A Seshadri Sekhar

In aircrafts and missiles, actuators are commonly utilized for driving various subsystems including flight control surfaces. Brushless DC motor (BLDC) based electromechanical actuation systems are getting increasingly popular due numerous advantages. Electromechanical Linear actuators (EMLA) are common due to their high torque amplification. The failure modes for these systems transcend electrical, mechanical, and electronic systems are masked by external forces and the dynamic properties of control systems making it difficult to use model based condition monitoring techniques. Generally these systems are stored for longer times before operation, so a comprehensive health monitoring strategy is required for their fault free operation as and when required. The aim of this paper is to identify gear faults signatures in motor current of closed loop electromechanical linear actuators by experimental approach. It also considers the effect of load on the Motor Current Signature Analysis (MCSA) signatures and their comparison with vibration signatures. An amplified error spectrum method devised for such feedback systems was also compared for its fault signature detection capability with vibration and MCSA signatures.

Introduction and Background: Spiral bevel gears are ubiquitous in numerous power transmission systems. The exigent demands such as high performance, more strength and less noise in helicopter drives can be met by selecting the optimal geometrical design parameters. However, a comprehensive study on the influences of geometric parameters on the transmission system dynamics is not found in the literature. Objective: Therefore, in this study, the effects of geometric design parameters such as spiral angle, pressure angle and pitch cone angle on the spiral bevel gear set dynamic characteristics for various boundary conditions are studied. Method: Firstly, the analytical modelling of coupled lateral-axial–torsional vibrations of the spiral bevel gear set is derived theoretically under few assumptions. Then the emphasis on the effects of geometric parameters on the dynamic characteristics such as critical speeds, coupled mode shapes and unbalance vibration response of the gear pair supported by flexible and rigid boundary conditions are investigated numerically. Results: Under rigid support conditions, it is observed that there is no remarkable influence of geometric parameters on the critical speeds. However, the unbalance response amplitudes at and around the resonance peaks are notably affected. Whereas the critical speeds and unbalance responses are essentially affected by the variation of axial and torsional stiffness at the supports. Meanwhile, the influence of geometric parameters on the critical speeds corresponding to torsional and axial modes is remarkable for the flexible support conditions. Furthermore, the steady-state response due to combined torsional and unbalance excitations is also studied, and the results show that the geometric parameters influence the axial, lateral and torsional responses. Conclusion: The results obtained through this study are useful in the design of the spiral bevel gear set.

In the present study the detection of the crack in rotor system has been carried out using the statistical features such as kurtosis, skewness and crest factor of vibration response. Such features, mainly kurtosis have been used in literature for identifying bearing and gearbox defects. The simulation results of crack detection have been compared with those of experiments carried out using laser vibrometer. Single and two cracks have been identified. It has been noted that the kurtosis and skewness changes are significant when the crack is closer to the bearing locations. The skewness is mostly negative but as the crack depth increases the positive values of skewness increase beyond the middle part of the shaft. The crest factor deviation curve is developed, which could successfully identify even cracks of small depths. © KRISHTEL eMAGING SOLUTIONS PRIVATE LIMITED.

In general damping is estimated from impulse response obtained by giving small impact when the system is stationary (like a civil structure or a beam), this approach is well documented in many research works. In case of rotor-bearing systems such as a steam turbine, the damping estimated by impact is not identical to the damping observed during operation because of the angular acceleration effects. Since, it is not possible to obtain impact responses from a rotor bearing system operating at high speeds, one has to relay on signals obtained during operation which contain damping information. The present study compares conventional and time-frequency methods for estimating damping in rotor bearing systems from transient characteristics. Advantages of using time-frequency techniques over conventional techniques are focused.

In this paper, coupled response measurements are suggested for detection of a transverse surface crack in a cantilever beam. The crack has been modelled by a local compliance matrix of six degrees of freedom and simulated for detection. The eigenfrequencies are obtained using finite element method (FEM) and are compared with those of experiments. However, the small changes in eigenfrequencies are not enough for crack detection. The flexibility matrix contains non-diagonal terms, and thus indicating the coupling among the different modes of vibrations. A harmonic force of known amplitude and frequency is used to dynamically excite the beam in bending mode. But due to the presence of crack, the bending excitation induced the axial and torsional vibrations also. The measurements of direct (bending) response and coupled (axial and torsional) responses are carried out using triaxial accelerometer and rotational laser vibrometer (RLV). The experiments are carried out for different bending frequencies, crack depths and locations. The results indicate that the coupled responses are very sensitive to crack; hence these measurements are suggested for crack detection. © 2012 Inderscience Enterprises Ltd.

