Browsing by Author "Bannister, R. H."
Now showing 1 - 10 of 10
Results Per Page
Sort Options
Item Open Access Acoustic emission detection of fatigue crack propagation in a power station steam chest environment(Cranfield University, 1997-02) Cook, J.; Bannister, R. H.This thesis addresses the problem of detecting and positively identifying the approximately known acoustic emission signatures produced through fatigue crack propagation in power station steam chests. This work includes extensive laboratory fatigue testing to produce and record signatures in specimens fabricated from the steam chest steel, on-site recording of the ambience noise levels from a fully operating oil-burning power station and develops and demonstrates the effectiveness of various signal processing techniques at extracting the signatures embedded in the noise. This noise is high amplitude, giving us a low signal to noise ratio, and is broad in the frequency domain, with both regular and irregular high-amplitude metallic noise transients that cover the entire frequency range of interest. It is therefore essential to use sophisticated signal post-processing techniques to detect and to identify the crack signatures. The post-processing techniques developed and employed include time-frequency transformations, matched filters and signal expansion filters implemented in both in the time domain and in various two-dimensional time-frequency domains. From a performance comparison, both on the experimentally recorded data and on data digitally generated for the purpose, we determine the optimum signal processing method for our requirements and provide an assessment of the relative computational efficiencies. Generated for comparison are spurious but similar signatures characteristic of the power station steam chest environment; oxide crushing within an existing dormant crack and stress corrosion cracking signatures produced by the same steel constantly loaded in a corrosive environment. It is demonstrated that there is sufficient distinction between these signatures and those produced by crack propagation.Item Open Access Acoustic emission pulse analysis: an integrated system for transducer calibration and signal processing(1982-10) Oliveras, Juan M.; Bannister, R. H.The present thesis covers the pioneering effort of establishing an infrastructure for conducting research into the analysis of Acoustic Emission signatures, with emphasis being placed upon the recovery of the true surface waveform for a given transient input. The topics covered in this work are: The p·rovi s ion of the necessary computer software for the handling and processing of the data. Here, a method is included for the correction of phase errors introduced by the sampling/multiplexing of ·the signals, which provides the basis for the implementation of data convolution.techniques in the frequency domain. J°he development of an experimental rig and source of excitation, later applied to the design of transducers. The development of a new transducer configuration, capable of producing a relatively flat frequency response (±7dB) over the frequency range 300 KHz - 2 MHz. The development of a procedure for the calibration of transducers, based upon a modified reciprocity method. The technique produces the calibration of transducers in relative terms, requiring a comple- . . mentary method to provide the reference level for the calibration. As a means of assessing the proposed techniques, the transducer calibration was applied to the de-convolution of pulses generated with a step forcing function, and the results compared with an equivalent theoretical model previously published.Item Open Access Acoustic Emission Signal Classification in Condition Monitoring Using the Kolmogorov-Smirnov Statistic.(2001-01-01T00:00:00Z) Hall, L. D.; Mba, David; Bannister, R. H.Acoustic emission (AE) measurement at the bearings of rotating machinery has become a useful tool for diagnosing incipient fault conditions. In particular, AE can be used to detect unwanted intermittent or partial rubbing between a rotating central shaft and surrounding stationary components. This is a particular problem encountered in gas turbines used for power generation. For successful fault diagnosis, it is important to adopt AE signal analysis techniques capable of distinguishing between various types of rub mechanisms. It is also useful to develop techniques for inferring information such as the severity of rubbing or the type of seal material making contact on the shaft. It is proposed that modelling the cumulative distribution function of rub-induced AE signals with respect to appropriate theoretical distributions, and quantifying the goodness of fit with the Kolmogorov-Smirnov (KS) statistic, offer a suitable signal feature for diagnosis. This paper demonstrates the successful use of the KS feature for discriminating different classes of shaft- seal rubbing. A hierarchical cluster algorithm was employed for grouping extracted KS values. AE rub signals were simulated with various metallic seals and measured at the journal bearings of a test rig rotating at approximately 1500 rev/min. Also, the KS classification results were directly compared withmore established AE feature vectors.Item Open Access Condition monitoring of slow speed rotating machinery using stress waves(1998-06) Mba, David Uzo; Bannister, R. H.Condition monitoring through the use of vibration analysis is an established and effective technique for detecting loss of mechanical integrity of a wide range and classification of rotating machinery. Equipment rotating at low rotational speeds present an increased difficulty to the maintenance engineer, since conventional vibration measuring equipment is not capable of measuring the fundamental frequency of operation, also, component distress at low operational speeds does not necessarily show an obvious change in vibration signature. This thesis presents a study of high frequency stress wave analysis as a means of detecting early stages of loss of mechanical integrity in slow speed machinery, the first of its kind. Investigations were centred