Rolling bearings are high-precision machine elements whose service life directly determines the performance of machines. However, the actual service life is determined by many factors. Premature bearing failures cause costly equipment downtime, sometimes even with very serious consequences. Bearing experts provide some simple yet practical tips to optimize bearing performance.
Start with the right choice
Right from the very beginning, design engineers could enhance the bearing service life by selecting the right bearings for the application. Many factors — such as loads, rigidity, bearing life expectation, operating environment, etc. — need to be considered. Renowned bearing manufacturers have years of experience in different industrial applications. Developing bearing solutions with their assistance can contribute to optimal bearing and equipment service life.
Bearings from renowned manufacturers are produced with the latest technology and undergo stringent quality assurance procedures. Nevertheless, to guarantee the optimum bearing service life, special attention should be made in the following aspects: proper storage, careful mounting and dismounting, adequate lubrication and re-lubrication, appropriate condition monitoring, timely maintenance, and sound personnel training.
In principal, all bearings should be stored in their original packaging until being mounted. They should be kept in a clean, non-humid environment at a fairly stable room temperature. Rolling bearings should be stored away from dust, water and aggressive chemicals. Vibrations and shocks could permanently damage the bearings mechanically and therefore must be avoided during handling and storage.
Basically all bearings must be stored flat. Particularly larger and thus heavier bearings might be deformed by their own weight if they are left standing vertically for a long period.
Special care should be taken for the storage of pre-greased (sealed or shielded) bearings. Such grease could change in consistence over a long storage period. This could raise the running noise to a certain extent when put in operation for the first time. Therefore the shelf life of such bearings should be controlled by an FIFO-system (First In First Out).
Cleanliness is paramount when dealing with rolling bearings. The running surfaces and rolling elements usually have a surface finish roughness of tenths of microns (1/10 µm or 0.0001 mm). Such smooth surfaces are very sensitive to damages by contaminants. The lubrication layer between the running surfaces has usually a thickness between 0.2 to 1.0 µm. Impurities with particle size larger than the lubricants could get over rolled by the rolling elements and thus build up localized stresses in the bearing steel and eventually cause premature material fatigue. Normal environment dust has a grain size of up to 10 µm, which could already damage the bearings.
Therefore, a clean, dust-free environment is extremely important for bearing storage and mounting.
Thorough preparation for mounting
Bearings should be mounted and dismounted carefully by means of appropriate tools. Industry experts estimate that improper fitting causes 16 percent of all premature bearing failures.
For volume mounting in the production assembly the conditions are usually strictly controlled, and the suitable equipment is available for bearing installation. However, for maintenance or replacement work, the environments could vary. Therefore, thorough preparation for bearing fitting is necessary in order to ensure the optimum bearing service life. First of all, the relevant documentation, such as drawings, maintenance manuals, specifications, etc., should be carefully studied. All components, such as shafts, distance rings, housings, cups, flanges, etc., must be thoroughly cleaned and protected from contaminants. The conditions of such adjacent components should also be checked carefully.
Careful mounting and dismounting
Depending on the application, size and type of the bearing, an appropriate mounting method — mechanical, thermal or hydraulic — and tools should be selected. Here are some basic rules for bearing mounting:
Mounting forces should never be applied through rolling elements. This could easily lead to localized overloading in the contact area between the rolling elements and raceways which in turn causes premature bearing failures.
The bearing surfaces should never be hit directly with any hardened tools such as hammers, cotter pin drives, etc. This could cause a breakage or fragmenting of the bearing rings.
The instructions from the respective mounting equipment supplier should always be followed.
About 90 percent of rolling bearings are never removed from the equipment where they are built in. Usually only the larger bearings would be removed as part of the scheduled preventive maintenance programs. Same as mounting a bearing, dismounting should also be thoroughly prepared. During the dismounting, ensure the adjacent components such as the shaft or housing are not damaged. Appropriate methods and tools should be used for dismounting, depending on the bearing type, size and application.
The lubricant separates the metallic bearing surfaces such as rolling elements, rings, and cages and thereby reduces friction, preserves the metal parts and guards off contaminants and impurities. A wide range of lubricants — including grease, oil, and solid — is available for different operating conditions. The correct selection of lubricant is crucial to ensure optimal bearing and equipment service life.
Bearing lubricants undergo permanent mechanical stressing caused by the over-rolling of rolling elements. Moreover, lubricants change their chemical properties over time, particularly at high operating temperatures and in humid or polluted environments. All these lead to a gradual loss of lubricating quality.
Therefore bearings have to be re-lubricated at regular intervals to ensure maximum service life. The re-lubrication interval depends on operating conditions such as temperatures, running speeds, loads, environment, etc.
Only in case of pre-greased bearings (shielded or sealed bearings), i.e. "greased-for-life" bearings, the bearing service life is determined by the lubricant service life span.
Lubricants must be stored properly according to manufacturers’ instructions. Particular attention must be paid to keep the lubricant clean from any contamination. Prior to each application, the condition of the lubricant should be checked carefully.
Condition monitoring and maintenance
Generally rolling bearings are extremely reliable although they do not have an indefinite life. Like all other important components in the machinery, they should be inspected and maintained regularly. How often the inspections and maintenance should be carried out depends on the importance of the particular application and operating conditions of the individual equipment.
For bearing arrangements with critical functions, it is advisable to incorporate a condition monitoring feature at the design stage. Important parameters of the machine operation such as vibration and noise can be monitored continuously. Preventive measures could be planned before breakdowns.
Practice makes perfect. But proper training provides the basis for the practice. Reputable bearing manufacturers offer various training programs for commercial, technical and workshop staff. Costly human errors can be avoided if maintenance technicians possess fundamental knowledge in handling bearings. Design and product development engineers can maximize the equipment performance and minimize life-cycle costs by optimal design of bearing locations.
Bearings are often critical components in all machines. Proper storage, careful mounting and dismounting, adequate lubrication and re-lubrication, appropriate condition monitoring, timely maintenance and, last but not least, sound personnel training are essential to improve bearing service life, and therefore enhance equipment performance.