Problems caused by particles
So what are the problems caused by deleterious particles in grease? Broadly speaking, we can view the problems in three categories (although they are not mutually exclusive): Damage to bearings, noise in bearings, and clogging of distribution equipment.
Damage: If the hardness and size of the particles is sufficient, over rolling of the particles in a rolling element bearing can result in permanent damage to the bearing. This causes denting of the raceway, which in turn results in high stresses at the edges, leading to spalling and reduction in the fatigue life of the bearing (Figure 5: see right. Debris denting). Additionally, if sufficiently hard, the particulate contamination may cause abrasive wear, also leading to premature failure of the bearing.
Noise: Even if the particles are not hard enough to result in permanent damage to the bearing, the over rolling of these particles will cause an increased level of noise in the bearing. Noise will result from over rolling of even relatively soft particles, such as undispersed thickener. Why do we care about noise anyway? The property of “quietness” or “low noise” is becoming more important in the world of grease. There are several reasons for this, the most well known being the growth in consumer electronics and appliances. Consumers don’t want to listen to bearing noise when they are using a fan or a piece of audio equipment. A second reason is not consumer related, but industrial in nature. Many types of equipment such as grinders used in metal processing produce high levels of noise, which can be a health and safety hazard. Grease which is “noisy” can contribute to this already challenging problem. And finally, a third area where noise can be a problem is during bearing manufacturer quality control processes, where grease noise can mask noise or vibrations caused by defects in the bearing itself.
Clogging of filters and distribution equipment by excessive particulates is also a significant problem, particularly where the grease is distributed by a centralized lubrication system. These systems usually have very small clearances, so may incorporate filters to prevent particulate material entering and plugging the grease lines and distribution blocks. If the filters become clogged, or if the distribution system itself becomes clogged, this can lead to lubricated equipment failures due to lubricant starvation in the bearings.
Testing for deleterious particles
There are several different tests which can be used to determine the presence and / or type of particles in grease. First, we’ll discuss the tests which can be used to determine the amount (size and concentration) of particulate. A test in use by the paint and coatings industries, which is designated ASTM D1210 uses a device called the Hegman gauge. In this method, the particles in a grease are counted and sized from zero to 100 microns using a machined depth gauge. This method is not in common use in the grease industry, but is being seriously considered as part of a draft grease cleanliness definition under development by a European Lubricating Grease Institute (ELGI) – National Lubricating Grease Institute (NGLI) Joint Working Group. Another method in use to determine size and concentration of particles in grease is designated DIN 51813, Solid Matter Content of Lubricating Greases. In this method, a sample of grease is passed through a 25 micron filter, and then the residue is solvent washed, dried and weighed to give the milligrams of particulate (greater than 25 microns) per kilogram of grease (Figure 6). And yet another method which finds occasional use is FTM 3005.4, Dirt Content of Lubricating Grease. In this method, a small sample of grease is examined under a microscope to determine the size and number of particles present (Figure 7).
Figure 6: DIN 51813.
Figure 7: Microscopic comparison of typical and "clean" greases.
Equally as important as determining the size and concentration of particles is evaluating their abrasiveness. Only one standardized test is in existence today to do this, designated as ASTM D1404, Estimation of Deleterious Particles in Lubricating Grease. In this method, the number of scratches that appear on the surface of highly polished acrylic plates under specified test conditions are counted. This method is also being considered as part of the draft grease cleanliness definition under development by the aforementioned ELGI – NLGI Joint Working Group.
It should be noted here that the Joint Working Group is moving towards adopting a definition of grease cleanliness which is “a measure of the absence of particulate matter that has the potential to cause damage in certain applications.” As such, both the amount (or concentration) and size distribution, along with a measure of the abrasiveness of the contamination are likely to be proposed as part of a draft standard.
In order to measure the noise level of a grease, again there are no industry standard test methods in existence. Various bearing manufacturers have developed their own in-house methods, some of which are available to the general public. There are two different approaches to measuring noise, one using anderometry, and the other employing acoustic techniques. In anderometry, which appears to be the preferred technique, noise is evaluated by measuring the vibrational velocity of a bearing’s stationary outer ring while the inner ring is rotated (Figures 8, 9, 10). Included in this category are: FAG MGG11, SKF BeQuiet+, and other OEM proprietary methods. In the acoustic techniques, noise is evaluated using sound amplification, and requires the use of a special sound chamber (Figure 11: see above right, Acoustic grease noise measurement). Included in this category are JIS B 1548 and various OEM proprietary methods.
Figure 8: Anderometry.
Figures 9 & 10: Noise recording on a "dirty" grease vs. "clean" grease.
Specifications and Standards for Cleanliness
There is a growing trend to define and include cleanliness requirements in a number of grease industry standards and OEM specifications. A few examples include:
The next and final installment of this article will cover measures to control deleterious particles. You can view the first installment of the article, which covers types and sources of particles here.
Chuck Coe is the President of Grease Technology Solutions, LLC, President of NLGI, and a member of STLE. You can find his contact information in the STLE member database. He holds both the NLGI CLGS certification and the STLE CLS certification. He is chairman of the STLE/NLGI Grease 101 course, which has been offered at past Annual Meetings, and will be offered again at the 2014 STLE Annual Meeting in Orlando, FL.
Other articles in this issue: