How clean is ‘clean enough’ for grease lubricants destined for industrial equipment?
Noria promotes the proposition that solid contaminant levels should be driven as low as necessary to achieve optimum cleanliness targets for the most sensitive components in a system.
With OEM guidance, it is not too difficult to determine which components are the most sensitive in most hydraulic and circulating systems, though it can be a vexing challenge to drive the contaminants down to that optimum level, and then hold that level for the life of the machine. Fortunately, ideal contaminant levels for oil-lubricated bearings are relatively easy to determine.
But what about contaminant levels for greased bearings? It’s no easy task to filter solid, abrasive contaminants out of the grease after it enters the bearing housing. Are bearing greases delivered in a cleaner state than the lubricating oils used in manufacturing? How are solid contaminant levels measured for fresh greases? How clean are fresh greases? How can solid contaminants in in-service greases be measured? Let’s explore some of these issues.
Where Are We and How Did We Get Here?
The primary purpose of grease cleanliness is to safeguard the longevity of the lubricated parts or systems, because the lubricant film which separates the contacting surfaces - whether rolling or sliding - is usually very thin (less than 1m up to 10m). It is clear that the presence of any particles whose size is larger than the film thickness can be detrimental to the smooth running and service life of the bearing. The shape and hardness of these particles must also be considered. The film thickness is affected by the viscosity of the base oil; the nature, microstructure and particle size of the thickener; as well as the presence of solid lubricants.
Historically, the analysis of grease has been confined to new grease testing for product acceptance and quality control. Because the majority of bearings are lubricated with grease, it would serve us to understand more about grease testing and, for the purpose of this article, solid contaminant level testing in industrial grease products.1
Solid particles in grease lubricants can come from many sources; the four most common culprits are the formulation of raw materials, the packaging containers, the grease manufacturing process and the environment. Raw materials - the base fluids, additives and thickeners used to make lubricating oils and grease - may contain paper fiber, plastic debris, iron oxide, and chips from a drum liner. Without stringent process control, all can find their way into a lubricant.2
Most grease manufacturers exercise appreciable caution to assure that they do not inflict solid debris contamination abuse on the grease products they manufacture. Selection of basic materials is based on the belief that their supplier is exercising caution to maintain a high level of product purity, for base oils, additives and thickener components.
From this starting point the grease manufacturer visually inspects raw materials before and as they are blended together. The materials are loaded into the blending kettles by hand and through material hoppers. Screens are used to catch unwanted materials (paper, cloth, handling utensils, etc.) that could fall in during this process.
A final filter at the outlet of the kettle catches any remaining large debris from the batch prior to milling the product. These filters are designed to catch items that represent a hazard to milling machinery, and consequently target items larger than the 3-to-7-thousandths of an inch range, which is equivalent to filtering particles 75 microns and larger. Final filtration may be applied to remove any agglomerated thickener particles following the grease milling procedure. However, final filtration is not likely to be performed at appreciably tighter tolerances (that is, to less than 75 microns) unless there is a specific customer requirement.
There are plenty of recognized applications that require ultra-pure and low-noise greases for rolling bearings. Specialized greases are required to comply with very low noise limits and narrow operating tolerances, and ensure an economic as well as energy-saving operation. Some of the typical candidates include:
The central issue for these applications is reducing noise (particularly for audio and computerized electronic drives), reducing time for final quality control and extending bearing life.3
While noise is not a particular concern for heavy manufacturing, the solid contaminants and particulates that create noise are, and the purity that supports the low-noise objective would fit strategically within the industrial reliability engineering plan.
Figure 1. Principle of Solid-borne Sound Measurement
How are Solid Contaminant Levels Measured in New Greases?
The ISO Solid Contaminant Code (ISO 4406:99) rating system is the measure of choice for rating the amount of solid material in either a new or used (in-service) oil lubricants.
Similarly, there are a few approaches designed to provide a solid contaminant rating for grease. The measures are not part of an ISO designated method, but are still useful to the practitioner. There are both direct measures that look for and measure quantities of solid particles and indirect measures that observe the effect of solid particles and characterize the results. Whether the lab selects a direct or indirect measure depends on the product’s technical requirements as defined by the reliability goals of the user with respect to machine design and application.
There are a few military standards and test methods for aircraft grease contaminant limits. MIL-G-81322 is used to rate aircraft grease and MIL-G-81937 is used to rate (ultra-pure requirement) instrumentation grease. The MilSpec for aircraft grease is less than 1,000 particles per cubic centimeter with particle sizes between 25 and 74 microns, and no particles larger than 75 microns. Assuming the particle distribution (and density) in the grease is similar to Air Cleaner Fine Test Dust (ACFTD), this would mean that the total amount of particulate contained in each milliliter of aircraft grease would be similar to an oil with an ISO Cleanliness code of 22/19.
The specification for the ultraclean instrument grease is less than 1,000 particles per cubic centimeter with particle sizes between 10 and 34 microns, and no particles larger than 35 microns. This would roughly equal an ISO code of 19/16.
The European DIN standard 51-825 is also a recognized standard for rating solid contaminants in lubricating greases. The result is a gravimetric rating that measures the amount of particles greater than 25 microns per kilogram of grease.
The grease test used to quantify solid particles in grease, as defined in test procedure DIN 51-813, is performed by pressing 500 grams of grease through a 25-micron mesh screen, capturing the remains from the screen, dissolving the grease portion and then draining this portion through a filter. The filter is weighed and compared to the original filter weight to determine the quantity of particles 25 micron and larger that were captured. A successful test result would be less than 20 mg of particles greater than 25 microns per kilogram of grease.3 This test result limit would be equivalent to an ISO Cleanliness code of 23/19.
