Grease cleanliness is a common topic of discussion among grease suppliers, bearing manufacturers and end users, all of whom tend to hold strong opinions on the subject. Numerous technical papers as well as magazine articles have addressed both the importance of cleanliness in greases as well as the frustration over the lack of an industry-accepted definition or measure of cleanliness.
The Draft Definition
To address these concerns, a joint working group has met at both NLGI and ELGI annual meetings for the past several years. Recently, in light of the various ways of defining and measuring grease cleanliness, the group has agreed on a draft definition to ensure focused efforts.
Before defining the term, the group had to sort through a series of cleanliness classifications based on specified levels of filtration and/or packaging. Other definitions were derived from existing ASTM, DIN or other test method standards - some of which count and size particles in the grease, measure noise in bearings caused by particles in grease, measure the amount (weight) of particles and evaluate the abrasiveness of particles.
The draft definition of grease cleanliness was defined by the working group as:
A measure of the absence of particulate matter that has the potential to cause damage in certain applications.
A measure of the suitability of a grease for applications in which contamination by particulate matter would cause damage to the pieces in contact with the lubricant. The extent of contamination can be measured by the size, number and/or hardness of the particulate matter.
Note: The types of particulate contamination could include scale and wear metals from the manufacturing process, burned soap and additives, and dirt and sand.
Test Method Evaluation
To meet this definition, the working group reviewed various standardized test methods for their applicability, simplicity, cost and availability of test equipment and level of precision (if available). During the ELGI annual meeting in April of 2006 (Prague), a proposal was made to combine two standardized test methods, the ASTM D1404 and Hegman gauge methods; therefore the number, size distribution and hardness of the particulate matter in a grease sample could be measured.
The ASTM D1404 "Deleterious Particles" method provides a means to count the number of hard particles in a specified sample size. In this method, the number of scratches that appear on the surface of highly polished acrylic plates under specified test conditions are counted. Greases would then be rated 1 through 3 as follows: 1 = less than 10 scratches, 2 = 11 to 40 scratches and 3 = more than 40 scratches.
The second test is the Hegman gauge, in which particles are counted and sized from 0 to 100 microns using a machined depth gauge. The greases would then be rated as follows: A = clean (0 particles > 100 microns), B = moderate (1 to 5 particles > 100 microns) or C = dirty (more than 5 particles > 100 microns).
The results of the two tests are combined into a 3x3 grid, and the grease is given one of nine ratings, from 1A to 3C. The ratings indicate both the number and size of particulates in the grease as well as the hardness of the particles.
Data from a number of samples is being evaluated on a mini round-robin basis to assess how different labs compare for both clean and contaminated greases. So far, data has been collected from three participating labs, and no conclusions will be drawn until data is received from three more participants.