Article Summary:Maintenance professionals often report that the biggest problem with their tribology program is antifriction bearing lubrication specifically grease. These issues include over greasing, under greasing, using the wrong grease, and sometimes no grease at all. Over greasing causes high temperatures and results in shedding of oil from grease. Under greasing causes inadequate lubricant delivery. Using the wrong grease can also have the same effect it doesn't properly deliver oil to the loaded rollers. All these will eventually result in lubricant starved bearings and will cause increased energy loss due to high friction. This energy will manifest itself as mechanical energy (ultrasonic) and thermal energy (heat). Several methods for condition monitoring of grease have been published, and many tend to contradict each other to some degree. Some sources maintain tribology as the best method. However, in most circumstances, this is not practical or recommended. Many bearings do not have an access port to the grease. Those that do present their own problems. The grease samples taken most likely do not represent the true condition of the grease inside the bearing and may also contain particulate and contamination picked up during the sampling. The traditional lubrication method has been time-based preventive maintenance. This method can result in over or under greasing, depending on the periodicity of greasing, operating conditions and run time of the machinery.

Article Details:Antifriction Bearing Lubrication Rules of Thumb Overview: Maintenance professionals often report that the biggest problem with their tribology program is antifriction bearing lubrication specifically grease. These issues include over greasing, under greasing, using the wrong grease, and sometimes no grease at all. Over greasing causes high temperatures and results in shedding of oil from grease. Under greasing causes inadequate lubricant delivery. Using the wrong grease can also have the same effect it doesn't properly deliver oil to the loaded rollers. All these will eventually result in lubricant starved bearings and will cause increased energy loss due to high friction. This

energy will manifest itself as mechanical energy (ultrasonic) and thermal energy (heat). Several methods for condition monitoring of grease have been published, and many tend to contradict each other to some degree. Some sources maintain tribology as the best method. However, in most circumstances, this is not practical or recommended. Many bearings do not have an access port to the grease. Those that do present their own problems. The grease samples taken most likely do not represent the true condition of the grease inside the bearing and may also contain particulate and contamination picked up during the sampling. The traditional lubrication method has been time-based preventive maintenance. This method can result in over or under greasing, depending on the periodicity of greasing, operating conditions and run time of the machinery. Objective: The objective of any grease lubrication should be to optimize and maintain the lubrication condition of equipment that falls within the program. The results of non-optimal lubrication include increased friction and loading, higher temperatures, introduction of grease into non-grease areas, introduction of contaminants, and eventually premature bearing failure. To achieve optimum lubrication it is important to be able to determine: The lubrication condition at any given time The conditions when re-lubrication is necessary The volume of grease required for re-lubrication Lubrication Regimes: A schematic representation of lubrication regime as a function of specific film thickness "Improving the Reliability of Machines by Understanding the Failure of Their Moving Parts", Master Series Course taught at CSI by M. Neale and D. Summers-Smith, October 1997 shows the lubrication regimes. Regime Lubricant Thickness D Dry contact 0.00 microns (possible 0.01 micron oxide) B Boundary Lubrication 0.01 micron M Mixed Lubrication 0.01 micron E Elastohydrodynamic Lubrication 1 micron H Hydrodynamic lubrication 20 microns Roller element bearings are designed to take advantage of elastohydrodynamic lubrication regime at the roller-to-race interface, and hydrodynamic lubrication regime at the roller-to-cage interface. Lubrication starvation often causes

