Marta Zamorano

Mechanical looseness is one of the most common faults affecting rotating machinery and may lead to severe degradation or catastrophic failure if not detected at early stages. This paper presents a vibration-based methodology for looseness diagnosis in rotating shafts using Wavelet Packet Transform (WPT) energy analysis and supervised feature relevance evaluation. Experiments were conducted on a rotating machinery test bench under healthy and defective conditions with an artificially introduced diametral clearance of 0.5 mm. Vibration signals acquired at different rotational speeds were processed through WPT to extract packet energy features from the time-frequency domain. Neighborhood Component Analysis (NCA) was employed to evaluate the discriminative capability of the extracted features and to investigate the distribution of looseness-related information across the WPT packets. The results revealed that mechanical looseness generates characteristic energy redistribution patterns concentrated within specific frequency regions and that rotational speed significantly influences the localization and separability of the fault-related vibration components. The influence of different mother wavelets was additionally analyzed, showing clear operating-condition-dependent differences in the discriminative relevance of the extracted WPT energy features.

Railway axles are among the most safety-critical components in rolling stock, as their failure can lead to catastrophic consequences. One of the most subtle damage mechanisms affecting these components is fretting fatigue, which is a particularly challenging damage mechanism in these components, as it can initiate cracks under real service conditions and is difficult to detect in its early stages, which is vital to ensure operational safety and to optimize maintenance strategies. This paper focuses on the development of fretting fatigue damage in solid railway axles under realistic service-like conditions. Full-scale axle specimens with artificially induced notches were subjected to loading conditions that promote fretting fatigue crack initiation and growth. Acoustic emission techniques were used to monitor the damage progression, and post-processing of the emitted signals, by using wavelet-based tools, was conducted to identify early indicators of crack formation. The experimental findings demonstrate that the proposed approach allows for reliable identification of fretting-induced crack initiation, contributing valuable insights into the in-service behavior of railway axles under this damage mechanism.

La digitalización del mantenimiento ferroviario como consecuencia de la adopción de la Industria 4.0 puede detectar prematuramente la presencia de defectos en el material rodante y notificárselo a los operadores ferroviarios antes con tiempo suficiente para actuar antes de que se produzca un incidente. Esta digitalización implica la instalación de sistemas de medida en sistemas críticos como son los elementos de rodadura de los vehículos ferroviarios y, más concretamente, los bogies. Aunque la inversión necesaria para la digitalización del material rodante puede ser elevada, a la larga debería reducir los costes del extensivo mantenimiento que realizan los operadores ferroviarios, así como los posibles efectos sobre las personas y el medio ambiente. En este trabajo se pretende determinar la condición de funcionamiento de los elementos de un bogie ferroviario mediante la combinación de técnicas clásicas de tratamiento de la señal vibratoria con sistemas de clasificación automáticos propios de la Industria 4.0. Para ello se han instalado acelerómetros en las cajas de grasa de uno de los ejes del bogie, registrándose las medidas de vibración en diferentes condiciones operativas y en las tres direcciones del espacio. Las señales de vibración se procesan en los dominios del tiempo y de la frecuencia, extrayendo una serie de hiperparámetros que posteriormente se introducirán en un clasificador basado en redes neuronales. Previamente a la introducción de los hiperparámetros en el sistema de inteligencia artificial, se utilizan técnicas estadísticas para seleccionar el conjunto de hiperparámetros que, a priori, deben garantizar la mayor tasa de acierto del sistema experto. Así mismo, se optimizan los parámetros del sistema clasificador para determinar la configuración que permite obtener la mayor tasa de acierto en la identificación de la condición del eje ensayado. Los resultados muestran un elevado nivel de acierto del clasificador basado en redes neuronales.

