Proper Bearing Maintenance From Start to Finish
For any type of bearing in rotating machinery, applying best maintenance practices and using the correct enabling tools can help contribute to maximum bearing service life. Courtesy of SKF.
The primary messages: Bearings should always be properly stored, mounted, adequately lubricated when and where required, monitored, dismounted, and ultimately inspected to uncover root causes of any damage.
While particular applications will present unique factors influencing a bearing’s service life, the following general maintenance-oriented rules of thumb can make all the difference in the world – before bearing installation, while the bearing is performing, and when it may need to be replaced.
Making the initial right moves
From the outset, proper storage is critical. Bearings should always be stored in a cool, clean, low-humidity environment free of dust, shocks, and vibrations. (For these reasons, storing bearings directly on a floor should be avoided.) They should ideally be stored flat rather than on end and be kept in their original, unopened packages until just before mounting. If kept in a standing position, the likelihood of false brinelling (wear of the raceways and rolling elements caused by residual vibration) increases significantly. False brinelling is much less likely to occur in bearings laying flat.
In the cases of sealed or shielded types of bearings, operators should be cautious when storing them over long periods of time. The lubricating properties of the grease used to fill these types of bearings may deteriorate, causing potential lubricant-related problems when a system is up and running. (Most bearing manufacturers have specific shelf-life limits, based upon the greases used in their bearings.)
The importance of cleanliness cannot be over-emphasized. All bearings should be kept clean, because contamination and corrosion will shorten the life of any bearing.
When a bearing is ready to be mounted, maintenance staff should confirm that shaft and housing are clean, undamaged, and dimensionally accurate (with proper fit and tolerance); lubricant is clean and correctly specified; necessary tools and equipment are on hand, and safety precautions are in place.
When mounting a bearing, never strike it directly with any hard object, such as a steel hammer or a chisel, and never apply the mounting force through the rolling elements.
Because they are precision components, bearings should be handled and mounted with care using correct techniques and technologies. An estimated 16% of all premature bearing failures are caused by poor fitting, usually using brute force, and being unaware of the availability of the suitable mounting tools and methods.
The primary methods for proper mounting of a bearing are commonly referenced as “cold” or “hot,” consistent with their enabling technologies. Cold mounting, or mechanical mounting, generally is recommended for small and medium sized bearings (with outside diameters up to 4 inches); methods involving heat mounting will be appropriate for relatively larger bearings; and hydraulic techniques should be considered when mounting especially large bearings. Tools have been developed to accommodate each particular method.
In cold mounting, the misguided practice of using a standard hammer and pipe for the job has long been discredited due to the damage that can occur. This practice can cause debris to enter the bearing or, if not done properly, a pipe can slip and impact the internals of the bearing. Best practice: The use of fitting tools to eliminate harmful brute force and apply the proper force to both bearing rings to isolate the rolling elements from impact force for a more reliable installation.
Hot mounting, where the bearing is pre-heated, provides a practical solution to allow for a bearing’s expansion and subsequently easier installation, while maintaining specified interference fit after the job is completed. Induction heaters can integrate various features to help prevent bearing damage during the heating process. These solutions stand in direct contrast to less effective (and potentially dangerous) methods, including an open flame, hot oil baths, and ovens or hot plates.
For the larger sized bearings, hydraulic techniques and compatible tools and equipment deliver the goods. Hydraulic techniques allow for more control and further help to maintain precision, accuracy, and repeatability; minimize the risk of damage to bearings and shafts; require less manual effort; and promote greater operator safety.
Choosing the proper bearing lubricant will help bearings perform as long as intended. Good lubricants primarily provide a separating film between a bearing’s rolling elements, raceways, and cages to prevent metal-to-metal contact and undesired friction that otherwise would generate excessive heat that could lead to adhesive wear and subsequent metal fatigue and spalling of the bearing contact surfaces. The proper lubrication further acts to inhibit wear and corrosion and helps guard against contamination damage.
