Three main journal bearing types, their selection

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.

This article contains excerpts from "High performance bearing comparison" at the 2015 Turbomachinery Symposium.

Pressure Dam Bearing[/caption]

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.

Offset Halves Bearing[/caption]

Tilting Pad Journal Bearing[/caption]

Tiling 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.