New trends in maintenance techniques are oriented to digitization and prognosis. The new electronic devices based on IoT (Internet of Things) technology among others that support the industry 4.0 paradigm let enhance the traditional condition monitoring techniques to better understand and predict the state of a machine in service. Related to maintenance applications, one of the important steps in condition monitoring tasks for fault diagnosis is the selection of the optimal pattern to provide accurate results (avoiding fault positives/negatives) with adequate computation time. When implementing this, the selection of optimal parameters and thresholds for setting alarms are important to detect problems in the machine before the failure occurs. Vibratory signals have been proved to be a good variable to determine their mechanical behavior. Nevertheless, parameters obtained from time domain measurements are not computationally efficient nor good patterns to compare different machine conditions. In this sense, tools that represent the frequency domain or time–frequency domain have been useful to detect defects in rotating elements such as bearings. In this work, defects in ball bearings are studied using wavelet packet transform. For this, a methodology will be developed for the optimal selection of the mother wavelet, incorporating intelligent classification systems, and using a medium Gaussian support vector machine model. In this way, it will be verified that the correct selection of this function influences both the results and the ease and reliability of detection. The results using the selected mother wavelet will be compared to those using Daubechies 6, since it is the mother wavelet that has been used in previous works and which was selected based on experience. For it, vibratory signals are obtained from a testbench with different bearing conditions: healthy bearings and defective bearings (inner and outer race).

The detection of cracked railway axles by processing vibratory signals measured during operation is the focus of this study. The rotodynamic theory is applied to this specific purpose but, in practice and for real systems, there is no consensus on applying the results obtained from theory. Finding reliable patterns that change during operation would have advantages over other currently applied methods, such as non-destructive testing (NDT) techniques, because data between inspections would be obtained during operation. Vibratory signal processing techniques in the time-frequency domain, such as wavelet packet transform (WPT), have proved to be reliable to obtain patterns. The aim of this work is to develop a methodology to select the optimal function associated with the WPT, the mother wavelet (MW), and to find diagnostic patterns for cracked railway axle detection. In previous related works, the Daubechies 6 MW was commonly used for all speed/load conditions and defects. In this work, it was found that the Symlet 9 MW works better, so a comparative study was carried out with both functions, and it was observed that the success rates obtained with Daubechies 6 are improved when using Symlet 9. Specifically, defects above 16.6% of the shaft diameter were reliably detected, with no false alarms. To validate the proposed methodology, experimental vibratory signals of a healthy scaled railway axle were obtained and then the same axle was tested with a transverse crack located close to a section change (where this type of defect typically appears) for nine different crack depths.

Air quality can be affected by merchant ships, so it is important to regulate emissions that are produced, as well as to use energy efficiently. In this sense, the cleanliness of the oil used in lubrication is essential to achieve a better use of energy and reduce losses in marine engines. For that, it is vital to carry out good maintenance strategies. Therefore, it is important to develop techniques that allow condition monitoring during engine operation. In order to detect potential problems as soon as possible, it is common to analyze vibratory signals, since sustainable changes in the rotating frequency and its harmonics can be detected, which was the objective of this work, by analyzing the time-frequency domain using wavelet packet transform. A methodology to select the optimal function (mother wavelet) and the best patterns to monitor, in order to determine the state of the purifiers of the marine lube oils, was carried out, including intelligent classification systems. Specifically, this document considers centrifugal oil lubricant separators systems, since the monitoring of these systems can determine the condition of different mechanical systems.

Nowadays, there are many methods to detect and diagnose defects in mechanical components during operation. The newest methods that can be found in the literature are based on intelligent classification systems and evaluation of patterns to obtain a diagnosis; however, there is not any standard method to assess features. Wavelet packet transform allows to obtain interesting patterns for evaluating the condition of rotating elements. To perform this calculation, it is necessary to select a series of parameters that affect the resulting pattern. These parameters are the decomposition level and the mother wavelet function. A detailed methodology for the selection of the mother wavelet is proposed, which is the aim of this work, to obtain the most suitable patterns in the diagnostic task. This proposed methodology is applied to data obtained from a rotating shaft with a crack located at the change of section. These signals were measured at low rotation frequency (below the critical rotation frequency) and without eccentricity, where detection becomes more complex.