Bearing Grease Consistency
The consistency of bearing grease can greatly affect bearing performance. As most people know, the consistency of oil lubricants is measured in terms of "viscosity." The consistency of "Bearing Grease," on the other hand, is measured in "penetration levels".
Grease Penetration Definition
Penetration is the depth, in tenths of millimeters, to which a standard weighted cone sinks into the grease under prescribed conditions ("unworked" versus "worked"). Therefore, higher penetration numbers indicate softer greases, since the cone has sunk deeper into the sample.
How is Unworked Penetration Measured?
Unworked Penetration is measured when a sample of grease is brought to 77°F and transferred to a standard cup; its surface is smoothed and the cone, in its penetrometer assembly, placed so that its tip just touches the level grease surface. The cone and its movable assembly, weighing 150 Grams (0.33 Lbs.), are permitted to rest on top of the grease for exactly five seconds. The distance dropped is measured and recorded as the unworked penetration level.
How is Worked Penetration Measured?
Most greases change significantly in consistency when worked (sheared or kneaded). Thus a worked penetration level is considered as significant as the unworked penetration level in regards to the service behavior of the grease. To measure the Worked Penetration Level, the grease is first churned for 60 round-trip strokes a standard grease worker. Air is driven out of the sample and the penetration of the cone is again measured; this reading is almost always higher than the unworked penetration level.
Comparing Worked and Unworked Penetration
It is not only important to know how high or low the unworked penetration level is, but also how much of a spread there is between the unworked and worked levels. A typical, non-channeling (slumping type) instrument bearing grease might have a unworked penetration level of 290 and a worked penetration level of 295. This means that the grease is of fairly light consistency to begin with, and with only a five point spread between the unworked and worked levels, maintains a fairly uniform consistency throughout its service life.
Channelling Grease Characteristics
Other grease lubricants with a heavy consistency, called channeling greases, have typically very low penetration levels in the low 200 range and the most common ones will have no more than a 5 to 10 point spread between the worked and unworked penetration levels. These heavy consistency greases (channeling greases) are the lubricants of choice for high speed applications. The initial torque incurred in mechanisms lubricated with channeling greases is very high; however, after a limited amount of movement the majority of the grease will channel (be pushed to one side) and a thin film will be left to adequately lubricate the moving members of the mechanism with a minimum of resistance.
Grease Consistency Plays Role in Preventing Fretting Corrosion
Another small group of greases has yet to be officially named. These greases are typically used to reduce or prevent the possibility of fretting corrosion, and have a very large spread between their worked and unworked penetration levels. Typically they will have an unworked penetration level of about 285 and a worked level of 315 or higher. This large spread means that the grease is quite thick initially and when worked becomes the consistency of a heavy oil. Once the working action has stopped, the grease will return to its original heavy unworked consistency.