A.R. Mohanty

APPLICATION OF DISCRETE WAVELET TRANSFORM FOR DETECTION OF BALL BEARING RACE FAULTS

Bearing race faults have been detected by using discrete wavelet transform (DWT). Vibration signals from ball bearings having single and multiple point defects on inner race, outer race and the combination faults have been considered for analysis. The impulses in vibration signals due to bearing faults are prominent in wavelet decompositions. It is found that the impulses appear periodically with a time period corresponding to characteristic defect frequencies. It has been shown that DWT can be used as an effective tool for detecting single and multiple faults in the ball bearings.

Fault Detection in a Multistage Gearbox by Demodulation of Motor Current Waveform

Demodulation of vibration signal to detect faults in machinery has been a prominent prevalent technique that is discussed by a number of authors. This paper deals with the demodulation of the current signal of an induction motor driving a multistage gearbox for its fault detection. This multistage gearbox has three gear ratios, and thus, three rotating shafts and their corresponding gear mesh frequencies (GMFs). The gearbox is loaded electrically by a generator feeding an electrical resistance bank. Amplitude demodulation and frequency demodulation are applied to the current drawn by the induction motor for detecting the rotating shaft frequencies and GMFs, respectively. Discrete wavelet transform is applied to the demodulated current signal for denoising and removing the intervening neighboring features. Spectrum of a particular level, which comprises the GMFs, is used for gear fault detection

Detection and monitoring of cracks using mechanical impedance of rotor-bearing system

The dynamic behavior of structures, in particular, that of a rotor, containing cracks is a subject of considerable current interest. In the present study, the influence of a transverse surface crack, both open and breathing conditions, on the mechanical impedance of the rotor-bearing system, has been investigated by FEM analysis for flexural vibrations. The results show that the mechanical impedance changes substantially due to the presence of crack and follow definite trends with the crack parameters (depth and location) and force location. The normalized mechanical impedance of rotor system is more sensitive to the breathing crack when compared to the open crack. The sudden changes in mechanical impedance have been observed at the multiples of the running frequency for the breathing crack. The measurement of mechanical impedance has been suggested for crack detection and condition monitoring of rotor-bearing systems.

Vibration analysis of a misaligned rotor—coupling—bearing system passing through the critical speed

Abstract The transient response of a misaligned rotor—coupling—bearing system passing through the critical speed has been analysed by using the finite element method (FEM) for flexural vibrations. The coupling has been modelled in two ways: a frictionless joint and a joint with stiffness and damping. From the vibration analysis, the subcritical speeds at one-half, one-third and one-fourth the critical speed have been found when the misaligned rotor—coupling—bearing system passes through its critical speed. The continuous wavelet transform (CWT) has been used as a tool to extract the silent features from the time response of the rotor system. A parametric study has been carried out to investigate the transient response of this rotor system for different angular accelerations in different types of misalignment.

Transient lateral analysis of a slant-cracked rotor passing through its flexural critical speed

The vibrational characteristics of a slant-cracked rotor passing through its flexural critical speed have been analysed by using finite element method for flexural vibrations. The transient response of a cracked rotor has been obtained by applying an unbalance force and a harmonically varying torque on it. When a crack is present in the rotor, the sub and super harmonic frequency components at an interval frequency corresponding to torsional frequency were found to be centered on critical speed of the rotor system. A parametric study has been carried out for transient response of the rotor for different crack depths, rotor angular accelerations and torsional frequencies. Some suggestions for detection and monitoring of the slant cracks in the rotor are described.

Technical Note: Gearbox Health Monitoring through Multiresolution Fourier Transform of Vibration and Current Signals

The multistage helical transmission gearbox in the present study is driven by an induction motor and is coupled to a DC generator, which is connected to an electrical resistance bank for loading purposes. One and two teeth are artificially removed in one gear of the gearbox to simulate actual fault condition. When the gearbox is operated under several loads, the vibration signals are acquired from the tail-end bearing of the gearbox, and simultaneously the current drawn by the induction motor is acquired and monitored. For signal analysis, discrete wavelet transform is applied to the current and vibration signal. Subsequently, short-time Fourier transform (STFT) was applied to the level that contains information of the gear mesh frequencies and their sidebands of rotating shaft frequencies. This is a corrected multiresolution Fourier transform (MFT), which helps in further characterization of the signals. It is inferred that MFT coefficients of vibration and current signals can predict a consistent trend in the energy level possessed by the gear mesh frequencies with an increase in the severity of defects, and hence can be a useful tool in gearbox health monitoring.

Fault detection in a multistage gearbox by time synchronous averaging of the instantaneous angular speed

The paper deals with an instantaneous angular speed (IAS) based algorithm for fault detection in a multistage gearbox. Fast Fourier transform (FFT) is a well established technique for analysis of a stationary signal. However, the IAS signal from the multistage gearbox is a combination of periodic signal and structure borne noise. This structure borne noise makes the IAS signal non stationary. Hence, FFT is no longer suitable for this non stationary signal analysis. Therefore in this paper, the IAS signal is time synchronously averaged with respect to lay shaft speed and then followed by FFT reveals the gear mesh frequency and rotational frequencies which are synchronous with the gearbox’s rotating speed. Thus, the algorithm can accurately and reliably detect the fault in multistage gearbox using the IAS. The paper also investigated the behavior of IAS signal for three different gearboxes (healthy, one tooth and two teeth broken) under two different speeds and three different loading conditions.

Crack detection by measurement of mechanical impedance of a rotor-bearing system

Measurements were done to determine the influence of a transverse crack on the mobility of a rotor-bearing system. The results show that the mobility changes substantially due to the presence of crack and follows definite trends with the crack depth and direction of force application. The significant changes in mobility were observed at the natural frequency and at the running frequency of the rotor system. Good agreement between the experimental results and numerical simulations has been observed. The measurement of mobility has been suggested for crack detection and condition monitoring of rotor-bearing systems.

Noise Control of Home Appliances — The Green Way

Material used for noise control applications should possess good sound absorption and dampening properties. In this work, the authors have focused on noise control applications of novel material made of jute which is a readily available, cheap and biodegradable natural fiber. Based on physical, flammable and acoustical properties, jute felt shows good sound absorption properties which has been used for noise control in a domestic clothes dryer and a vacuum cleaner. The noise sources in these appliances were ranked by measuring their radiated sound intensity levels. An overall reduction of radiated noise levels of 6 dB and 10 dB were obtained in the domestic dryer and the vacuum cleaner, respectively, by using the developed jute felt.

Fault detection in a centrifugal pump using vibration and motor current signature analysis

Due to growth of mechanisation and automation, todays industrial systems are becoming more complex. A small breakdown of any non-redundant machine component affects the operation of the entire system. To increase the availability and reliability, automated health monitoring and self-diagnostic capability (SDC) becoming essential to many industrial machineries like pumps, motors, etc. Condition monitoring does not prevent the failure, but it can predict the possibility of future failure by measuring certain machine parameters. Though there are various condition monitoring techniques, vibration analysis and motor current signature analysis (MCSA) are most suitable for detection of faults and abnormalities in machine systems. This work attempts to develop an SDC framework and diagnose the impeller condition of a centrifugal pump using MCSA. Time and frequency domain analyses are done for different impeller conditions of the pump, such as normal impeller and defective impellers. Significant differences are observed and a fault prediction strategy is recommended.