While it may seem a dry topic, it is important to educate yourself about bearing grease consistency because it is one of the main factors that can have a positive or negative impact on the performance of your machinery. Of course, the performance of your machinery can also have an impact on the consistency of your bearing grease, so it can be something of a vicious circle at times.
Unlike oil lubricants, which are measured by their viscosity, bearing grease consistency is measured in terms of its penetration levels.
Grease penetration refers to the depth, measured in tenths of milometers, that a weighted cone will sink to in the grease under worked or unworked conditions. The higher a penetration number is, the further it will sink, and the softer the grease will be.
Measuring unworked penetration
To measure the penetration of unworked grease, it is standard practice to heat the substance up to 77 degrees Fahrenheit and then pour it into a standard cup, smoothing the surface in the process. Then, a cone weighing roughly a third of a pound is placed on top of the grease for a total of five seconds. How far it sinks is measured as its penetration level unworked.
Measuring worked penetration
To measure worked penetration levels, grease is churned around 60 times as a standard grease worker to ensure that it is changed in consistency to a similar level that it would be when worked in machinery. Air is then removed from the grease and the cone is placed into the grease to take a correct measurement.
Comparing the two
Once you have the worked and unworked measurements, it is important to also work out how much spread happens between the two. If the difference between the two figures is relatively low, that means the grease is of a light consistency, whereas a bigger difference would indicate a heavier grease.
Grease lubes that have a heavy consistency are known as channelling greases, and they tend to have penetration levels in the lower 200 range, and have a 5 to 10 point spread maximum. These greases are best used for high speed machinery, where they will create a thin film of lube to protect high-speed components.
Greases, whether they be from Mobil, Shell or Fuchs, which have a much bigger spread when tested worked and unworked are very useful for preventing fretting corrosion because they start off thick and mature to a good working heavy oil.