Maintenance professionals often report that the biggest problem with their tribology program is antifriction bearing lubrication - specifically grease. 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 nongrease areas, introduction of contaminants, and eventually premature bearing failure. These issues usually are initiated by 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. The latter two will eventually result in lubricant starved bearings and will cause increased energy loss due to high friction.
Approximately 90% of all roller bearings are lubricated using grease; the remainders 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.
The mixing of greases with incompatible thickeners such as polyurea and lithium stearate greases has become common place as most motor manufacturers use polyurea fills. This information is not often relayed to maintenance individuals in time prior to the next lubrication resulting in cross contamination and grease degradation and ultimate lubrication failure.
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. 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 and the quantity to re-lubricate. To succeed in this endeavor the lubrication engineer must establish a comprehensive grease program consisting of a sampling procedure / method, an analytical testing program and the use of ultrasonics/sonics, vibration or other technology in establishing the volume of grease required for re-lubrication.
In under lubricated 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.1
In over lubricated roller element bearings sound analysis has proven ineffective for determining over lubrication conditions. The best method for determining over lubrication is temperature. Some studies show 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 condition2). As temperature rises, the rate of grease oxidation and deterioration will increase.
Obtaining the samples-the next challenge in standardizing grease analysis
As a "rule of thumb", in most circumstances, procedures for obtaining representative grease samples from bearing housing and gears are not consistent and most likely do not represent the true condition of the "worked" grease near the bearing. It may also contain particulate and other contamination picked up during the sampling process.
In-service grease samples from motors, valves, and various bearing housings, typically require the equipment to be out of service. A key factor is that a large volume of sample is needed to perform current analytical testing methodologies and along with this issue is that it is extremely difficult to obtain a representative "worked" sample from near the bearing while the component is still sealed.
Therefore our next challenge in optimizing a grease analysis program is the development of test methodologies to measure in-service grease conditions utilizing a smaller amount of grease and a sampling process that enables representative grease samples be taken without disassembling of the component.
For motors, a design has been developed to allow a replaceable fitting to be installed at the motor drain port. This fitting serves two purposes: it takes the place of a drain plug, allowing displaced grease to drain from the cavity without building up pressure and compromising the bearing shield/seal; and it provides a protected pathway for representative grease draining from the cavity to be captured and submitted for analysis. Not all motor configurations provide a ready quantity of grease from the bearing at this drain point when normal greasing procedures are used. Further research is ongoing to identify those motor configurations where sampling at the drain does not provide sufficient quantity or a representative sample of grease. Alternative methods of grease extraction will be required in these cases.
Figure 1 - Grease sampling fitting and volume of grease.
In the current designs, the sampling fitting is also optimized for the subsequent laboratory analysis. By providing a sealing surface in the fitting cylinder, the entire volume of grease is available for analysis.
Extraction of the grease is done under variable pressure and force conditions, and the response of the grease can be measured and related to the grease consistency and pumpability, important characteristics for inservice greases. As the grease is extracted, it can be delivered in a thin film for accurate analysis by FTIR, RULER, and emission spectroscopy, giving detailed information about grease oxidation, contamination, and mixing.
For MOVs, more advanced sampling tools are under development. Thought of as a "Grease Thief", the device must be able to travel from the access hole to the active lubrication location, near the gear mating area. This requires the device to push grease out of the way in the space between the access hole and the gears, and then capture a small amount of grease close to the mesh point. Because certain greases are relatively stiff, it is also important to close around the grease at this point to capture it in the sampling device and be able to extract it externally. Such a device has been developed, and should significantly improve the confidence and capabilities of MOV grease analysis.
Figure 2 - Gearbox grease sampling thief, with extension tube.
Still other applications, such as pillow block bearings and reservoirs for pumps, compressors and fans, will require some modification of motor sampling fitting designs in some cases. In others, bearing design is for uncontained release of grease to the surroundings (such as certain pillow block designs) which may require cruder sampling methods. However, wherever there is a critical machine, regardless of configuration, the demand for reliability drives us to develop improved sampling methods to enable extracting the valuable information present in grease analysis.
Some of the following tests can be used to determine the condition of the grease in components:
Comparative FTIR - FTIR spectrums are created from new grease samples for all greases in a facility's program. Using an HATR (horizontal attenuated transverse reflectance ) rig, we apply a thin film of grease across the crystal and use the auto-gain function to maximize signal and get a representative spectrum. We then test the sampled in-service greases, and compare them to the spectra of new grease. In particular, for different families of greases, the FTIR spectra are quite different, and can be compared to see if significant mixing has occurred. In other cases, similar greases (two different polyurea greases) might not have a significant difference in their spectra, but there is less likelihood of compatibility issues in that case. Still, many greases within the same family from different manufacturers can be differentiated with FTIR analysis.
RDE Spectroscopy - The grease is weighed out and added to a glass vial where it is diluted and dissolved with a filtered mixture of grease solvent. This liquid mixture is then analyzed by RDE spectroscopy, and the results are PPM normalized to 1 gram of grease based on the measured weight of grease that was dissolved. The concentration of metals in the grease can be compared to the new grease for the purpose of identifying significant differences in additive metals that could point towards grease mixing. Also, the presence of wear metals can also be deduced. Rotrode Filter Spectroscopy can be performed to evaluate the size influence of the wear, as this detects metal particles larger than 6 micron.
Analytical Ferrography- this test can also be performed on the dissolved grease (prepared as for RDE Spectroscopy) to visually identify the amount, shape, composition and wear severity of the particulate in the sample.
RULER - The RULER instrument works on the principle of linear sweep voltammetry. By applying this test method, in which a variable voltage is applied to the sample while measuring the current flow, the presence and concentration of various antioxidant additives (including but not limited to ZDDP) can be determined based on their unique electrochemical oxidation potential and the magnitude of the induced current. This procedure has recently been developed as a full ASTM test procedure under ASTM D6971.
Dropping Point - a modified test can be performed that is usually employed to find the melting point of various substances with a capillary tube. While not directly linkable to the published dropping point values, comparing the temperature at which in-service grease might liquefy under increasing temperature can provide insight into the effect of mixing of greases or in-service degradation on its properties at operating temperatures and ability to continue to provide reliable lubrication.
Grease Rheology - some previously published reports discuss the use of cone and plate rheometers to measure grease consistency and relate to standard tests such as cone penetration. Additional similar tests are under development to measure grease response to deformation and flow, and to relate these characteristics similarly to consistency, pumpability, and other characteristics of rheology that relate to changes undergone by the grease which may compromise its performance.
Grease analysis presents a significant opportunity to expand machinery diagnostic capabilities. The historical challenges of obtaining representative and trendable samples are being addressed through technological developments and new approaches. The further development of repeatable analysis methods that utilize smaller quantities of grease will produce greater value, and encourage the sampling of greases from locations where only small quantities are accessible. By designing grease sampling equipment appropriately, the matter of proper grease purging may also be addressed through the establishment of sampling programs.
1 - From www.compsys.com, "Antifriction Bearing Lubrication Rules of Thumb", May, 2003, Emerson-CSI.
2 - From www.compsys.com, "Antifriction Bearing Lubrication Rules of Thumb", May, 2003, Emerson-CSI
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