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Tribology and Lubrication Technology Lubrication Fundamentals December 2013 : Page 14

tion or shear stress. To determine this property, we drive a perforated plate through the grease via a so-called stro-ker for a specified number of cycles, usually 60 strokes. But for applications where shear stability is critical, 100,000 strokes are applied. This equipment is shown in Figures 4 and 5. The National Lubricating Grease Institute (NLGI) has developed a clas-sification system for rating the consis-tency of greases based on the millime-ters the cone penetrates the grease, as shown in Table 1. Table 1 | NLGI Classification System for Rating Grease Consistency. Worked Penetration 445-475 400-430 355-385 310-340 265-295 NLGI Grade 000 00 0 1 2 3 4 5 Hard/Block Figure 5 | Perforated Plate Table 2 | Reproducibility and Reproducibility of ASTM D217. Penetration Unworked Worked Prolonged worked Block Penetration Range 85 to 475 130 to 475 130 to 475 Under 85 Repeatability, One Operator and Apparatus 8 units 7 units 15 units 7 units Reproducibility, Different Operators and Apparatus 19 units 20 units 27 units 11 units Smooth Figure 4 | Stroker Grease Consistency Semifluid Consistency is the one property that is always specified on purchase (except in the retail world). Grease with a penetration of 265-295 millimeters is classified as a NLGI No. 2. This grade is far and away the most common, certainly the default grade for most equipment designs (and usually what you buy in retail stores). It has a nice smooth consistency that allows the grease to be easily spread onto surfaces, pretty much stays put and yet is thin enough that it can be pumped in many systems operating at moderate ambient temperatures. Every time we discuss test methods and the results that we obtain, we need to take into consideration the preci-sion and accuracy of the test method. Accuracy seeks to answer the ques-tion, “Did you get the correct measure-ment value?” Bias is a systemic error that contributes to the difference be-tween the mean of a large number of test results and an accepted reference value. The accepted reference value is considered the true value and there-fore, bias has to do with the degree to which that true value is not obtained. Thus, in general, through calibration, bias can be reduced or eliminated. Precision has to do with the variability of the test method itself. 14 220-250 175-205 130-160 Does the method provide the same number every time, or does it vary, and if so, by how much? In ASTM parlance, precision is stated in their published standards as the repeatability and the reproducibility of the method. Repeat-ability has to do with the difference between two test results performed by the same operator in the same labora-tory with the same apparatus on the same test material under constant operating conditions. Reproducibility has to do with the difference between two single and independent results ob-tained by different operators working in different laboratories on identical test material in the normal and correct operation of the test method. The method for measuring consis-TRIBOL OG Y & L UBRIC A TION TE CHNOL OG Y tency of greases, ASTM D217, states that the method’s worked penetration variability for greases, like a No. 2, for more than one lab and operator is 20 units (see Table 2). Thus, in a given sit-uation, the test results could vary by 40 units (+/-20) and statistically have the same result. So what we find at the end of the day is that to make a grease of consis-tently consistent consistency, we must use a method that in and of itself is not especially consistently consistent. For-tunately, for the industry’s needs, the method seems to be neither a blessing nor a curse; rather, it is good enough. But specification writers need to be aware of these kinds of issues when they write their specifications. WWW .S TLE. OR G

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