For the sake of clarity, the author characterizes an indirect measure as a measure that would quantify the consequence of a particle, rather than actually quantifying particles themselves. There are a handful of recognized and quasi-standardized tests.
ASTM D1404 measures the amount of solid abrasive material in a quantity of grease by using a scratch test. The rating is a subjective evaluation of the number of arc-shaped scratches that would appear on highly polished acrylic plates after 0.25 grams of grease is compressed between the plates and stressed in a circular motion under 200 PSI force. The plates are graded as:
This does not correlate well to other known industrial contaminant measures.
Both FAG and SKF have devised internal test methods to rate the contaminant levels in a grease by observing the indirect effect of contaminants in the grease on a closely monitored system. Both approaches measure the noise or decibel (dB) of a bearing under a set of test conditions. The SKF approach automatically injects grease into a bearing in a test stand.
The SKF noise rating method is designed to assess particle effects on rolling and grease damping characteristics. Called the “Be Quiet +” rating method, the test uses an automated test stand that injects grease into a deep groove ball bearing (type 608 at 1800 RPM), operates the bearing for 15 seconds before testing, operates the bearing for an additional 30 seconds after testing and then dispels the grease and repeats the exercise. An acceleration transducer measures energy readings in three windows (Table 1).
The high bands reflect shortwave disturbances, which can be caused by particles in the lubricant. A peak value is also recorded in all frequency bands to record unique or irregular disturbances that are mainly due to the lubricant.
The FAG ‘MGG’ and Shell ‘GMN’ test methods use similar techniques, including the same bearing, bearing load and energy range measures. The FAG-MGG method requires the test results to be based on readings from five bearings vs. just two bearings for SKF and three bearings for the Shell-GMN method.
Figure 2. Noise Recording on Dirty Lubricant Illustrating Permanent Damage
Figure 3. Noise Recording on Clean Grease (Polyurea Grease)
As Figures 2 and 3 demonstrate, there can be an appreciable difference in the amount of contaminant and noise levels a grease can generate in a bearing. Notice that the overall noise level in Figure 2 is gradually rising over the prolonged test cycle. The increased amplitude suggests that as the debris in the grease rolls through the bearing clearances the surface damage increases. The reading averages will get higher as more damage occurs.
How Clean are Fresh Greases?
The common-sense answer is the products are as clean as grease buyers require. There is obviously a reasonable differentiation in grease cleanliness quality, depending on the questions that the user asks.
SKF offers four classes, and further divides the classes with its “Be Quiet +” quantitative rating division.
The four classes are as follows:5
The particles’ hardness and size is such that over-rolling [particles passing through the loaded surfaces] leads to permanent damage giving increased overall noise and reduced bearing fatigue life.
The hardness and size of over-rolled particles [generally undispersed thickener] may damage the bearing surfaces which gives a noticeable increase in overall bearing noise but not to the degree that the bearing fatigue life is adversely affected.
The hardness and size of over-rolled particles will produce noticeable vibration peaks but the bearing surfaces are not permanently damaged.
The highest degree of cleanliness due to a minimum number of particles giving vibration peaks.
The more quantitative GN rating system is shown in Table 2.
The majority of today’s greases fall into the clean class. An example of a typical clean grease would be a polyurea product that could have large agglomerates of thickener present, producing large vibration noise peaks but have little influence on bearing and race surfaces.5 For a grease to fit into the quiet category, the manufacturer would take extra measures to filter raw materials and final products, manufacture in a clean environment, and ship final products in isolated containers.
If the lubricant user wishes to get to the bottom line then the Kluber-modified version of DIN 51-813 may provide a degree of clarity and certainty, with a correlation to a gravimetric reference.
The Kluber test method requires that the tester dissolve 20 grams of grease and filter the solution through a 1.2-micron filter. The effective mesh size would be smaller than 1/20th of the standard 25-micron mesh size. The filter would be weighed before and after the test.
According to the referenced paper by Helmut Miller,3 most lubricating greases manufactured for low-noise applications test out by the Kluber-modified method at more than 300 grams of solid particles per kilogram. This represents a gravimetric equivalent of approximately 80 mg/L (calculation based on a nominal grease weight of 7.2 lbs. per gallon), which translates into an ISO cleanliness rating of roughly 23/19.
This, or a similar method, may be used to measure solid contaminants in in-service greases, with the caveat that sampling problems could sorely mislead the maintenance team unless accounted for.
Stefan Daegling with Shell Research noted4 in 1999 that there were approximately 18 billion bearings in operation somewhere in the world, and the majority, perhaps 90 percent as noted by Bill Herguth, are grease-lubricated.
Bearing manufacturers have done a good job of demonstrating to bearing users how vitally important contamination control is in achieving L10 (L10 is a statistical life-cycle measurement method used to predict bearing life) bearing lifecycles. However, until now, there has not been a great demand for super pure or quiet greases outside of high-tech applications such as computer hard-drive reader motor bearings.
Large machinery operators in automotive, steel, paper and power generation industries put a significant amount of effort and money into cleaning bearings to achieve better reliability. Based on what we have seen, it appears that the same machine that may be lubricated with oil operating in the ISO 15/12 range would be supported by grease-lubricated bearings with cleanliness ratings in the range of ISO 23/19.
So, where do we go from here? A few suggestions include: