Ashish K. Darpe

Bearing damage classification using instantaneous energy density

A methodology is proposed for classification of type of defect in a bearing using vibration data. Hilbert-Huang Transform (HHT) is used to obtain Instantaneous energy density (IE) values corresponding to interaction of rolling elements and the defect. IE values are treated as a time series and autocorrelation coefficients (ACs) with varying lags are calculated. Based on the ACs, a Defect Occurrence Index (DOI) is proposed to rank the possibility of a type of bearing damage. Based on the DOI, an adaptive filtering process is used to filter the maximum IE corresponding to the peaks generated during the defect interaction. The main feature of the present approach involves capturing the phenomenon of amplitude modulation (using the approach of autocorrelation) for an inner race defect and uses its absence in the case of outer race defect to distinguish between the inner and outer race defects. Statistical techniques (Chi-square test and Coefficient of variation (CV)) on the filtered IE are used for damage type identification. A new parameter, Defect Severity Value (DSV), is proposed for assessment of the severity of the bearing defect. The proposed methodology is validated on simulated outer/inner race defect, and on two different vibration datasets obtained from seeded defect experiments. The proposed methodology helps uniquely identify the bearing damage.

Influence of flow velocity and flexural rigidity on the flow induced vibration and acoustic characteristics of a flexible plate

A numerical investigation on the influence of structural flexibility and flow velocity on the flow-induced acoustic and vibration response of a plate is presented. Simulations are performed on a test geometry of rigid square bluff body with a trailing flexible plate in low Reynolds number flow stream. The focus of the study is to characterize the flow-induced vibration and associated aerodynamic far field sound radiation from a flexible structure in flow. The role of flow velocity and level of structural flexibility on the acoustic radiation is thoroughly investigated. A linearized Euler equation based computational aeroacoustic hybrid method and a surface source approach for coupling the flow and acoustic domains are implemented with a bi-directional fluid structure interaction. The vortex shedding frequency of the coupled fluid-structure system synchronizes with the fundamental frequency of the trailing plate and steady-state vibration of the plate is observed. The results indicate that the relation between vibration level and the flow velocity as well as structural flexibility is not linearly related. For a particular combination of flow velocity and plate stiffness, the coupled fluid-structure system shows the resonance condition. The observed resonance frequency is slightly different from the free vibration (natural) frequency of the plate. Computation of the acoustic shows that the magnitude and spectral nature of the far field sound depends on the amplitude of the vibration and a higher acoustic pressure and the sound rich in tones is observed at resonance condition.

Experimental Investigations on Torsional Vibrations of a Rotor During a Rotor-Stator Rub

Theoretical investigations on rotor-stator rub phenomenon are available on wide range of problems. However the experimental studies are limited in numbers. Further, the measurement studies on torsional vibration during rub are hardly any. The present work reports and discusses measurement of rotor’s torsional vibrations during its contact with the stator and examines the possible rub diagnostic features. A simple rotor bearing system is made to interact with a stator pin mounted on a stiffer stator frame. The rotor assembly, with two discs and two support bearings, intends to model a turbopump where the central disc simulates the impeller and the overhanging disc is meant to represent turbine. A flexible coupling between the driver and the driven unit is provided to reduce the effects of any misalignment remaining in the system and to isolate the rotor from the motor. The measurement of torsional vibration is carried out using a torsional laser vibrometer. The measured signals during rub and no rub conditions are compared in time as well as in frequency domain to bring out the torsional vibrations features related to rub. Excitation of various torsional modes of the rotor system is experienced due to the frictional torque caused by the occurrence of rub. The presence of rotor’s torsional mode frequencies, rotational harmonics and the bending natural frequency in the spectrum of torsional velocity signals are indicative of rub.

Detection of Gear Tooth Crack in a Wind Turbine Planetary Gearbox

A planetary gearbox of a horizontal axis wind turbine drive train is modelled as a vibratory system and vibration response is investigated for detecting a typical gear tooth flaw. A detailed dynamic model involving two translational and one rotational degree of freedom for each component of the planetary stage is formulated. The gearbox stage considered in the study is a low speed planetary gear stage (three identical planets with spur teeth, sun and fixed ring gear) as the typical arrangement commonly used in wind turbine industry. The effect of gravity is incorporated in the mathematical formulation as the mass of the drivetrain components is considerable in such application. The vibration response of the elements is influenced by the gear mesh stiffness variations. The presence of a tooth crack shifts the localized mean value of gear mesh stiffness to a lower value. The localized change in turn influences the vibration response of all the components. In order to extract fault-induced vibration features, a difference signal is generated from the synchronous time domain vibration signals for the healthy and cracked tooth. The time domain and frequency domain data of the proposed difference signal are studied. They reveal useful information for the purpose of detection of gear tooth crack. The spectral characteristics can be used for condition monitoring and early detection of gear tooth crack for a wind turbine gearbox.