A.S. Sekhar

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.

Condition monitoring of cracked rotors through transient response

In the present study a simple Laval rotor with a transverse crack is considered. The analysis assumes that the vibrations remain small in comparison to the sag of the rotor under its own weight. If a cracked shaft rotates slowly under the load of its own weight, then the crack will open and close once per revolution. i.e. it breathes. Considering a simple hinge model which is a very good model for small cracks with the breathing action of crack, the transient vibration response of a cracked rotor passing through its critical speed is analysed. as an attempt for crack detection and monitoring. Effects of different factors such as crack depth. unbalance eccentricity with phase and acceleration influencing vibration are investigated.

Crack identification in a rotor system: a model-based approach

The dynamics and diagnostics of cracked rotor have been gaining importance in recent years. In the present study a model-based method is proposed for the on-line identification of cracks in a rotor. The fault-induced change of the rotor system is taken into account by equivalent loads in the mathematical model. The equivalent loads are virtual forces and moments acting on the linear undamaged system to generate a dynamic behaviour identical to the measured one of the damaged system. The rotor has been modelled using finite element method, while the crack is considered through local flexibility change. The crack has been identified for its depth and location on the shaft. The nature and symptoms of the fault, that is crack, are ascertained using the fast fourier transform.

Model-based identification of two cracks in a rotor system

Abstract The dynamics and diagnostics of cracked rotor have been gaining importance in recent years. Relatively few authors have addressed the problem of multi crack assessment for rotors. In the present study a model-based method is proposed for the on-line identification of two cracks in a rotor. The fault-induced change of the rotor system is taken into account by equivalent loads in the mathematical model. The equivalent loads are virtual forces and moments acting on the linear undamaged system to generate a dynamic behaviour identical to the measured one of the damaged system. The rotor has been modelled using finite element method, while the cracks are considered through local flexibility changes. The cracks have been identified for their depths and locations on the shaft. The nature and symptoms of the fault, that is crack, are ascertained using the FFT.

The effect of roughness parameter on the performance of hydrodynamic journal bearings with rough surfaces

The influence of the roughness parameter (C) and the roughness patterns (longitudinal, transverse and isotropic) on the steady state and dynamic characteristics of hydrodynamic journal bearings with rough surfaces has been studied using the model of Christensen and Tonder (The hydrodynamic lubrication of rough journal bearings. Trans. ASME, Journal of Lubrication: Technology 1973;95:166–72). Reynolds-type equations have been solved using the stochastic finite method. It is seen that the transverse roughness tends to increase significantly load carrying capacity and stability with roughness values, whereas in the case of other roughness patterns the effect is seen to be very small.

Effects of cracks on rotor system instability

The stability threshold of a rotor-bearing system having a transverse crack has been studied by using FEM and considering the various crack parameters and shaft internal damping (viscous and hysteretic damping) and geometric parameters. It has been observed that the instability speed has reduced considerably with increase in crack depth and influenced more with hysteretic damping compared to viscous damping. The stability of the rotor system including shaft with two open cracks has also been carried out and observed the influence of one crack over the other for threshold speed limits.

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.

Elastohydrodynamically lubricated ball bearings with couple-stress fluids, Part I Steady-state analysis

Conventional elastohydrodynamic lubrication (EHL) analysis of point contacts is extended to include couple-stress effects in lubricants blended with polymer additives. A transient pressure differential equation, generally referred to as a modified Reynolds equation, is derived from the Stokes microcontinuum theory and solved using the finite difference method with a successive over-relaxation scheme. The solution is obtained under isothermal conditions, assuming a suitable exponential relation of pressure-viscosity variation. A nondimensional couple-stress parameter, which can be considered the molecular length of the additives in the lubricant, is used in the analysis. From the results obtained, the influence of the couple-stress parameter on the EHL point contacts is apparent and cannot be neglected. Lubricants with couple stresses provide an increase in the load-carrying capacity and reduction in friction coefficient as compared to Newtonian lubricants. Empirical formulas for the calculation of central and minimum film thicknesses of lubricated point contacts with couple-stress fluids are derived with the nonlinear least-squares curve-fitting technique using different numerically evaluated data. This may help to avoid the time-consuming numerical calculations.

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.