the nlgi consistency number (sometimes called ânlgi gradeâ) expresses a measure of the relative hardness of a grease used for lubrication, as specified by the standard classification of lubricating grease established by the national lubricating grease institute (nlgi). reproduced in standards  (âstandard classification and specification of automotive service greasesâ) and  (âautomotive lubricating greasesâ), nlgiâs classification is widely used. the nlgi consistency number is also a component of the code specified in standard  âlubricants, industrial oils and related products (class l) â classification â part 9: family x (greases)â. the nlgi consistency number alone is not sufficient for specifying the grease required by a particular application. however, it complements other classifications (such as  and ). besides consistency, other properties (such as structural and mechanical stability, apparent viscosity, resistance to oxidation, etc.) can be tested to determine the suitability of a grease to a specific application. ==test methodology== nlgi's classification defines nine grades, each associated to a range of astm worked penetration values, measured using the test defined by standard  âcone penetration of lubricating greaseâ. this involves two test apparatus. the first apparatus consists of a closed container and a piston-like plunger. the face of the plunger is perforated to allow grease to flow from one side of the plunger to another as the plunger is worked up and down. the test grease is inserted into the container and the plunger is stroked  while the test apparatus and grease are maintained at a temperature of . once worked, the grease is placed in a penetration test apparatus. this apparatus consists of a container, a specially-configured cone and a dial indicator. the container is filled with the grease and the top surface of the grease is smoothed over. the cone is placed so that its tip just touches the grease surface and the dial indicator is set to zero at this position. when the test starts, the weight of the cone will cause it to penetrate into the grease. after a specific time interval the depth of penetration is measured. ==classification== the following table{{cite book |last=rudnick |first=leslie r. |title=synthetics, mineral oils, and bio-based lubricants: chemistry and technology (chemi
Lubricants