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Posted on Jun 30, 2021
Journal bearings (more commonly known as plain bearings or sliding bearings) are constructed with a shaft (journal) that rotates in a supporting sleeve or housing. There is no rolling element in these bearings. Their design and construction are relatively simple, but the operation can be quite complex.
The shaft within the journal bearing is rotating in a shell of curved washers. Each washer can rotate independently to follow the curvature of the shaft. The advantage of this design is the more precise alignment of the bearing shell with respect to the rotating shaft and the resulting increase in shaft stability. Besides the design, another factor that influences the operation is lubrication.
(Oil and grease) lubrication forms a film between the contact surfaces of the moving elements to reduce friction and dissipate the heat. The moving elements, including pins and bushings, can be fully lubricated, semi-lubricated (or dry). Oils are used in journal bearings when cooling is required or when contamination must be flushed out of the bearing.
High-speed journal bearings are always lubricated with oil, not grease. Oil is supplied to the bearing by a pressure pump. Grooves in the bearing shell are used to distribute the oil throughout the bearing surfaces. Let's take a look at how lubrication affects the operation of the moving elements in the journal bearings.
A hydrostatic jack, produced by an external pressurized oil supply, can be used to lift large, heavy pivots before shaft rotation. In normal operation, the shaft rotates at a sufficient speed to force oil between the curved surfaces of the shaft and shell, thereby forming an oil wedge and a hydrodynamic oil film.
This hydrodynamic fluid layer enables the bearings to withstand extremely high loads and operate at high rotational speeds. The common operation speeds range from 175 to 250 meters per sec. The rotating shaft is not centered in the bearing shell during normal operation. This offset distance is the bearing eccentricity and it creates a unique location for the minimum oil film thickness.
When the bearing incorporates low-speed bushings, the shaft it supports and the bushing do not rotate fully. Partial rotation of the moving elements at a low speed prevents the formation of a complete fluid film. This results in the metal-to-metal contact (scroll down for more info) in the bearing. These types of bearings are usually lubricated with high-pressure grease to help support the load.
The grease contains solid molybdenum disulfide to increase the load capacity of the grease. Many outdoor construction and mining equipment applications include pins and bushings. Consequently, shock loads, as well as water and dirt contamination, are often the main considerations when lubricating them.
Dry journal bearings consist of a shaft rotating in a dry sleeve, usually a polymer that can be mixed with solids such as molybdenum, graphite, PTFE, or nylon. These bearings are limited to light-load applications with low surface velocity. Without lubrication, the mechanism cannot withstand extreme applications.
Below is an in-depth video showing the lubrication system in the journal bearing.
Introduction to Journal Bearings, KTU Web
Journal bearings operate in the limit mode (metal-to-metal contact) only during equipment startup and shutdown when the rotational speed of the shaft (journal) is insufficient to form an oil film. It is during startup and shutdown that almost all bearing failure occurs.
In addition to the lubrication coverage, the minimum oil film thickness is also important. It indicates the dynamic running clearance of the bearing. Oil film thickness and dynamic clearance are useful in determining filtration and surface finish requirements for the metal. Minimum thickness during operation from 1 to 300 microns is common. 5 to 75 microns are more common in medium-sized industrial equipment. The thickness is greater in equipment that has a larger diameter shaft.
:: Read More: Sleeve Bearings are Super Important Components!
Oil vortex is a phenomenon that can occur in high-speed journal bearings where the position of the shaft inside the shell becomes unstable and the shaft continues to change its position during normal operation due to the fluid forces generated in the bearing. The oil vortex can be reduced by increasing the load or by changing the viscosity, temperature, or oil pressure of the bearing.
A permanent solution may include a new bearing with different clearances or designs. Oil runout occurs when the oil spinning frequency coincides with the natural frequency of the system. The result can be a catastrophic failure.
The pressure in the contact area of radial bearings is much lower than in rolling bearings. This is due to the larger contact area created by the matching surfaces of the journal and the shell. The average pressure is determined by the force per unit area, or the weight/load divided by the approximate load area of the bearing.
At low pressures, there is no increase in oil viscosity in the bearing contact area. Bearings for automotive and high-stress industrial applications have average pressures ranging from 20 to 35 MPa. At these levels, the viscosity is slightly increased and the maximum pressure encountered is typically about twice the mean value to a maximum of about 70 MPa.
Radial bearings are a form of journal bearing, which includes a sleeve, plain, shell, and Babbitt. The term Babbitt refers to the layers of softer metals (lead, tin, and copper) that make up the metal contact surface of the bearing shell. These metals add to the stronger steel bearing shell and cushion it in front of a harder rotating shaft.
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