While every type of rolling bearing in rotating equipment offers the expectation of a certain service life, sometimes the life can be threatened for any number of reasons along the way. As safeguards, implementing proper maintenance practices and using the correct enabling tools at every stage of a bearing’s life cycle ultimately can make all the difference in maximizing bearing service life and promoting plant productivity and efficiency.
Handle with Care
Regardless of application, 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 to avoid contamination risks. If bearings are kept in a standing position, the likelihood of false brinelling (wear of the raceways and rolling elements caused by residual vibration) will increase significantly.
In the cases of sealed or shielded bearing types, caution should be exercised when storing them over long periods of time. This is because the lubricating properties of the grease used in these types of bearings may deteriorate, resulting in potential lubricant-related problems down the road. (Most bearing manufacturers have specific shelf-life limits, based upon the greases used in their bearings.)
In addition, 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.
Prior to mounting a bearing, the maintenance staff should confirm that shaft and housing are clean, undamaged, and dimensionally accurate (with proper fit and tolerance); lubricant is fresh and correctly specified; necessary tools and equipment are on hand, and safety precautions are in place.
Bearings should be handled with care using the appropriate gloves (especially helpful for hot or oily bearings) and appropriate carrying and lifting tools to enhance safety and readily facilitate bearing positioning onto a machine’s shaft.
Because they are precision components, bearings should be mounted using correct techniques and technologies. The 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 for reliable installation: Employing fitting tools to avoid harmful brute force and applying the proper force to both bearing rings, isolating the rolling elements from impact.
Hot mounting, where a 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 mounting larger sized bearings, hydraulic assist tools are recommended. Hydraulic devices 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.
Selecting and applying the proper lubricant in the correct amount at the required time interval is essential in realizing optimized bearing performance, reliability, and service life.
Lubricant provides a separating film between a bearing’s rolling elements, raceways, and cages to prevent metal-to-metal contact and friction that otherwise would generate excessive heat leading to adhesive wear and subsequent metal fatigue and spalling of a bearing’s contact surfaces. Proper lubrication additionally acts to inhibit wear and corrosion and helps guard against damage from contaminants.
Half of all bearing failures attributed to poor lubrication are caused by the use of an improper grease type for the operating conditions or by mixing incompatible greases exhibiting different properties. Therefore, it is imperative that the correct type of grease be used with the necessary thickener and base oil viscosity in the proper amount at the prevailing operating temperature of an application. Greases can further be formulated to achieve distinct characteristics by varying oil viscosities, soap, and additives to accommodate application requirements and operating conditions.
In service, sufficient lubrication of bearings is just as important as selecting the proper type of lubricant. Lubricant-delivery methods include technologies for manual relubrication (such as grease guns) and continuous relubrication systems providing quantities of fresh lubricant on a regular basis, while increasing safety and minimizing maintenance attention. Ready-to-use or tailored automatic lubricator systems can deliver a consistent, correct, and contamination-free grease supply.
Over time, lubricant in any bearing arrangement can gradually lose its lubricating properties due to mechanical work, aging, and/or the buildup of contamination. This underscores the maintenance-related need for grease to be replenished or renewed or for oil to be filtered and changed at regular intervals.
Align the Application
After a bearing has been mounted in an application, such as a motor connected to a pump, the components should be properly aligned. If not, the misalignment can cause the bearing to suffer additional load, friction and vibration, which can accelerate fatigue and reduce the service life of bearings and machine. Even worse, increased vibration and friction can significantly increase energy consumption and the risk of premature bearing failures. An assortment of shaft and belt alignment tools and machinery shims have been developed for this purpose.
Industrial maintenance strategies have clearly shifted over the years from reactive to proactive approaches. The historical strategy of preventive or time-based maintenance has largely given way to predictive and/or proactive maintenance, which is based on the condition of rotating equipment when in operation. This strategy is grounded in the principle that while machinery failure may be unpredictable, emerging failure is detectable.
When equipment fails without warning, the unplanned downtime, unanticipated deployment of maintenance staff, related costs, and resulting lost productivity will disrupt any operation. Advance warning of developing faults will always be a plus in realising optimised machine productivity and reliability—not to mention supporting a safe work environment.
Condition monitoring technologies support predictive maintenance objectives by equipping operations with the surveillance tools to collect data reflecting the health of machinery and uncover faults for timely fixes before they can worsen.
Data-collection options range from basic handheld or stand-alone units to more sophisticated, connected, and networked systems. Some devices and instruments focus on assessing the more significant physical operating parameters, such as vibration and temperature, while highly engineered systems at the other end of the spectrum can be configured to monitor and diagnose many more. The capabilities expand with integration into computerised maintenance management systems, the use of specialised software programs, and application modules and accessories targeting specific types of analysis.
For especially critical assets, continuous on-line condition monitoring systems offer indispensable lifelines. On-line systems (hardwired or wireless) deliver up-to-the-minute information by looking at data 24/7 and storing scheduled and/or alarm data via permanently mounted sensors. The data can then be transmitted via the hardwired or wireless system to a host computer running relevant software packages.
All these combined initiatives can optimize the potential of a bearing throughout the life cycle in any application. Further insights can also be gained when partnering with an experienced bearing manufacturer with the know-how and expertise to help keep bearings performing reliably as intended.
By Paul Sadimas