A perfect fit: the importance of mounting bearings properly

Published:  11 January, 2021

The way in which bearings are mounted has a profound effect on both the performance and reliability of these vital machine components. Mark Townsend* lays-out the key principles for mounting bearings successfully and describes a tool that ensures such operations can be carried-out quickly and accurately

The reliability of bearings is quite remarkable. Each year, approximately 10 billion bearings are manufactured around the world and only a small fraction of them will ever fail. Indeed, over 90% of bearings will outlive the equipment in which they are installed. Only around 0.5% of these bearings are replaced because they are damaged or fail in service, but when this happens, it can cause significant problems and costly downtime.

Generally speaking, one third of bearings fail due to fatigue and one third because of issues with lubrication. Another one sixth of bearings fail due to contamination. The rest fail because they have been handled or mounted incorrectly—issues that are entirely avoidable.

Fatigue-related failures

Impact damage caused to bearings during their handling, mounting and storage is primary cause of early fatigue-related failures. Mounting force, for instance, when applied to the wrong ring, can pass through the rolling elements of the bearing, creating dents in both the raceways and the rolling elements themselves. Damage propagates from the points of these deformations and ultimately results in premature failure.

Another cause of early fatigue failures is the presence of contaminants introduced into the bearing or housing during mounting. Further, cylindrical roller bearings can be damaged easily during assembly. In the case of NU-design bearings, this can happen, for example, after the inner ring is fitted to the shaft and the outer ring with the cage and roller assembly is installed in the housing. If the shaft is askew during assembly and not rotated, the rollers can scratch (plough) the raceway of the inner ring, causing indentations in the form of long, transverse streaks.

Proper preparation prevents poor performance

The proper mounting of bearings calls for experience, care, cleanliness, accuracy, theselection of the correct mounting method and the use of the appropriate tools for the job. Preparation is key. Prior to mounting, drawings and/or instructions must be studied to determine the correct order in which the various parts of the bearing must be assembled, the type, size and variant of the bearing to be installed, the appropriate lubricant and the quantity in which it should be applied, the correct mounting method and the tools required.

Keep clean

Cleanliness is essential to ensure that a bearing has a long service life and it begins in the storage room. Bearings must be stored in a cool, dry area on shelves that are not subjected to vibrations from near-by machinery. Packaging must not be removed from the bearing until it is time for it to be installed.

Normally, the preservative applied to new bearings does not need to be completely cleaned away—it is only necessary to wipe-off the outside diameter and bore surfaces. Whenever possible, bearings should be installed in a dry, dust-free area, away from metalworking or other machines that produce swarf and dust. The bearings and all mating parts, including the lubricant, must be clean and free from damaging contaminants.

Where a bearing has to be mounted in an unprotected area, steps must be taken toprotect it and the mounting position from contaminants, such as dust, dirt and moisture, until installation work has been completed. This can be achieved by covering or wrapping the bearing and machine components with plastic or foil.

The importance of accuracy 

A bearing will only perform satisfactorily if its associated components are geometrically accurate and if the prescribed tolerances are adhered to. When measuring these, it is important that the components and the measuring instruments are at approximately the same temperature. This is particularly important where large bearings, and their associated components, are concerned.

The right tools for the job

When performing installation work, gloves should be worn, and carrying and lifting tools specially designed for mounting bearings should be used. Further to keeping operatives safe, using the proper tools will save them time and effort.

A good dismount

Likewise, there is always the potential to damage an otherwise good bearing during dismounting. Therefore, whenever possible, such operations should be avoided. However, if a bearing must be dismounted and it is intended for re-use:

No part of the bearing should be hit directly.

The dismounting force must not be allowed to be transmitted through the rolling elements.

The bearing must not be heated with an open flame.

After the bearing has been dismounted, it must be cleaned with a suitable solvent and dried carefully. All parts of the bearing, and especially the raceways, rolling elements and cage, should be inspected for wear or damage. If the bearing can be reused, it can be protected against corrosion with a thorough coating of grease, oil or an anti-corrosive fluid, and repackaged.

As the old saying goes, if you look after your tools, then they will look after you. In the case of bearings, this duty of care begins with their mounting. Mounted correctly, bearings will provide many years of reliable service.

Mobile assistance

There is much to remember when mounting and dismounting a bearing—even before we consider the specifics of the many different methodologies that could be used. As such, SKF has developed a mobile application to ensure that these processes can be carried out quickly, accurately and easily. The application, called SKF Bearing Assist, can guide operatives step-by-step through the entire mounting process. In the first instance, the bearing needs to be scanned. SKF Bearing Assist will then provide all of the information needed by the operative to plan and prepare for the mounting of the bearing, listing the tools and parts required, and providing detailed instructions. SKF Bearing Assist will help in the selection of the most appropriate bearing for a given application and can be used to keep track of bearing exchanges. Further, the application enables mounting procedures to be identified, documented and quickly shared with maintenance teams—meaning that mounting knowledge can be transferred 

The calculated life expectancy of any bearing is based on eight assumptions:

1.The bearing is of high quality and has no inherent defects.

2.The bearing is the right one for the job.

3.The dimensions of the parts working with the bearing, such as the shaft and housing seats, are appropriate.

4.The bearing is mounted correctly.

5.The correct lubricant, in the proper quantity, is always available to the bearing.

6.The bearing arrangement is properly protected (sealed).

7.The operating conditions are matched to the bearing arrangement.

8.The recommended maintenance is performed.

*Mark Townsend is SKF’s service engineering manager.

https://twitter.com/SKFgroup        https://www.linkedin.com/company/skf/

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