Achieving optimum bearing performance in gearboxes

Power transmission bearings cover a vast range of applications, from domestic washing machines to wind turbines and mining machines. Many of these will involve the use of a gearbox and, as new gear types and configurations are developed, so too must bearing selection be considered crucial to this development process.

When selecting suitable bearings for a gearbox the special operating conditions of the respective bearing locations have to be taken into account. For example, full complement cylindrical roller bearings may be best for low speeds and high radial loads, whereas cylindrical roller bearings will be more suitable for high speeds and high loads.

Some configurations may include, a four-point bearing, acting as the locating bearing for the high speed stage, while a cylindrical roller bearing takes the radial load. And where ultra-high loads have to be borne in an oblique arrangement, spherical roller bearings should be considered.

The load, or input torque, must be determined and established as either variable or constant. The speeds and direction of rotation need to be evaluated as well as the interaction between the load and the speed.

The arrangement of the gearbox shafts is another important consideration, since the requirements of a vertical configuration are very different to those for a horizontal or inclined arrangement. This, along with any considerations for axial movement will affect the design of the lubrication and sealing systems.

Where reliability is concerned, one crucial factor is bearing life, and the application will have a considerable influence on this. Using the data it has accumulated over decades studying real world applications and combining this with data for lubrication systems, environmental conditions, contamination factors and material analysis, NSK has developed a useful tool for bearing life prediction called ABLE-Forecaster (ABLE being an acronym for Advanced Bearing Life Equation).

The materials of construction of the bearing will have implications in terms of overall reliability and longevity. Both the material of the bearing itself, as well as any sealing materials, must be carefully considered to ensure the drive system performs as expected.

Operating environments will generally dictate materials of construction, while exposure to dust, moisture, temperature extremes or aggressive agents will also affect the choice of lubrication and sealing arrangement. Ultimately, the choice of lubrication will have a major impact on bearing life.

Finally, some attention must be given to the method of assembly of the gearbox and its ongoing maintenance, particularly as some bespoke designs will be assembled by hand and there may be a need to disassemble a gearbox at regular intervals.

All of these design considerations will have an impact on the costs of production and these must be balanced to ensure that benefits of improved performance and reliability are justifiable in terms of the application requirements.

Free copies of the Bearings for Industrial Gearboxes publication are available from NSK: Freephone 0500 2327464. Callers should quote the publication reference: IG/C/E/12.11 and also the title of the publication.