To maintain reliable performance of a rolling element bearing, proper maintenance and inspection should be performed. It is suggested that periodic maintenance include operator inspections, preventive maintenance and predictive maintenance.
Scheduled maintenance encompasses supervision of operating conditions, lubricant replenishment and regular periodic inspection using simple to more advanced techniques. Items that should be checked regularly during operation include noise, vibration, temperature and lubricant condition.
Grease analysis is an important part of periodic inspection; however, it has traditionally been difficult to test grease on-site, especially during equipment operation.
Hence, this article will present some basic on-site inspection techniques that provide simple and accurate means of assessing the condition of used grease samples. The methods can also be used to perform quality checks on new greases and those that have been in storage for a prolonged period.
Grease consistency, or stiffness, is an important property of grease. New grease stiffness or penetration number is measured in the laboratory by a test such as ASTM D217.
Certain greases tend to harden during use and, if this proceeds too far, can lead to impending bearing failure. It is therefore often necessary to monitor the consistency of greases in service. However, a grease penetrometer is not generally at hand in an industrial plant environment, thus the need for simpler inspection methods.
Figure 1. Simple Grease Consistency Tester
The National Lubricating Grease Institute (NLGI) defines grease consistency grades based on the ASTM D217 worked penetrations. The in-house system for estimating grease consistency involves collecting a series of unused greases of known NLGI grades.
The time it takes for a fixed volume of a specific grade of grease, contained in a glass syringe, to completely expel from the syringe’s chamber at a controlled temperature is measured. Consequently, a calibration curve can be created. Trend analysis can then be performed on used grease samples to monitor change in consistency.
Figure 2. Preliminary Results
Figure 1 is an illustration of a simple used grease consistency tester. Figures 2 and 3 show preliminary results and the correlation of the results with the data extracted from the standard penetrometer.
Figure 3. Correlation Between Two Techniques
(Standard Penetrometer vs. a Simple Grease Consistency Checker)
This simple grease consistency test can be performed on-site to obtain quick answers that will help detect problems and their degree of severity, thus prompting necessary corrective maintenance. The prime advantage of this approach is that the method can be used quickly and samples need not be sent to a remote laboratory for analysis.
This test is a procedure for detecting and estimating the concentration of harmful, gritty contaminants that may be found in new and/or used lubricating grease. A small amount of the grease sample is placed between two polished acrylic plates held rigidly and parallel to each other in metal holders.
The assembly is pressed together, squeezing the grease to a very thin layer. The apparatus is constructed so that the top plate may be rotated with respect to the bottom plate while under pressure (Figure 4).
Figure 4. Exploded View of Test Apparatus
The plates are then removed, washed and the characteristic arc-shaped scratches are counted. The procedure and materials used are similar to ASTM D1404. These scratches show when wear debris and foreign particles are present in the grease sample (Figure 5).
Figure 5. Typical Results
Wear debris analysis has recently become a more widely applied technology in machinery health condition monitoring. Wear is the primary mechanism by which rolling element bearings deteriorate. By observing the amount and mechanism of wear periodically, the rate of bearing deterioration can also be monitored.
Traditionally, this has been done with atomic emission spectrometry (AES). However, this technology is deficient in analyzing greases in industrial drives, because the most important size fraction, the particles larger than 10 µm, are ignored. In a normal bearing wear cycle, the average wear particle size is 5 µm to 15 µm.
In highly distressed situations, these particles can become even larger and as such, traditional elemental techniques have limited effectiveness in monitoring these changes.
Apart from AES, the most commonly used methods for wear debris analysis are microscopic analysis (analytical ferrography), ferrous density analysis, such as direct reading ferrography, patch test, chip collectors and used filter inspection.
These techniques, if properly selected, may overcome the particle size limitation and provide additional information on the mechanism, location and extent of wear, and to some degree, the state of the lubricant and any contaminants.
Figure 6. Analytical Ferrography Analysis
The basic principle of inspecting wear debris in grease is as follows:
Obtain representative used grease samples from rolling element bearings.