The equations of motion, with four degrees of freedom, taking into consideration the flexibility, damping and cross coupling of the fluid film bearings are derived for a cracked Jeffcott rotor supported on fluid film bearings. Dimensionless equations are developed for dynamic radial load, dynamic pressure developed in the fluid film bearings and coefficient of dissipation considering the journal vibrations in two harmonics; bearing fluid film stiffness and damping coefficients. These are applied to a cracked Jeffcott rotor supported on different types of bearings, i. e., cylindrical journal bearings, offset cylindrical bearings, tilting pad journal bearings and three-lobe bearings. Based on the allowable dynamic pressure developed in the fluid bearings, the severity of cracked shaft and allowable crack depths are estimated in this study. Measurement of dynamic pressure and dissipation for monitoring the crack growth is suggested. However, 2x vibration is the best indicator of cracks in the shafts. © 1995 Taylor & Francis Group, LLC.

Modern rotating machinery is prone to fatigue cracks due to the severe working and continuously varying loading conditions. To avoid failure of the rotating systems due to these cracks, the system needs to be continuously monitored. The reliability of the machinery can be enhanced with the fault identification at the early stages of their occurrence. Model based methods are used for crack identification in rotating systems successfully. Model based methods are of different types. One of the methods, the equivalent loads minimization method is studied here. It has a limitation that the error in identified crack depth increases with decrease in number of measured vibrations. In the present work, the theoretical fault model loads used in least squares minimization algorithm are transformed using modal expansion to reduce error in the identified fault parameters. The crack is identified accurately even in the case of less number of measured vibrations. The method is applied for crack identification in a centrifugal pump rotor using transverse vibrations. The results presented are obtained purely from FEM simulations. Copyright © 2010 by ASME.

Composite materials are having good dynamic properties, specific strength, and specific stiffness. The analytical formulations for dynamic analysis of composite tube shafts are mainly equivalent modulus beam theory (EMBT) and layerwise beam theory (LBT). The EMBT is improved by including coupling effects in the formulation. In the present study, the natural frequencies are calculated using modified EMBT for hybrid composite tube shafts and results are in good match with numerical and experimental work. The of position of CNT inserted layer and amount of CNT added to CFRP, GFRP, and hybrid of CFRP and GFRP composite tube shafts are analyzed.

The wind turbine drive train dynamic analysis is carried out with and without existence of gear tooth breakage. The loads acting on the wind turbine are considered based on Danish Standard DS472 which is a simplified method of taking loads in wind turbine. A broken gear tooth is modelled by the change in the gear tooth thickness. The gear mesh stiffness of broken tooth is evaluated by the change in cross sectional area and area moment of inertia due to change in tooth geometry. From the frequency representation of vibration data, the fault information is not clearly observed due to the insensitiveness of the dynamic system to the stochastic loads. However, by further analysis, fault information is obtained with the use of the gear tooth contact force from which a clear sub harmonics are observed at the defective pinion tooth frequency.

An important rotor fault, which can lead to catastrophic failure if undetected, is fatigue crack in the shaft. The cracked rotor problem received the first attention in 1970, since when the interest among the researchers started. Vibration behavior of cracked structures, in particular cracked rotors, has received considerable attention in the last two decades. The problem of damage and crack detection in structural components has acquired important role in recent years. However, the studies are mainly dealt with single crack. If the structure is cracked in at least two positions, the problem of crack sizing and location becomes decidedly more complex. Relatively few authors have addressed the multi-crack assessment for structures. The objective of this present study is to summarize the different studies on double/multi-cracks and to note the influences, identification methods in vibration structures such as beams, rotors, pipes, etc. And thus this paper brings out the state of the research on multiple cracks effects and their identification. © 2007 Elsevier Ltd. All rights reserved.