The consistency of bearing grease can greatly affect the component’s performance. While the consistency of oil lubricants is measured in terms of viscosity, the consistency of bearing grease is measured in penetration levels.
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.