How to achieve optimum bearing performance in gearboxes

NSK products are known across the globe for their reliability and long life, as well as their economic and environmental credentials. The design of drive systems can be a complicated process, as can diagnosing the cause of a power transmission failure. NSK has developed a useful guide to assist engineers and designers alike in this complex field.

Bearings used in modern industrial drive systems must meet the most demanding requirements. The range from domestic washing machines to wind turbines to mining machines is vast, and each application requires a different combination of values to be met. The NSK product range varies from miniature bearings with a bore of 1mm up to rolling bearings with a diameter of 5m. The development of heat and seizure resistance, improved material and lubrication technology have been at the forefront of the NSK ethos.

Designers of industrial drive systems continue to develop new gear types and configurations so the bearing selection process must be considered as crucial in maintaining the desired performance of the drive system.

When selecting a suitable bearing type, the special operating conditions of the respective bearing locations have to be taken into consideration, illustrated as follows:

• Full complement cylindrical roller bearing for low speed and high radial loads
• Spherical roller bearings for ultra-high loads and components in an oblique arrangement
• Cylindrical roller bearings for high speed and high loads
• Four-point bearing, acting as the locating bearing for the high-speed stage, while the cylindrical roller bearing takes the radial load

Interaction between load and speed

All the determining factors need to be carefully considered in order to ensure the correct specification is produced for each application. The load, or input torque, must be determined and established as either variable or constant. The level of speed and direction of rotation need to be evaluated as well as the interaction between the load and the speed.

The arrangement of the shafts is a vital issue to be considered, 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.

One crucial factor is bearing life, which is mainly driven by the application itself. NSK has developed the ABLE-Forecaster (Advanced Bearing Life Equation) which uses data from actual applications and tests, which has been accumulated over decades of work in this field, and combines it with data for lubrication systems, environmental conditions, contamination factors and material analysis.

A further crucial consideration in the selection process is the choice of materials for the construction of the bearing. Both the material of the bearing itself as well as any sealing materials must be carefully considered to ensure the drive system performs to the desired level.

The environment in which the bearing must perform has a major impact on the final design of the component. Exposure to dust, humidity, temperature extremes or aggressive agents must be considered, and accounted for, in the process of selecting the correct bearing. This also affects the type of lubrication that can be used - and may determine the sealing required on each component. Lubrication has a major impact on bearing life, so care is required to strike the right balance.

In addition, the method of assembly which is to be used for the drive system is an important factor in determining the bearing design, since some bespoke designs need to be assembled by hand. This is also considered alongside the maintenance requirements for the drive system, which may need to be disassembled at regular intervals.

All of the factors reviewed so far will have an impact on the costs of production and these must be balanced to ensure that benefits of improved performance and reliability can be justified for the application in question.

NSK produce more than 35,000 different types of ball bearings and roller bearings, but they continue to work on new developments and improvements, to ensure they stay one step ahead of the field. Free copies of the publication Bearings for industrial gearboxes 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. Please visit www.nskeurope.com for further information.