Journal bearing design criteria - VII Measuring oil film thickness

Journal bearing design criteria - VII Measuring oil film thickness

As stated in the previous articles, radial journal bearings have operating parameters sufficient to carry the bearing load between the journal (shaft) and the bearing surface. These parameters include speed, load, oil flow, oil viscosity, bearing width, bearing diameter, bearing and oil temperatures, surface texture, etc. One of the main objectives in bearing design is to separate the bearing surface from the journal surface by means of an easily transformable elastic oil film with a suitable thickness. This oil film layer acts as a suspension. However, its easily transformable nature, the intensity and direction of the load, the size and direction of the shaft speeds and changeable nature of the thickness of this oil film at any time pose a risk in terms of the life of the bearing - shaft.

In terms of bearing design, it is a phenomenon that there should be an eccentricity (amount of offset of the centers) between shaft and bearing centers. This misalignment allows easier assembly, creates a place for oil volume and a dynamic operation and pressure-generation mechanism. As a result of this eccentricity, an oil layer that narrows and expands between the two surfaces is formed. In the lubrication literature, this oil layer is called oil wedge. An inclined wedge touches the shell (hub) and shaft surfaces and connects the surfaces to each other, and with this compression of the wedge the surfaces can move together. Again, the transverse wedges serve to adjust the position between the connected elements, the oil wedge moves the shaft away from the bearing surface and almost floats the shaft in the body of the oil, keeping the shaft suspended in the oil. Therefore, the shaft surface transfers the load to the bearing surfaces while rotating via this suspension. Thus, the bearings serve as the shaft supporting element. With the variable thickness of the oil layer, direct contact of the shaft and bearing surface is cut, ensuring a longer life of the system.

The nominal minimum film thickness in radial journal bearing, which carry variable loads such as the engine bearings in vehicles, is defined according to the surface roughness. Depending on the operation environment, the roughness of the shaft and the bearing surfaces must be greater than the sum. Here, in general, the average roughness (Ra) as well as the maximum surface roughness (Rt, Rz, etc.) should be taken into account. In the lubrication literature, there are tables and charts that recommend minimum oil film thicknesses at micrometer level for operation areas (engine bearings, electric motors, general purpose machine applications, slides, etc.). These charts should be used in bearing design. In addition, oil filtration and compliance in the system is another important factor that affects the minimum oil film. (To be continued)

25.11.2020 12:28:00
Prof. Dr. Ertuğrul Durak     1019

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