Half of all bearing failures attributed to poor lubrication are caused by selection of an inadequate grease type for the operating conditions or to mixing incompatible greases with different properties. Therefore, it is imperative for optimized bearing performance that the correct type of grease be selected to deliver the necessary base oil viscosity in the proper amount at the prevailing operating temperature.
Grease has traditionally served as the preferred lubricant for rolling bearings. The practical benefits become apparent: Grease is easy to apply, can be retained within a bearing’s housing, and offers protective sealing capabilities.
Greases are classified by their stiffness or consistency according to the U.S. National Lubricating Grease Institute (NLGI) and are graded from NLGI Class 000 (very soft) to 6 (very stiff). These classifications are based on the degree of penetration achieved when a standard cone is allowed to sink into the grease at a temperature of 25°C for a period of five seconds.
Grease composition is roughly 85% base oil (mineral or synthetic) and 15% soap or thickener and will vary from grease to grease. The base oil is the oil inside the grease, which provides the lubrication under the operating conditions. The soaps or thickeners hold the oil and/or additives together to enable the lubricating grease to function and, in some cases, may enhance the lubricant film. (The type of thickener gives the grease its typical characteristics, retaining the oil in a similar manner to a sponge retaining water.) Additives provide additional characteristics such as wear/corrosion protection and friction-reducing effects.
By varying oil viscosities, soap, and additives users can benefit from greases with distinct characteristics able to suit particular applications and operating conditions.
Turning to bearings in service
In service, sufficient lubrication is essential. Maintenance goals: Deliver the right lubricant in the right amount at the right time.
Among lubricant delivery methods, manual lubrication (with grease gun) typically can present major challenges for maintenance technicians if the appropriate tools, practices, and knowledge are absent – and reliability can further be affected by under- or over-greasing. As a practical alternative, automatic lubrication can be employed to provide quantities of clean lubricant on a regular basis, while increasing safety and saving time for staff. Ready-to-use or tailored systems can be engaged, depending on application, lubricating points, and similar considerations.
Over time, the lubricant in a bearing arrangement gradually will lose its lubricating properties due to mechanical work, aging, and/or the buildup of contamination. This underscores a maintenance-related necessity for grease to be replenished or renewed and for oil to be filtered and changed at regular intervals to help promote maximum bearing service life.
To gain long bearing life it is imperative to determine the condition of machinery and bearings while in operation. This can be accomplished with a process known as “condition monitoring.”
Condition monitoring allows for the repair of components detected as problematic prior to their failure. This is accomplished by performing condition-based maintenance. The approach not only reduces the possibility of catastrophic failure, but also allows plant personnel to order parts in advance, schedule manpower, and plan unrelated repairs during the downtime.
The most significant machine-condition parameters to help monitor the health of a bearing include (in no specific order) noise, temperature, speed, vibration, and alignment. A variety of measuring instruments will enable users to analyze all factors.
When a bearing must be taken out of service, for whatever reason, proper dismounting practices should be followed.
One reason for dismounting an old bearing is to replace it with a new one. When proceeding, care must be taken not to damage the shaft in the process, which can result in compromising a machine's efficiency. A damaged shaft can greatly influence the service life of the new bearing.
Another reason to dismount bearings is for maintenance or replacement of other machine components. Since these dismounted bearings will be mounted again (unless they are damaged during dismounting), proper dismounting methods and tools should be enlisted. Choice of tools will depend on bearing type, size, and fit.
In situations where a bearing must be replaced due to premature failure, detective work focusing on bearing and grease analysis can help point to root causes. Among common root causes: inadequate lubrication, contamination, errors in mounting/dismounting, and/or electrical damage. Pinpointing the actual cause(s) will help prevent a repeat of history.
When it comes to bearing maintenance, every decision and practice can impact a bearing’s performance, reliability, economy, and service life. Partnering with an experienced bearing manufacturer can open the door to the knowledge and technologies that will support maximized bearing life and reduced maintenance time, labor, and costs.
By Daniel Juchniewicz, Applications Engineer for SKF USA Inc.