Grease penetration tests are conducted by Nye technicians in order to determine the stiffness of a grease. The P0, or unworked and P60 or worked penetration tests follow ASTM D-217 method for full scale or ASTM D-1403 method for half scale or quarter scale. They are two of the four required tests performed on every single batch of grease at Nye prior to shipping.
The penetration test begins with the grease at 25 degrees Celsius, plus or minus 1 degree Celsius, being levelled into a cup. The sample size and cone weight for this test are determined by your application’s needs. Using a penetrometer, a cone is dropped into the cup for 5 seconds, creating a hole in the grease. The technician records the depth in a tenth of a millimeter of this hole. This value is known as the P0 or unworked penetration.
The grease is then sheared (or worked) using a mechanical hand operated device through 60 double strokes, simulating the use of the grease. When this is completed, the technician repeats the cone test to determine the P60 value, also known as a worked penetration. Nye technicians can also carry out P10,000 and P100,000 tests, depending on the application’s specifications.
The penetration values of P60 are rated using the NLGI grade chart from 000 to 6. These values determine the stiffness of the grease and how a grease will react over time to service and wear. The higher the penetration value, the softer the grease. The lower the penetration value, the stiffer the grease. A grease that is too soft may migrate away from the area that needs to be lubricated. A grease that is too stiff may not effectively migrate into those areas that need to be lubricated.
So how to the penetration values equate to the NLGI grades? A chart explains below explains but it is important that you know the NLGI grades are allocated from the P60 Worked Penetration figures:
So for example, take a look at the penetration numbers below but focus on the P60 number:
You will see that the P60 (or 60X) value in this example is 280, which if you refer to the NLGI grade guide above, 280 is in the NLGI grade 2 range. Bingo!
For base oil characteristics, Kinematic Viscosity testing is a key data point to understand.