on the Rotating Biological Contactor (RBC) which is used for sewage treatment in small communities and rotates between 0 .6 to 1 revolutions per minute. The mechanism of stress wave generation was the relative movements between mating components that were experiencing loss of mechanical integrity, for instance, the loss of tightening torque between clamped components. Development of the stress wave monitoring technique was accomplished by firstly, simulating a variety of mechanical faults on the test-rig, and secondly, by numerous ‘on-site’ tests on operational RBCs and their bearings. In application of this technique to operational RBCs and bearings, as well as cataloguing evidence of mechanical deficiencies associated with RBCs operated within the Severn Trent Water region, the author inspected 261 RBCs and covered over 35,000 miles, contributing to the uniqueness of this research programme. This research has provided Severn Trent Water limited with a PC-based monitoring system comprising an AE transducer, pre/post amplifiers, ADC board and software on which all diagnosis was undertaken. Furthermore, during the process of investigating reasons for the mechanical failures of the RBC, technical information was uncovered thereby assisting Severn Trent Water in perusing a more robust design of RBC. Moreover this work has assisted Severn Trent Water to maintain that envious reputation that over a number of years not one of the RBC sewage treatment works has failed its consent level as set by the Environmental Agency.Item Open Access Condition monitoring of slow-speed rolling element bearings using stress waves(Professional Engineering Publishing, 2001-01-01T00:00:00Z) Jamaludin, N.; Mba, David; Bannister, R. H.Condition monitoring of rolling element bearings through the use of vibration analysis is an established technique for detecting early stages of component degradation. However, this success is not mirrored at rotational speeds below 16r/min. At such speeds the energy generated from bearing defects might not show as an obvious change in signature and thus becomes undetectable using conventional vibration measuring equipment. This paper presents an investigation into the applicability of stress wave analysis for detecting early stages of bearing damage at a rotational speed of 1.12r/min (0.0187 Hz). Furthermore, it reviews work undertaken in monitoring bearings rotating at speeds below 16r/min.Item Open Access Ensuring effluent standards by improving the design of Rotating Biological Contactors(Elsevier Science B.V., Amsterdam., 2007-04-05T00:00:00Z) Mba, David; Bannister, R. H.The Rotating Biological Contactor (RBC) is used for both municipal and industrial wastewater treatment; however, the RBC has been plagued with mechanical deficiencies since its conception. The unpredicted loss of mechanical integrity results in loss of operation and the discharge, into the environment, of effluent that does not meet consent standards as set by governmental bodies. The consequence of the latter on aquatic life cannot be understated. Whilst maintaining an effluent standard is of vital importance this has to be balanced against the capital costs of RBC units. In addition to detailing reasons for defects associated with RBC’s this paper presents a novel design approach for shaftless RBC’s. This novel design takes cognisance of the mechanisms and reasons for mechanical failure, thereby reducing occurrence of loss operation of RBC units and ensuring that the effluent continually meets discharge standards. The added significance of the design is a 50% reduction in coItem Open Access Mixed modal balancing of flexible rotors without trial runs(Cranfield University, 1998-09) Preciado Delgado, E.; Bannister, R. H.The subject of this thesis is about the balancing of large flexible rotors which exhibit mixed modal characteristics. The objective of the research was to develop a balancing procedure to determine correction masses without trial runs. This required the determination of(a) the modal vibration vectors for each resonance, (b) the modal damping ratios,(c) the mode shapes and(d) the equivalent mass of the rotor for each mode. It was made clear from the beginning that trial runs are unavoidable either, when the mode shapes cannot be determined using an analytical or numerical method, or when there is dual vibration at normal operating speed, produced by the influence of higher unbalanced modes, is too high to allow continuous operation of the machine. Therefore, the scope of the project was limited to the possible determination of correction masses without trial runs for the vibration modes included within the normal operating range. Some studies about the minimisation or complete elimination of trial runs have been published by several authors, but a literature search revealed no reports of systematic application of these procedures to field balancing of large turbo generators. This suggested that some practical difficulties had still to be overcome, opening the possibility for further research on this area. Analysis of the rotor response demonstrated the necessity of considering the angular position of the transducers when registering the rotor vibration. It was shown that measuring in a direction other than those of the principal axes of stiffness introduces errors when determining the magnitude and phase of the correction masses. That is to say, failing to consider the effects of the transducer angular position eliminates the possibility of balancing the rotor without trial runs. This is the first time that this problem has been recognised. The procedure developed was verified using an experimental rotor rig. The successful application of the procedure to the balancing of this rotor demonstrates that balancing withouttrialrunsisnotonlyatheoreticalbutalsoapracticalpossibility. The dynamic characteristics of the rotor rig, however, were some what limited and did not cover all the possibilities considered during the project. Therefore, a more complete numerical example was also successfully solved using the computer model of a rotor with characteristics