Papers by Author: A.W. Lees

Abstract: Recent EPSRC funded research at Glasgow University, Swansea University, and Virginia Polytechnic and State University, and collaborative work with the Karlsruhe University of Applied Sciences, on the application of shape memory alloy (SMA) elements integrated within glass epoxy composite plates and shells is currently leading to the design of a novel smart bearing based on the principle of antagonistic action. In this system a ball bearing is fitted halfway down a glass epoxy composite tube, entering through one end of the tube. The tube has both ends rigidly built in to the support frame. The tube is divided into two regions, one on each side of the centrally located bearing. SMA strips are bonded in two independent sets of four, each set running axially along half the length of the tube and separated by 90 º around the tube. The four strips in each set are electrically connected in series to a high current power supply that can be switched in or out, and the current set, as required. This provides a convenient and fast way of heating each set of SMA strips through the martensite-to-austenite transformation temperature, and provides a significant axial contraction load on the tube in either direction. Previous FE analysis has provided predictions for converting an axial contraction load into useful stiffening of the structure in the radial and hoop directions. This introduces the potential for modification of the dynamic performance of the flexible rotor. In addition to separate heating each half of the active bearing has its own independent forced-air cooling system. Previous work by one of the authors, and others, has shown that a single SMA/composite active bearing can be very effective in both altering the natural frequency of the fundamental whirl mode as well as the modal amplitude. The drawback with that design has been the disparity in the time constant between the relatively fast heating phase and the much slower cooling phase which is reliant on forced air, or some other form of cooling. This form of design means that the cooling phase of one half, still using forced air, is significantly assisted by switching the other half into its heating phase, and vice versa, thereby equalising the time constants, and giving a faster push-pull load on the centrally located bearing; a loading which is termed ‘antagonistic’ in this paper. The experimental system is discussed in terms of potential performance and control issues.

Abstract: Misalignment of multi-bearing rotor systems is one of the most common fault conditions yet it is still not fully understood. There are numerous (and sometimes confusing) accounts in the literature asserting the presence of harmonics in the vibration signal, but no quantitative descriptions are offered. Harmonics may arise, of course, from the nonlinearities in fluid film journal bearings or from the kinematics of flexible couplings, but in this paper only rigidly coupled rotors mounted on idealised linear bearings. It is shown that even for this case, excitation at twice synchronous speed is developed and an expression for the magnitude and phase of the response is derived. Several examples are then studied to give some insight into the magnitude of these harmonic terms which can arise. It is argued that it is precisely because the harmonic terms can arise from diverse sources, a full description of the phenomena has proved somewhat elusive. A brief discussion of the type of rig required to validate the model is given. There is a need to separate the phenomena discussed here from the nonlinearities found in real machines. Some features of the new facility are described.