Componente Perceptual – Paisaje

In conclusion, concepts of general maintenance techniques and condition based maintenance are described in Chapter two. In addition to that vibration based condition monitoring which is described in Chapter three is applied to investigate the vibration signals obtained from different operating parameters, bearing starvation and worn journal bearing. Based on theoretical study and experimental evaluation, it has found that vibration of a journal bearing is influenced by a number of factors including radial load, shaft speed and lubricant characteristics.

High frequency vibration responses are mainly from two frictional effects in a journal bearing. The surface asperity collision in the boundary lubrication explain the main vibration responses, whereas fluid shearing induced asperity deformation and recovery in the hydrodynamic lubrication regime can also be an effective vibration generation mechanism. Analytic and experimental studies show that these vibration responses in the

Condition Monitoring of Journal Bearings for Predictive Maintenance Management Based on High Frequency Vibration Analysis [Ch.10]

DEGREE OF DOCTOR OF PHILOSOPHY (O. Hassin) 167

high frequency range are complicated and difficult to separate according to the oil types and lubrication regimes.

A mathematical model is used to successfully calculate natural frequencies of the journal bearing. This model considers both conventional fluid forces and asperity contact forces. The natural frequencies of the system are around 6 kHz and 13 kHz. Furthermore, the natural frequencies gradually increase while the radial load increase and oil viscosity decrease.

Abnormal operating conditions are one of the problems that cause a journal bearing to fail. Based on the analytic studies and experimental verifications, it has been demonstrated that popular vibration monitoring can detect abnormal oils and operations in wide operating conditions.

Through a hierarchical clustering approach, the similarity and difference between the spectra of test samples can be recognised step by step in a relatively narrow frequency band. Finally, it obtains a classification result in the frequency band around 10kHz that allows different oils and operating conditions to be separated in consistent with lubrication regimes.

This successful diagnosis is achieved based on modulation signal bispectrum analysis which allows the nonlinear vibration responses of journal bearings to be characterised into two distinctive patterns to correspond to the instable lubrication and the asperity interactions respectively. Furthermore, the MSB magnitudes in different frequency bands can be based to differentiate the asperity interactions between asperity collisions and the asperity churns. A higher magnitude in the lower frequency band can indicate the excessive asperity contacts due to lowering viscosities. Meanwhile a higher magnitude in the higher frequency band indicates the extreme fluid frictions.

Based on the vibration mechanisms established and experimental studies, the vibration signals collected under different operating conditions and lubricant levels can be separated adequately for the purpose of condition monitoring. Journal bearing vibration responses are induced by the combination of the conventional hydrodynamic effect and the asperity churns and collisions between the lubricated rough surfaces the combination of which can be characterised by modulation bispectrum analysis. The hydrodynamic effect can interfere with the asperity effect in the low frequency range (3.5kHz to 5.5kHz), resulting in good

detection of the instability but an inconsistent diagnosis of oil levels. Meanwhile, the structural resonances in the high frequency range (5.5kHz to 11kHz) can better reflect the excitations and result in a more agreeable separation of different levels under a wide variety of operating conditions.

A journal bearing is designed to use lubricant oil film to reduce asperity collision by keeping two surface separation. Lubrication can reduce friction but it can avoid it completely. Thus if the wear happened the geometry of bearing will change and wear will change the bearing’s clearance. RMS values of raw data are good indication; they present that a worn bearing releases less vibration. This result presents only vibration power caused by solid friction, even though the sources of vibration in a journal bearing are mechanical and asperity collisions. MSB technique can present two main sources of vibrations in journal bearing. Low frequency band combined with harmonics of shaft frequency which increase with a worn bearing and cause more fluctuating of the shaft. On the other hand, a high frequency band has less combination with shaft frequency and this band might relate to asperity contact. Vibration decreases with a worn bearing because the space between surfaces is greater.

A worn bearing means that the bearing gradually increases its radial clearance and its geometry is changed. Due to that, thick oil film will be formed which avoids asperity contact but increases the shafts fluctuation. MSB technique successfully presents two main sources of vibrations in journal bearing: fluctuating shaft and asperity collisions. Mean values of low frequency band of MSB shows the instability of the shaft inside the worn bearing. In contrast, mean values of a high frequency band of MSB present asperity collision vibration which has less effect at worn bearing. More clearance leads to less metal to metal contact but this is not always it is good for a journal bearing.