boundary and mixed regimes instead of the desired hydrodynamic and elastohydrodynamic modes. The first to go is usually the hydrodynamic lubrication at the roller-to-cage interface. This will result in high friction between the rollers and the cage. Why is the roller-to-cage typically the first to show high friction when grease stops delivering oil to the bearing? It takes about a 20 micron thick oil film to support hydrodynamic lubrication of the roller-to-cage, compared to the 1 micron thick oil film to support elastohydrodynamic lubrication of the roller-to-race. Fortunately, the roller-to-cage interface is lightly loaded so friction does not necessarily mean immediate, high wear. Eventually, however, the boundary lubrication will take its toll on the cage and adhesive wear debris will be released causing secondary damage and accelerated failure progression. If the load and speed are high, the rollers can over heat. Finally, without adequate oil film between the roller and race, friction and torque increase. The increased torque is focused on damaging the surfaces of the rollers, race and cage. Mixing Grease: Approximately 90% of all roller bearings are lubricated using grease; the remainder are oil lubricated. Lubricating grease is produced by suspending mineral or synthetic oil in a thickener, which carries the oil within a network of fibers. Popular thickeners include polyurea, aluminum complexes, and calcium, sodium, and lithium soaps. During normal operation, oil gradually bleeds from the grease thickener, lubricating the bearing's contacting and sliding surfaces. As a general rule, greases that have different thickeners should never be mixed. When incompatible greases are mixed, the resulting lubricant is generally softer than either of its components. The softer the mixture tends to slump in the bearing, and in extreme cases the oil will bleed completely out of the grease mass. In some instances, such as with mixtures involving aluminum complex greases, the opposite effect can occur. The grease can harden. In this case, the base oil is bound tightly in the grease's lattice-like fiber network and is unable to bleed properly. Both softening and hardening have negative effects on grease performance and can lead to premature bearing failure. Greases that have the same thickener and similar base can sometimes be mixed without harming grease effectiveness. However, technicians should be aware that even greases belonging to the same family can differ in formulation and internal chemistry.

Test Equipment for Lubrication Analysis:

As stated previously, lubrication problems will produce energy in the form of ultrasonic sound and heat. Therefore, test methods should include both. A multi-frequency ultrasonic/temperature probe should be used to collect data. The CSI model 7100 SonicScan has the ability to measure decibel (dB) energy levels in three independent frequency ranges. The low, medium and high ranges are approximately 4 kHz, 30 kHz and 40 kHz respectively. The sensor has tuned frequency responses in these three ranges that correspond to common faults. By knowing what fault types show up in each frequency range, technicians can identify, quantify and isolate faults. Characteristics of Lubrication Sound: Most bearing impacts will fall in the 4 kHz range, while lubrication and minor impacting problems will appear at 30 kHz. These are the frequency ranges that need to be examined when analyzing bearings. Under lubrication: In roller element bearings, lubrication sound is created by friction induced stress waves from the interaction of the roller-to-race and the roller-to-cage. As lubrication starvation occurs, the film thickness will decrease resulting in a greater coefficient of friction. The increased friction coefficient creates more energy in the form of heat and sound. At 30 kHz, under lubricated bearings will sound like white noise and has little periodicity common to bearing mechanical faults. The sound will be similar to a rushing river or standing next to a waterfall. Temperature is not generally a good indicator of under lubrication unless lubricant is absent altogether. Studies have shown that partial lubrication starvation has little or no effect on temperature. Over lubrication: Sound analysis has proven ineffective for determining over lubrication conditions. The best method for determining over lubrication is temperature. Studies have shown that at the 30 kHz range, over greasing has little effect on dB levels. However, temperatures can increase dramatically in a relatively short period of time (one test resulted in a 7% temperature rise 12 minutes after inducing an over greased condition). As temperature rises, the rate of grease oxidation and deterioration will increase. Methods for Detecting Lubrication Condition: The CSI Model 7100 has several attachments for measuring ultrasonic noise. The probes pertinent to lubrication analysis include the Range Isolation Magnet (RIM) sensor and the Range Isolation (RI) probe. The best sensor for bearing testing is the RIM sensor. The magnetic mount provides more repeatable data because the contact angle and pressure remain constant. However, the RIM mount does not measure temperature. The RI sensor may not provide the repeatability of the RIM sensor because the pressure applied and the contact angle may vary, but it does measure temperature and can easily fit into tight

