Shaft whirling in a twin-spool jet engine system

Modern jet engines have two co-axial shafts which rotate usually in the same direction but are not coupled mechanically. In common with other types of high speed machinery, shaft vibration, particularly that excited by unbalance and known as whirling, is a serious problem. Despite the enormous investment involved in the design and development of a new type of jet engine, relatively little research has been done on shaft whirling in jet engines. It is believed that no detailed investigation of whirling in a two shaft system has been carried out. The work described in this thesis was designed to establish the accuracy with which the whirling frequencies could be predicted and in particular to examine how these were influenced by the speeds of the two shafts as a result of gyroscopic effects. The experimental rig constructed for this investigation was a simple full size model of the rear half of a typical twin-spool jet engine. After extensive testing of the components of the rig, a computer model was developed which would predict the natural frequencies of the rig to an accuracy of better than 5%. It was concluded that this accuracy was not likely to be approached in the prediction of the frequencies of an actual jet engine and such calculations should not be relied on in the design of an engine to ensure that whirling did not occur within the working speed range. The computer model was used to explore the variations of the frequencies over a wide range of combinations of shaft speed. Under certain conditions the frequencies were found to be influenced greatly by the speed of one or both shafts. At certain combinations of shaft speed the character of the mode of vibration of the two shafts was changed drastically by relatively small variations in the shaft speeds. Methods of determining the critical points of vibration in the working range of a typical jet engine were examined. It was found that each frequency which lay within the working range was liable to be excited by the unbalance of either shaft. The results suggest that more critical points Of vibration are likely to be encountered in the working range of an engine in which the shafts rotate in opposite directions. In the course of the investigation a careful examination of reverse whirl, in which the bent form of the shaft rotates in the opposite direction to the shaft rotation, was conducted. A plausible explanation of the cause of this phenomenon is given which suggests that it deserves further study. One of the shafts exhibited a severe vibration which was unaffected by the unbalance of the shaft. The mode of vibration was that of a subharmonic and further examination suggested that it was caused by non-linearity of the support of the shaft as a result of bearing clearance. It would appear essential that research into the control of this type of vibration is conducted