Previous Work

A central theme of my professional career has been the application of physical principals to optimise the performance of equipment and plant. Apart from a brief period in the Electronics Industry, the industrial part of my experience has been in the power generation industry mainly the Central Electricity Generating Board and later with Nuclear Electric plc. The work could loosely be described as condition monitoring, but always with the emphasis firmly placed on the physics of processes giving rise to observed phenomena. The work I have carried out has focused largely, but not exclusively on vibration and in the early year the emphasis was on modelling. More recently the main challenge has been on inverse problems but of course this builds on the experiences of direct modelling. My change in emphasis reflects a general shift in direction in the study of vibration, and I can claim some influence on this shift.

Major problems were also being experienced at two Power Stations with main boiler feed pumps. Theses pumps had a gearbox which exhibited a number of failures. Despite a number of conventional investigations the problems were not understood. High fluctuations in transmitted torque were causing the gear teeth to loose contact, and the re-meshing of the gears gave rise to very high stresses which led to tooth failure. A model was developed from first principles to describe the torque variations as a function of speed. The model was described in terms of the geometric and material properties of the shaft components together with the declared manufacture tolerances of the gears. This model led to a successful redesign of the system and publication of the calculation method.

As a result of a number of investigations, I became acutely aware although modelling of systems is invaluable in gaining insight into a system’s behaviour, it is rare to have adequate data in practice. With this realisation, my interest in inverse problems began and this is an area in which I am still engaged. With this viewpoint I began to re-formulate the problem of machine vibration with flexible supports. The treatment results in a very large least square problem which can yield the effective properties of the supporting structure - properties which are required for accurate condition diagnosis. This formulation was published, but investigation of the detailed issues was suspended when I moved into a more managerial position.

I became Head of the Heat and Fluids section and was responsible for the work of 18 staff. This was the beginning of a nine year period during which I was managing the work through various re-organisations of between 12 and 30 staff. Throughout this period I had considerable managerial responsibility including the separation of part of my group to PowerGen in 1989 and later with the approach of privatisation of Nuclear Electric. During my time in the Power Industry, I was author/co-author of about 85 format reports, notes and memoranda in addition to 15 published papers.

Progress On Recent Projects

In recent years, work in the Rotor_Dynamics and Vibration area has made substantial progress in four main areas.

1. The overall method of machine diagnosis, SIMFONE, which uses System Identification to optimize agreement between measurements and model, has now been extended to yield estimate of bearing loading and condition. The bearing loads yield, essentially the machine alignment and our method is the only method currently available for such estimates during machine operation. This is a significant step because a detailed knowledge of bearing loads during operation permits the calculation of some very important operational information such as minimum clearance between stator and rotor. These developments have been validated on simple rigs but now a much more comprehensive facility, among the best in the world, has been designed, built and commissioned. The overall method is now attracting attention from Rolls-Royce and SKF.
2. We have also made significant developments in the area of Acoustic Emissions. Whereas most practitioners in this area base judgments on the number of AE events, at Swansea we have considered the waveform and frequency content of the acoustic signal and this has lead to some very interesting results. By testing some ball bearings to destruction it was found that the predominant frequency of the signal, in our case 330khz shows a change of something like 10% prior to failure. Furthermore this technique was used to identify a subsurface crack in a ball bearing, as was shown by subsequent destructive examination. As a result of our work, one manufacturer of AE equipment is now modifying their system to give waveform information. The physic behind this frequency shift has yet to be understood fully, but it offers the potential of detection of faults at a very early stage.
3. Rotor misalignment is one of the most common difficulties in the operation of rotating machinery and is at the heart of Mechanical Engineering, yet is remains incompletely understood. Despite the rapid increase in understanding of rotordynamics, no satisfactory analysis explains the range of observed phenomena. There are, in the literature, reports of vibration signals at twice rotational speed and higher harmonics, yet other authors who report only synchronous excitation. Even recent discussions fail to reach consensus on the true nature of the phenomena. At a time when rotordynamics may be regarded as a mature technology, misalignment remains as an outstanding area where basic understanding is somewhat lacking. At Swansea a completely new theory of the effects of rigid coupling misalignment has been derived and it has been shown that under certain conditions, a harmonic at twice shaft speed can be generated. This is due to a coupling of the flexural and torsional degrees of freedom. The method has bee well received in three recent conference presentations and the first of a series of journal papers is currently in review at the Journal of Sound and Vibration.
4. The fourth area is the development of ‘Smart’ bearing housings using Shape Memory Alloys. This work involves Glasgow University and VirginiaTech (USA) . Although at early stage, a laboratory trial has demonstrated an ability to modify a machine’s natural frequency. The next stage will be to develop and optimize an appropriate control system. This, and indeed all of our work is basically aimed at the concept of a ‘Smart Machine’.

