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Power measurements and noise & vibrations levels in composite shafts in the maritime industry

Guijs, R.P. (2018) Power measurements and noise & vibrations levels in composite shafts in the maritime industry.

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Abstract:In the maritime industry, a shift from conventional materials such as steel towards more sophisticated materials such as composites can be distinguished. The reasons for this are numerous; it can be either from or cost- or weight-saving perspective, but also the reduction in maintenance required can be reason to apply different materials. An example of the application of composites in the rather conservative maritime industry are composite shafts. In the car industry they have been applied for quite some years, mainly because of the significant weight-saving which can be established. Also, in some special vessels composite shafts have been applied, especially for long spans when conventional shafts were simply not a possibility. For tugs composite shafts are applied because of the fact that they eliminate the need of bearings for support of the shaft. The reduction is both expenditures and time spend weighs up against to slightly higher purchase costs. However, after application of the shafts in various vessels, three problems were encountered. Because of the anisotropic nature of carbon fibre composite shafts, the method for power measurements on the shaft with strain gauges could be not be applied anymore and there were concerns about increased noise & vibrations levels because of the reduced uncoupling in the drive-line. Furthermore, because of the flammable nature of the composite, it was thought that this could cause problems with classification based on the fire integrity of the water-proof bulkhead. The latter was quickly resolved, since this scenario is considered as the stacking of two worst-case scenario’s, namely an internal fire and penetration of the hull. Furthermore, measures such as the installment of additional sprinklers had already been made and were accepted by class. To resolve the issues with the power measurements, an extensive literature research has been performed where multiple options are discussed. Examples of such are the application of Fibre Bragg Grating, SAW resonators, the measurements of the rate of twist or the use of a torque transducer. Furthermore, the application of strain gauges directly on the composite has been considered as well. The latter would be possible, however, it requires a high accuracy of alignment with the shaft to acquire the desired measurement accuracy. Furthermore, the mechanical and dimensional properties of the shaft itself are only known with a deviation of 10%, which would mean that calibration of every individual shaft is required to achieve the accuracy required by class. The most promising alternative, the application of a temporary torque transducer, will be a rather costly option because of both the purchase costs as the additional alignment required after removing the torque transducer from the drive-line. Hence, there has been chosen to enlarge the steel insert of the composite shaft to which the flange is attached. Benefits of these solution are that the additional weight is limited, no additional parts are required and the measurements can be performed, with strain gauges, on an isotropic material. This is the most cost-effective way, since no additional education for the engineers is required and the testing with strain gauges is a cost-efficient method. Concerning the expected increase in noise & vibration levels, a theoretical model has been developed. The background of these concerns was that because of the decreased weight of the shaft, the uncoupling of the drive-line would be limited and the eigenfrequencies would increase. This would increase the transmissibility of the vibration and could potentially lead to dangerous vibrations in the system. A dynamic model of the shaft itself, based on free harmonic vibrations, unveiled that the composite shaft actually shows a decrease in eigenfrequencies compared to the steel shaft, which can be contributed to the highly flexible couplings it is mounted with. Afterwards, a dynamic model of the entire drive-line has been created, which involves the engine and rudderpropeller as well and is based on an ASD Tug 2009. By replacing the steel shaft and its couplings for a composite counterpart, the dynamic behavior could be compared with as the only parameter the influence of the replacement of the shaft. Both the eigenfrequencies and mode-shapes displayed a high degree of similarity between the two set-ups. However, some eigenfrequencies were in order with the nominal speed of the engine (30 Hz), which could potentially cause resonance. A calculation on the amplitudes, both damped and undamped, revealed that this was not to cause any problems, for neither the composite or steel shaft. Noted should be that these are simplified, theoretical calculations which can not fully represent the highly dynamic and complex situation aboard a vessel in operation. Hence, the preparations for a validation step based on an ASD Tug 3212 is prepared to check whether the conclusions from the theoretical analysis are practically viable. All in all it can be concluded that the use of composite shafts instead of their steel counterparts will not impose any problems on the vessels concerning either power measurements or the noise & vibration levels, as long as the proper couplings and steel inserts are applied.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Programme:Mechanical Engineering MSc (60439)
Keywords:dynamic analysis, power measurement, maritime industry, modal analysis, composite shafts
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