ArticleJournal Bearing Analysis
by Ray Kelm
This paper provides an excellent introduction to journal bearings. A description of how they work as well as the many different designs and their applications with different machinery are discussed. Comparisons to anti-friction bearings are made. The importance of lubrication and its function within the bearing is described. Basic considerations in journal bearing design such as the L/D ratio, Somerfeld number, dynamic stiffness & damping are also discussed. The ratio of bearing to support (pedestal) stiffness is mentioned and why this ratio should be considered when performing vibration analysis of machines running on journal bearings. Vibration analysis methods such as time waveform analysis, orbital analysis & shaft centerline position and their application to machines operating within journal bearing are described in detail.
The use of both linear & exponential curve fitting to vibration trend data is explained as well as using this data to estimate time to alarm. Determining the accuracy of curve fitting to actual data using both graphical and mathematical techniques is discussed. This paper is of value for someone wanting to improve the relevance of their machine alarm limits and learn how to quickly identify when these limits have been violated.
Journal bearings are a subset of bearings used to support rotating shafts that use the principle of hydrodynamic lubrication. Opposed to anti-friction bearings that operate using rolling components inside the bearing (i.e. balls, rollers), journal bearings operate by using a self-generated hydrodynamic oil film pressure to support the shaft while preventing the shaft from contacting the bearing surface. Anti-friction bearings have predictable life cycles before they will fail that are known based on statistical testing of many similar bearings. In contrast, a journal bearing that uses hydrodynamic lubrication has an infinite expected life unless there is a loss of the oil film due to lack of lubrication or excessive forces.
In its simplest form, a journal bearing is a bearing in which a shaft or “journal” rotates in the bearing with a layer of lubricant (“oil”) separating the two parts through fluid dynamic effects. Since the bearing surface and the journal are moving with relative speed, lubricant will be drawn into the gap between them and forced to squirt out the sides of the hearing while the gap is converging. This can be observed in Figure 1 which shows the oil film thickness as a function of angular position around the bearing starting with the maximum gap (shown at 0°) through the minimum gap (shown at 180°) back to the maximum gap.
As the oil is drawn into the converging wedge by the relative motion, the pressure increases and the oil is forced out the sides of the bearing since the oil is relatively incompressible. The oil film pressure is shown in Figure 2 that is generated around the bearing. This self-generated pressure is what supports the load imposed by the shaft. While the oil film is converging (getting thinner) the pressure will be positive which will support the load of the shaft. If the oil film is diverging (getting thicker) the pressure will attempt to decrease. The pressure cannot fall below the vapor pressure of the oil, so the film will tend to cavitate in the diverging section.
The limits of the operation can be broken into two extremes: low speed and high speed. On the low end, there becomes a point at lower shaft speed where the oil becomes too thin and some contact between the shaft and the bearing surface will occur. This is dependent on the surface roughness of the shaft and bearing as well as the lubricant characteristics.”
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