Some Recent Papers

• J.Morris, S J Hardy, A W Lees & J T Thomas ,On the formation of residual stresses due to tension levelling of cold rolled strip, Ironmak Steelmak 28 (1): 44-52 2001
• M I Friswell & A W Lees, The modes of non-homogeneous damped beams, J Sound Vib, 242 (2): 355-361 Apr 26 2001
• M I Friswell, J E T Penny, S D Garvey & A W Lees, Damping ratio and natural frequency bifurcation in rotating systems. J Sound Vib , 245 (5): 960-967 Aug 30 2001
• ED Price , AW Lees , MI Friswell, Application of high frequency monitoring for classification of rolling element bearing failures, Key Eng Mat ,204-2: 173-182 2001
• J.Sinha, M I Friswell & A W Lees, The identification of the unbalance and the foundation model of a flexible rotating machine from a single run-down, Mech. Sys. Sig.Processiing, 16(2-3), ppp255-271, 2002.
• R. Tiwari, AW. Lees and MI. Friswell, Identification of Speed-dependent bearing parameters, J. Sound & Vib., 254(5), pp967-86, 2002
• C.W.Knight, S.J.Hardy, A.W.Lees and K.J.Brown, Finite element modelling of strip curvature during hot rolling, Ironmaking and Steelmaking, 29(1), pp70-76, 2002.
• S D Garvey, M I Friswell, E J Williams, A W Lees & I D Care, Robust balancing for rotating machines, Proc.I.Mech.E., 216(11), pp117-1130, 2002
• C.W. Knight, S.J. Hardy, A W Lees and K.J.Brown, Investigations into the influence of asymmetric factors and rolling parameters on strip curvature during hot rolling, J Mater Process Tech 134 (2): 180-189 , MAR 10 2003
• AW Lees, Recent work on Flexible Machines, Power Engineer, 17(3), p46, 2003.
• JK Sinha, AW Lees and MI Friswell, Estimating unbalance and misalignment of a flexible rotating machine from a single run-down, J Sound Vib 272 (3-5): 967-989 May 6 2004
• R.Tiwari, A W Lees and M I Friswell, Identification of Dynamic Bearing Parameters: A Review Shock & Vibration Digest, 36,2, pp99-124, 2004.
• AW Lees, JK Sinha and MI Friswell, The Identification of Unbalance of a Flexible Rotating Machine from a Single Rundown, Trans.ASME, J.Eng.for Gas Turbines and Power, 126(2), pp416-421, 2004.
• JK Sinha, AW Lees and MI Friswell, Estimating the Static Load on the Fluid Bearings of a Flexible Machine from Run-down Data, Mech.Sys.& Sig.Proc.,18(6),pp1349-68, 2004.
• M Palmonella, M I Friswell,JE Mottershead, and AW Lees, Guidelines for the implementation of the CWELD and ACM” spot weld models in structural dynamics, Finite Elements in Analysis and Design,41, 193-210, 2004.
• M Palmonella, MI Friswell, JE Mottershead and AW Lees, Finite element models of spot welds in structural dynamics: review and updating, Composites and Structures,83, (8-9), p648-661, 2005.
• SE Hirdaris and AW Lees, A conforming unified finite element formulation for the vibration of thick beams and frames, Int. J. Numer. Meth. Engng. 62 (4), p579-599, 2005.
• JJ Sinou and AW Lees The influence of cracks in rotating shafts, to appear in J.Sound and Vibration,285(4-5),,pp1015-1037, 2005
• ED Price, AW Lees & MI Friswell, Detection of severe sliding and pitting fatigue wear regimes through the use of broadband acoustic emission, IMechE Proceedings J, Journal of Engineering Tribology, 219(2),85-98, 2005.
• Knight CW, Hardy SJ, Lees AW, et al., Influence of roll speed mismatch on strip curvature during the roughing stages of a hot rolling mill, Journal Of Materials Processing Technology 168 (1): 184-188 SEP 15 2005
• Fonseca JR, Friswell MI, Mottershead JE, et al., Uncertainty identification by the maximum likelihood method, Journal Of Sound And Vibration 288 (3): 587-599 DEC 6 2005
• A W Lees & M I Friswell, Where next for condition monitoring of rotating machinery, Advances in Vibration Engineering, vol.5, no.4, Oct/Dec., 2006.
• Sinou JJ, Lees AW, A non-linear study of a cracked rotor European Journal Of Mechanics A-Solids 26 (1): 152-170 JAN-FEB 2007