What is ODS and how can it be used to solve structural non-conformities?

In general, the main objective of vibration analysis is to guarantee the availability for operation, safety and Structural Integrity a component, piece of equipment or structure. Equipment in operation generates vibrations, and even if it is at rest (out of operation), it can vibrate because it is excited by an external force (by a wind load, for example).

On a day-to-day basis in the field, vibrational analysis is carried out by collecting the vibrations that this equipment is subjected to. This is done using accelerometers. These accelerometers vibrate and, as they vibrate, generate low-current electrical signals that are read by a data acquisition system. This data is processed using specific algorithms, making it possible to analyze the vibration.

Nowadays, this vibrational analysis can take place online (with the accelerometers mounted directly and permanently on the object) or offline, with the engineer collecting this data in the field.

Using the data processed by the algorithms, those involved in the process try to detect faults prematurely or investigate the reasons for faults that are already known. Among the most common problems that can cause inconvenience due to vibration, encountered in the field on a daily basis, are: cavitation in pumps; electric motor failures; gearbox failures; imbalance, bearing failures, misalignment, resonance; among other inconveniences.

ODS is a methodology used in day-to-day vibration analysis. To understand it, some academic concepts are important to remember:

Degree of Freedom (GDL):

As determined by RAO (2009), for vibration analysis, the most simplified system is the model with a GDL, whose movement is determined by a specific coordinate or variable. Thus, the GDL is a certain minimum number of coordinate points for determining the positioning and movement of this system.

Free vibration:

Free vibration is characterized when a given object undergoes an initial disturbance and, after this moment, it continues to vibrate on its own, without the action of any external force causing additional vibrations in the system. The classic example of free vibration is a simple pendulum. Once it has moved to its starting point and is released, it begins to oscillate without any external forces acting on it.

Forced vibration:

Forced vibration differs from free vibration in that external forces act on the system under study. The classic example of forced vibration is a rocking chair which, after an initial disturbance, needs a continuous disturbance for the oscillatory movement of the child on the swing to continue.

In day-to-day industry, unbalances in components subjected to rotation such as pulleys, flywheels, fans, etc. are common examples. In practice, forced vibration can be caused by factors such as: breakage of a particular component that can excite the structure at a different frequency (breakage of a gear tooth in a gearbox, for example); inadequate structural sizing; misalignments in the structure above those foreseen in the design; unforeseen external and/or environmental excitations; unbalanced masses during the manufacture of the component (unbalanced masses in pulleys, drums, flywheels, fans, etc.).

Natural frequency:

It is the characteristic frequency or its particular set of frequencies of a given object in a situation of free vibration. This frequency is determined by factors such as the geometry, mass, dimensions and materials involved in forming the object under study. For simple objects, a relatively easy way to obtain the natural frequencies is to excite the object and then put it into free vibration, with the only external load being gravitational. In this way, the natural frequency can be measured by suitable instrumentation.

Modal analysis:

For complex equipment and structures (yard machines, conveyorsbuildings in metal structureetc.), a modal analysis must be carried out to find the natural frequencies of the structure. For this assessment, parameters such as the stiffness and mass of the object under study are used. Modal analysis is a very important tool for promoting the Structural Integrity an asset, whether during its design or operation. Once the object's natural frequencies are known, its excitation sources must then be investigated. Examples of these sources are motors, gearboxes, screens and rotating equipment in general that interfaces with the system. In this way, resonance phenomena can be avoided. Gif 1 shows a modal analysis carried out by Kot.

 

Gif 1: Modal analysis on a bridge - Source: Kot Collection.

It is important to note that this analysis gives the modal response of the object, in which each natural frequency is associated with a mode, a way of vibrating(shape). For field analysis of a system with several sources of excitation, ODS analysis is indicated and can work in conjunction with modal analysis, especially in cases where there is an overlap of frequencies. In these cases, resonance in multiple structural modes is possible.

Resonance:

This phenomenon occurs when the frequencies of the excitation sources in the equipment, structure or nearby regions are close to or equal to the natural frequencies of the system under study. When this occurs, if it is not damped, the object under study/analysis can show high displacements that were often not taken into account in the design of the equipment. These displacements can cause phenomena such as low-cycle fatigue, reduced component life and even the structural collapse of the object. Gif 2 shows the notorious case of the Tacoma Narrow Bridge resonating with wind loads.