spaces. Additionally, the zerk fitting attachment can be used to compensate for the pressure and angle variables. Set the operating mode of the Model 7100 SonicScan to Advanced. Set the application to Mechanical, and set the frequency to 30 kHz. The Display Mode should be set to Averaged. It is recommended that the zerk attachment be used if the RI sensor is to be used for measurement for more consistent data measurements. As a general rule of thumb, the optimum or baseline ultrasonic amplitude at 30 kHz should be 10 dB or less. Normal lubrication amplitudes should be 10 to 20 dB. Testing has shown that the critical level before permanent damage occurs was around 30 dB. This infers that greasing should be performed between 20 and 30 dB. These estimates are very general, and more exact levels should be determined by the type of bearing and the application. This should be accomplished through testing and trending. Re-Lubrication Procedures: Grease in a bearing gradually loses its lubricating properties during operation due to mechanical work, aging and build-up of contamination. It is therefore necessary for grease to be replenished or renewed periodically. Lubrication should always be performed when the lubrication of the bearing is still satisfactory (i.e., before the grease fails). In most cases, motor and bearing manufacturers recommend time interval greasing. Some manufacturers recommend 2 pole motors be greased twice per year and 4 pole and slower motors once per year. Others may recommend a periodicity in hours of operation. Neither recommendation normally takes into account the operating environment or the application, which can greatly effect grease condition. In high temperature environments, oil bleeds from the grease more rapidly than in cold environments. In fact, some sources cite that the time interval should be halved for every 15 oC increase above 70oC. Therefore, adhering to these recommended periodicities can sometime result in partial starvation or over greasing, depending on the environment. The volume of grease required to replenish a bearing in a partial grease starvation condition is the most difficult aspect of a lubrication program. As a rule, most bearing manufacturers recommend that the free space of the bearing be filled, while the free space of the housing should only be partially filled (usually 30 to 50%). Determining how much grease actually exists in the bearing and the required volume to top off the housing cavity is problematic at best. The only truly reliable method is to gain access to the interior of the bearing, which is not practical and may cause problems such as introducing contaminants to the bearing. Additionally, different bearing designs may require different procedures to ensure proper lubrication is maintained.

Manufacturers normally provide recommended periodicities and quantities for lubrication. However, their recommendations may not factor in high ambient heat or punishing applications/environments that can break down grease prematurely. One source, Cibro Agencies (Pte.) Ltd., recommends that the volume of grease used be derived from a formula. For bearings smaller than 300 mm and the lubricating interval is less than 6 months, Cibro estimates suitable quantities to be: G = 0.005*D*B Where G = grease quantity in grams D = outer diameter of the bearing in mm B = bearing width in mm. Technicians must ensure that exit holes are provided in the housing for removal of excess grease. Using this formula, used grease should theoretically be completely purged after three replenishments. For periodicities of longer than 6 months, or after every three replenishments, Cibro recommends that all the used grease be removed from the bearing and replaced with fresh grease. The downfall to this approach is the lack of periodicity of replenishment and the technical knowledge required for each bearing. Using the 7100 SonicScan, a greasing program can be created and tailored to individual applications and machine trains. However, the fine-tuning of a program can take significant time and commitment. Long term trending is required on individual machines to determine normal dB levels for each application and machine. Once these thresholds are established however, the time commitment is reduced dramatically. CSI recommends that if a bearing exceeds the 30dB threshold (for normal applications), grease should be added until the dB level drops to acceptable levels. If the noise level does not drop by the time manufacturer's recommended quantities are reached, stop greasing. Never exceed manufacturer's recommended quantities. If the dB level drops, stop greasing and note the amount added. Additionally, the temperature of the bearing should be monitored up to one half hour after greasing. Significant temperature rise is an indicator of over greasing. Recheck the bearing within a week and note the dB level. If the noise level has risen significantly, add more grease until the noise level drops, then add several more grams not to exceed manufacturer's recommended quantities and note the amount added. Recheck the bearing in another week. Repeat these functions until the greasing interval is close to manufacturer's interval recommendations, unless the application is severe. In this case, more frequent intervals may be required. Additionally, as stated, some bearings or applications will produce higher amplitude noise levels and this procedure will require some latitude in what are considered acceptable dB levels.

The procedure described above is simplified. CSI has an intensive program implementation plan that can be tailored to various facilities. Questions or comments can be directed to mailto:thomas.burnett@compsys.com. Interested in a greasing program for your facility? Contact CSI Services at 865-675-2400. Best regards, Tom Burnett Assistant Product Manager, Motor Management Services, Ultrasonics and Infrared Thermography Emerson Process Management CSI Division