyzable, and understandable even for less experienced or professionals from other fields. Loose machine elements (loose screw connections, broken welds), assembly and design errors (coupling error, resonance), as well as support and structural problems become easily noticeable. (It is important to note that although the evaluation is simple, modeling and measurement require appropriate expertise and practice.) However, the method is not omnipotent; do not attempt to analyze quickly occurring, difficult to reproduce, or time-varying (non-stationary) processes with it.

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

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