 

Gif 2: Tacoma Narrow Bridge in resonance - Source: https://tenor.com/view/tacoma-narrows-bridge-shaking-earthquake-gif-17537817

Mastery of time:

To make it easier to understand, vibration in the time domain can be interpreted as the sum of all the vibrational contributions that the equipment makes over time. Therefore, the sums of each piece of equipment can be perceived, and in practice correspond to the vibration that the human body feels when it is in a vibrating object. For example, when you're on a belt conveyor, it's the vibration that the employee feels on the walkway. It is the sum of the vibrations of the motor, gearbox, drums, belt, rollers, wind load, etc. interacting with each other.

Frequency domain:

When the frequency domain is analyzed, it is possible to separate the contribution that each vibration component makes to the object. In order to clarify this understanding, MASTRIANI (2018) shows in Figure 1 a comparison between the time domain and the frequency domain. On the left, you can see the vibration of two components acting on the object and the resulting vibration that represents the analysis in the time domain. On the right-hand side of the figure, you can see the separate contribution of the two components, which is the analysis in the frequency domain. The left side shows the amplitude of the signal in the time domain and the right side shows the amplitude of the signal in the frequency domain.

 

Figure 1: Vibration analysis in the time domain and frequency domain - Source: MASTRIANI (2018)

Fast Fourier Transform (FFT)

The FFT is a mathematical device used to discretize vibrations from the time domain to the frequency domain. In practice, an algorithm is developed to carry out this transform for processing the data collected, or a specific commercial program is used to carry out this data processing.

SDGS

As defined by DOSSING (1988), ODS (Operating Deflection Shape) can be measured on a given object directly, by relatively simple means. This methodology provides very useful information for understanding and evaluating the dynamic behavior of a component, piece of equipment or structure.

RICHARDSON (1997), has the following definitions for ODS:

• ODS is defined as the deflection of an object (structure, component or equipment) at a specific frequency. More generally, the methodology can be defined as any forced movement of two or more points on an object. In this context, when movement is determined from two or more coordinates, a deflection shape is defined;
• A shape is the movement of a coordinated point in relation to all the others. In this context, movement constitutes a vector quantity, having location and directions associated with it. This vector quantity is also known as an important concept in the study of vibrations known as GDL;
• The ODS can be defined and determined from a specific displacement of a forced vibration, either at a specific time or at a specific frequency. In this way, the ODS can be obtained for various types of responses in the time domain, whether these responses are sinusoidal, punctual or completely random.

In practice, how can ODS contribute to the Structural Integrity your asset? Kot suggests the following steps to take when the client is faced with a new phenomenon that impairs human comfort and/or could damage their asset:

1. Based on the client's account of their asset, drawings (structural, civil and/or mechanical) are used to study the entire system involved, as well as the battery limits for defining the model. In this study, possible frequency ranges and excitation sources are surveyed in order to plan the solution. It is important at this stage that the engineer has clear knowledge of the equipment's history and operating conditions, especially when the unwanted vibration phenomenon occurs;
2. Whether or not modal analysis is necessary for investigating the phenomenon is defined;
3. Based on the previous study, the accelerometers to be used are defined, as well as the available electronics with the compatible number of channels. Collection points are also defined and any deviations that may occur during the tests are identified for mitigation;
4. Data is collected in the field with specialized electronics to meet the demand. The equipment must be in the condition in which the inconvenient vibration phenomenon occurs, so that the model reproduces reality. Other frequency and operating ranges can be explored in order to detect other future hidden non-conformities;
5. Once back in the field, using specific mathematical tools, the data is processed and analyzed. The information obtained in the pre-study is compared with the data collected in order to diagnose the phenomenon. In this confrontation, various questions are asked, such as: what directions and displacements the asset makes; what is the behavior of the structure when excited; what are the differences in displacement between the various points of the asset; which sources of excitation contribute most to the perceived phenomenon, among others;
6. Based on the data studied and analyzed, a report is presented with the diagnosis obtained, as well as the actions required to resolve the problem.

Conclusion

If you need ODS and vibration analysis solutions to promote the Structural Integrity your asset, get in touch. Kot has qualified professionals and specific equipment available to contribute to the solution for your asset. Contact our team for more information!

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References:

[1] MASTRIANI, Mario. Quantum-classical algorithm for an instantaneous spectral analysis of signals: a complement to Fourier Theory. 2018;

[2] RAO, Singiresu S. Vibrações mecânicas. Pearson Education, 2009;

[3] DOSSING, Ole. Structural stroboscopy-measurement of operational deflection shapes. Sound and Vibration Magazine, v. 1, p. 18-24, 1988;

[4] RICHARDSON, Mark H. et al. Is it a mode shape, or an operating deflection shape?.Sound and Vibration, v. 31, n. 1, p. 54-67, 1997.