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The Effect of Vibration on The Pressure Distribution of Journal Bearing

The journal bearing is considered an important part of most rotary machines. It is used to support the radial loads. It is necessary to prevent metal to metal contact between the stationary part (bearing) and the rotating part (shaft). It suffers unwanted oil whirl due to the high self-vibration and a cyclic loads and this causes the bearing instability.
In this thesis, the performance of the journal bearing at the steady state and dynamic loading condition under the effect of self-excitation and forced harmonic excitation has been investigated.
For this purpose, the hydrodynamic of a plain cylindrical journal bearing system has been adopted.
The conventional form of the Reynolds equation is analyzed analytically and numerically using the central finite difference technique with a proper initial and boundary condition.
The analysis has been achieved for the short journal bearing approximation.
The numerical equations then have been intended and written in the computer program (FORTRAN-95) to obtain the results.
The validity of the journal bearing device in the mechanical engineering laboratory in the university of Kerbala has been tested to study the pressure distribution of the journal bearing. The results obtained were compared with those of numerical and found to be of high discrepancy. So the experimental technique is excluded to be compared with the dynamic loading results.
According to the numerical results obtained, the maximum oil pressure was obtained under the combine effect of forced harmonic vibration and self- excitation is (14.8778 MPa) which is increased by 36.66 percent from those obtained under the self- excitation only. This value was clearly increasing as the amplitude of vibration increases causing very high fluctuation which leads to failure.
It is found that the maximum value of lubricant pressure and the minimum film thickness under the effect of forced harmonic excitation was affected clearly by the frequency and damping ratios.
Where for the frequency ratio (ω⁄ω_n <1), it is found that, the maximum pressure increased causes decreases in the minimum film thickness. While The maximum film pressure decreases for((ω)⁄ω_n >1), making the minimum film thickness to be increased preventing the metal to metal contact.
It is also found that, the higher damping ratio causes decreases in the amplitude of vibration and this lead to decreases the maximum film pressure and therefore increasing in the oil film thickness. For damping ratio higher than 0.5, there is a little effect on the minimum film thickness.
It is noticed that, the maximum oil film pressure is higher under the effect of harmonic external forced vibration only than for both self-excitation vibration and under the combined effect by (51.92, 24.1) percent respectively for the same specifications for journal and the oil. However, the maximum oil film pressure dose not reach to the value of heavily loaded journal bearing (2GPa.) to take the elastohydrodynamics effect into the consideration.