An arbitrary standard measure of consistency (hardness) based on ASTM D217 (IP 50). The softer the consistency, the higher the penetration number.
The ASTM Definitions are:
Penetration â€“ The depth that a standard cone penetrates a sample of lubricating grease under prescribed conditions of cone weight, time and temperature. Penetration is measured in tenths of a millimeter, sometimes abbreviated dmm.
Undisturbed Penetration â€“ The penetration of a sample of lubricating grease that has not been stirred or disturbed measured at 25Â°C (77Â°F).
Unworked Penetration â€“ The penetration at 25Â°C (77Â°F) of a sample of lubricating grease that has received only minimum disturbance in transferring to a grease worker cup or dimensionally equivalent container.
Worked Penetration â€“ The penetration at 25Â°C (77Â°F) of a sample of lubricating grease immediately after it has been subjected to 60 double strokes in a standard grease worker.
Prolonged Worked Penetration â€“ The penetration at 25Â°C (77Â°F) of a sample of lubricating grease that has been subjected to more than 60 double strokes in a standard grease worker at a temperature of 15Â°-30Â°C (59Â°-86Â°F). Typical numbers of double strokes used for prolonged working are 10,000 and 100,000. After the prescribed number of double strokes, the sample is brought to 25Â°C (77Â°F), and worked an additional 60 double strokes, and penetration is then measured without delay.
Block Penetration â€“ The penetration at 25Â°C (77Â°F) of a sample of lubricating grease that is sufficiently hard to hold its shape, determined on the freshly prepared face of a cube cut from a block of grease.
Plasticity â€“ The property of an apparently solid material that enables it to be permanently deformed under the application of force without rupture. Plastic flow differs from fluid flow in that the shearing stress must exceed a yield point before any flow occurs.
Pumpability â€“ The ability of a lubricating grease to flow under pressure through the line(s), nozzle(s), and fittings(s) of a grease dispensing system. It is best indicated by the apparent viscosity at moderate shear rate. See ASTM D1092 and NLGI Publication, NLGI Steady Flow Charts for Grease.
Reversibility â€“ The ability of a grease to return to its normal grease-like consistency after temporary exposure to temperatures near or above the dropping point of that grease. Only a few types of greases have this property.
Rheology â€“ Study of the deformation and/or flow of matter in terms of stress, strain, temperature and time.
Rheopectic Grease â€“ A lubricating grease that undergoes an increase in consistency (hardens) when it is subjected to shear.