similar to those of a real turbine, and whose unbalanced distribution was not initially known by this author.Item Open Access Monitoring the lubricant condition in a low-speed rolling element bearing using high frequency stress waves(Professional Engineering Publishing, 2002-01-01T00:00:00Z) Jamaludin, N.; Mba, David; Bannister, R. H.This paper presents the results of an investigation into the development of a reliable, nonintrusive and effective technique capable of accessing the lubrication condition within a low-speed rotating bearing. This unique monitoring system is known as pulse injection technique (PIT) and involves transmitting a Dirac pulse through a bearing via a stress waves (SW) sensor. Analysis of measured stress wave signatures differentiated between a properly and poorly lubricated bearing. This method was found to be effective as stress wave propagation is sensitive to the transmission path, which in turn is affected significantly by the type and amount of grease present in the vicinity of the working elements of a bearing. The ability of this technique to monitor the lubrication condition within a bearing whilst in operation could be utilised to determine the appropriate time for grease replenishment or addition.Item Open Access The use of high frequency stress waves for monitoring gears(Cranfield University, 1995-09) Al-Balushi, K. R. N.; Bannister, R. H.The aim of this research is to investigate the feasibility of using stress waves for condition monitoring of gears. The project involved setting up an experimental rig, carrying out experimental work, acquiring stress waves signatures, and processing the signals. It has been shown that stress waves can successfully be employed for early detection of incipient gear failure. A experimental gearbox was employed during the experiments. Miniature ultrasound transducers, both sensitive and sufficiently small, were manufactured and installed on the stationary outer race of the rolling element bearing of the gearbox to detect stress waves from the meshing gears. The stress waves signals from the transducers were digitised and digitally processed to extract relevant information. The signatures were high-pass filtered at a cut-off frequency of 200 kHz, thus representing exclusive ultrasonic frequencies. A new statistical parameter, Energy Index, was developed and performed on the stress wave signatures which were segmented to represent individual gear teeth. Along with this new parameter, the classical statistical parameters, (Peaks, RMS, Standard Deviation, Kurtosis, etc.) were also performed. Conclusive results are presented in graphical form in terms of Cumulative Energy Indices' and Energy Indices in polar form for individual gear teeth. A new algorithrn was also developed and presented for the envelope detection of signal by iterative peak detection. Although no direct comparison was made between condition monitoring of gears using stress waves and methods such as low frequency vibration analysis and wear debris analysis, it is apparent that stress waves monitoring offers a much earlier warning of incipient gear failure because the technique can detect material defonnations which are precursors to changes in the dynamic properties of gears and the occurrence of wear debris. The technique, therefore, can predict incipient failure much earlier, extending the lead-time before failure, and as a result, minimising sudden failures which may have catastrophic consequences.Item Open Access The use of ultrasound measurement system to monitor the mechanical integrity of rolling element bearings and gears(Cranfield University, 1992-09) Tu, X.; Bannister, R. H.A theoretical design and study of production of various types of ultrasound transducers for acoustic emission research into the fatigue failure mechanism and application in machine condition monitoring of rolling element bearings and gears are reported. The transducer transmission modelling was presented and verified, theoretical transducer responses were exercised and their comparisons were made with experimental results. Transducer size, construction and location were considered important in the application of ultrasound. Advanced method of metallurgical attachment improved the bond durability and reliability, the signal transmission as well as the capability of withstanding a hostile environment (117 °C) within the vicinity of the component being monitored. In-Situ calibration provided a means of ensuring the in- service transducer performance and the sensing circuitry as a whole. _ The smallest fabricated transducer had a circular sensor of 2.6 mm diameter and 0.5 mm thick, which was highly sensitive to a few milli-meter stress wavelength. The ring type transducer was preliminarily found to improve the amplication range. r Five rolling element bearings were tested using the purposely designed ultrasound transducers, the experimental observations of the detected signal were then time- amplitude distribution displayed. The changing stages of failure development from a single fatigue spall of less than 3 mm to 1.5 mm in length and down to surface crackings of 25 m long, were appropriately explained. Two pairs of test gears having three different fault conditions (good, moderate and severe) were conducted on a Roll-Royce back-to-back gearbox test rig at 1000 RPM to 2000 RPM approximately. Transducers implanted inside the gearbox sensitively showed the gradual changes in amplitude distribution resulting from the fault incremental category. A study of signal transmission of the fault in a gear assembly revealed that half of the elastic stress wave energy was lost by the time it arrived at the support bearing. Statistical methods using RMS and kurtosis parameter a fault indicator were employed to evaluate the bearing and gearbox data. High frequency analysis confirmed that failed bearing or gear signatures were ultrasonic, in addition the signal frequencies below 50 kHz were also detectable by the purposely designed transducers. A defect simulation was also established to study the gearbox failure, its usefulness was then discussed.