Hydrodynamic journal bearings are currently the bearings of choice for most turbomachinery equipment including gearboxes. They possess several characteristics that are desirable such as high reliability and good rotordynamic damping characteristics. Several different designs of journal bearings are commonly utilized for gearboxes. The designs are all variations of a sliding bearing where a shaft journal slides on a thin film of oil. The design variations utilize different geometries and features in an effort to achieve rotordynamic stability and avoid sub-synchronous vibrations.
Pressure Dam bearings are essentially a plain journal bearing with a pocket cut in one half that has an abrupt stop at some point in the shell. Viscous and inertia effects result in a buildup of pressure at the dam, this localized high pressure area creates an artificial load on the shaft that helps stabilize the rotor.
Offset half bearings utilize geometry where two bearing shell pads are not concentric to each other. This creates a situation where oil is converged into a wedge helping stabilize the shaft in the bearing.
Tilting pad journal bearings are comprised of multiple pads that are supported by pivots. The pads support the shaft and pivot independently from each other. The pads are typically machined at a larger diameter than they are assembled to create a converging oil film. The tilting pad design is inherently very stable. The three previously mentioned bearing designs are by no means an exhaustive list of the possible options, but are common in turbomachinery and tested for this application.
Efficiency and oil flow are becoming a top priority for high performance gearboxes. Bearing selection can make a substantial difference in a gearboxes performance. This study shows that each bearing design tested has its strengths and weaknesses. Careful design and selection must be made by the engineer based on the desires and requirements of the end user. At lower journal velocities the pressure dam bearing will require the least oil flow but give the highest bearing temperature while maintaining a good power loss. The most demanding operating conditions require a tilting pad journal (TPJ) to handle the loads and speeds but needs the most oil flow. The offset half bearing seems to be a nice compromise of the three bearings being able to handle most of the high speeds and loads. It has slightly more power loss but requires less oil than the TPJ.
As anyone who works with rotating machinery knows, bearings are essential to machine functions. One of the first things to know about bearings is that there are two main types. We will go over these two main types and more in this installment of our Condition Monitoring Basics series of blog posts.
The two main types of bearings are:
journal bearings
rolling element bearings.
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Let&#;s start by talking about the simplest kind: journal bearings. Journal bearings have no moving parts, and the portion of the shaft in contact with the bearings&#;or journal&#;slides along the surface.
The pros of using journal bearings are that they are:
economical
compact
lightweight
have a high load-carrying capacity.
The other main type of bearing&#;rolling element&#;are most often used for industrial machinery and come in four standard types: ball, cylindrical, spherical, and tapered.
Some pros of ball bearings are that they separate the rotating shaft from the stationary mounting by rotating balls. This allows for a more even distribution of load and lower friction compared to journal bearings.
Cylindrical roller bearings function via the same principle as ball bearings, but have higher radial load capacity. They typically have lower axial load capacity than ball bearings.
Spherical roller bearings have rollers that are thicker in the middle and thinner at the ends. This type of roller can accommodate both static and dynamic misalignment better than other bearing types. Spherical rollers, however, are more difficult to produce and have a higher friction than cylindrical rollers.
Lastly, tapered roller bearings are designed to handle radial and axial loads simultaneously, whereas other bearings are designed to handle only one at a time.
These are all the main types of bearings and their functions. If you understand how the bearings in your machine function, you are more likely to be able to diagnose a machine&#;s problem, and perform required maintenance, which of course will contribute to the efficiency of your team and plant.
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