Selecting bearings for industrial gearboxes

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The requirement to improve the efficiency and performance of mechanical gearboxes has led to more powerful designs which, in turn, require bearings designed to deliver long service life. Understanding the role and design criteria for the different types of bearing within a gearbox is the key to ensuring the correct specification is used when constructing or repairing a gearbox, whatever the size. NSK is now using advanced materials and live predictive models to find the optimum product.

Selecting bearings for industrial gearboxes NSK has invested considerable time and money in developing new materials and designs to meet, and surpass, the requirements of industrial gearboxes. What follows is a review of the company’s mission and its recent achievements in the area.

In the modern industrial environment, mechanical drive systems are required to deliver reliability and performance in a wide variety of conditions. The requirements for the bearings employed in a precision operation such as printing press machinery are very different from those used in say, a wind turbine, where a high degree of maintenance-free operation is required. Understanding these requirements is crucial to developing bearings which will perform and provide reliable and cost effective service.

Within any gearbox there are a number of bearings, each with a specific purpose, and different operating conditions, therefore, requiring differing designs. Different gear designs make specific demands on the bearings, while the arrangement of the gear shafts and the lubrication system also dictate the requirements of the bearing design. The ability to collate all of the relevant information and translate it into a specific design is one of the first major challenges facing the bearing manufacturer.

A rotating gear shaft requires at least two bearings for optimum support and guidance with regard to the stationary element. Rolling bearings not only accommodate the respective radial and axial loads, but they also allow for shaft thermal expansion: a fundamental issue that concerns the arrangement of a thrust (fixed) bearing and floating bearing and any necessary set-up adjustments.

The thrust bearing is designed to accommodate the axial loads of the gear mesh, while the floating bearing takes the radial loads and accommodates the axial movement, as well as compensating for production tolerances and changes in operating temperature. This leads to making the selection of the basic bearing type and combination with other bearings on each drive shaft.

Arrangement of bearings

Calculating bearing load and direction requires considerable knowledge and experience, and is crucial in making the correct bearing selection. Bearing type is just the beginning of the process, which leads on to the arrangement of the chosen bearings as well as base material, size, cage design and lubrication systems.

The choice of materials for the construction of the bearing is also a crucial consideration in the selection process. Both the material of the bearing itself as well as the rolling element retention cage and any sealing materials must be carefully considered to ensure the drive system performs to the desired level.

NSK has examined vast numbers of failed bearings, using advanced inspection techniques and has used its research to develop its Super-TF material, which, thanks to a new material composition and special heat treatment, provides a significant increase in bearing life under contaminated lubrication conditions, up to 10 times that of standard material. In addition, it also improves resistance to flaking, wear and thermal stresses.

One other crucial factor which has to be predicted is bearing life, which is mainly driven by the application itself. To help on this front, NSK has developed the ABLE-Forecaster (Advanced Bearing Life Equation), the advanced software which uses data from actual applications and tests, which have been accumulated over decades of work in this field, and combines it with data for lubrication systems, environmental conditions, contamination factors and material analysis to provide a predictive model for making important design decisions.

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. By using in-depth knowledge of drive transmission systems, NSK can provide expert assistance to designers and maintenance engineers alike to ensure the balance between improved performance and cost effective reliability is found.

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 materials and lubrication technology make up a crucial part of the NSK ethos. NSK produce more than 35,000 different types of ball and roller bearings, but they continue to work on new developments and improvements, to ensure they remain one step ahead of the field.

In order to help those involved in the bearing selection process, NSK has produced a guide to help in understanding the basics as well as developments in bearing design. Free copies of the Bearings for Industrial Gearboxes brochure 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.

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