At the site, spread a thin layer of used grease sample on the back of the hand or the glass slide for inspection against the light, magnifying glass or optical microscope; or alternatively,
Place a measured amount of the grease in a clean beaker. Then liquefy the sample with a suitable solvent. The type of solvent to be used depends on the grease thickener and base oil used in the grease.
Draw the sample through a membrane filter. (See the patch test method described in the September-October issue of Practicing Oil Analysis magazine) or use a magnetic separation technique such as the ferrogram maker or rotary particle depositor slide maker to separate the solids from the grease solution.
Visually analyze the debris on the slide at 10X to 100X magnification under a reflected light microscope, analyzing the following particle features:
Particle outline shape
Particle surface texture
Particle thickness ratio
Particle edge detail
Particle composition (heating the slide may be required)
Reaction of particles when exposed to reflected and/or transmitted light
Analytical ferrography is one of the most effective and versatile tools for wear particle analysis. A typical ferrogram slide maker is shown in Figure 6. During this analysis, the ferrogram is scanning for evidence of an abnormal wear condition. Not only can the overall bearing condition be determined by the physical and/or chemical properties described in the table, but it can also be analyzed by looking for wear particle morphological features which in turn can be used to identify the wear mode, wear mechanism, severity of wear and possibly the composition of the wear particles.
Repeat the procedure at an “optimal” time interval for trend analysis. Although individual analysis can be performed, the routine used grease analysis normally provides better information, both for problem severity evaluation and/or problem diagnostic purpose. Wear debris analysis is a relatively simple procedure that does not require a high skill level to perform. Even so, the results directly indicate the level of threat and damage within industrial drives, which is absent from some of the more sophisticated techniques.
Table 1. Wear Mechanisms and Wear Debris Characteristics
Sizing the particles (both average, maximum particle size and the particle size distribution) is one of the more important aspects of testing. In general, the damage state of a rolling element bearing is proportional to the size of the particles. As a general rule, normal rubbing wear particles are no larger than 25 µm. Wear particles larger than 25 µm indicate a potentially more advanced state of wear, such as severe sliding, fatigue platelet and cutting wear (Figures 7 through 12).
Figure 7. Severe Sliding Wear Particle is Seen by Spreading
a Thin Layer of Used Grease on
a Glass Slide
Figure 8. Fatigue Platelet
Figure 9. Cutting Wear Particles
Figure 10. Cylindrical Particles (seal material/fiber) from Ferrogram Slide
Figure 11. Ferrogram Slide Shows Abrasive Media in Used Grease Sample
Figure 12. Spherical Wear Particles, Cutting Wear Particles and Contaminants on Ferrogram Slide
Visual and microscopic sample examination using blotter spot analysis can be an important source of information about the condition of used grease samples. Prior to blotting and/or filtering the sample, the liquefied used grease sample should be visually examined. Water in the liquefied used grease sample may be seen either as emulsified water or as a distinct water layer. The general cleanliness level of the grease may also be determined.
As a single drop of liquefied used grease placed on a piece of blotter paper, is absorbed, it will form a particular and characteristic pattern. This pattern is then interpreted to determine the condition of the used grease. A single spot test reveals certain properties. Prior to carrying out microscopic examination, the blotter paper should be visually examined (Figure 13).
Figure 13. Blotter Paper of Used Grease Samples
For example, the presence of water within the used grease can be detected from the blotter paper. This is seen in the form of light circular areas on the paper. Water also sometimes oxidizes the ferrous material, and therefore, the presence of rust indicates the ingress of water.
A series of blotter spot tests reveals the rate at which important properties are changing, and are more significant in a general evaluation of used grease conditions.
A low-cost used grease analysis is an important part of a predictive maintenance rolling element bearing program. Maintenance costs can be reduced by a confident extension of grease replenishment period, and critical wear production and grease physical/chemical deterioration rates can be recognized, enabling corrective action to be taken prior to catastrophic bearing and/or lubricant failure.
The author would like to acknowledge that the Thailand Research Fund (TRF) has supported this work.