The basics of lubricating grease and in-use testing
Written by Amy Rishell and John Sander
Applications by Thickener Type
Greases are usually marketed and recognized by thickener type. The OEM or manual often will specify that a grease of "X" thickener type is needed for an application. The three basic categories of greases are insoluble solid-thickened, polymer-thickened and soap-thickened.
Insoluble Solid-Thickened Greases
Usually greases thickened by insoluble powders, such as bentonite clay, are non-melt greases. In other words, they have extremely high or non-observable dropping points. These greases are typically used in high-temperature applications, such as kiln cars, ladle cars and furnace door bearings. These greases only have fair to good mechanical stability, so they are not recommended for applications where high shear is inevitable. Because this thickener system is composed of solids, the grease can eventually leave behind heavy amounts of residue in applications where the base fluid is driven off by evaporation.
Typical applications for insoluble solid-thickened greases include: open gears, high-temperature applications, EP with high-temperature applications, rolling element bearings, high- and low-speed applications, heavy loads, shock loads, ball and roller bearings, high oscillatory motion, high vibration, and low-torque.
Polymer-Thickened Greases
Many polymer-thickened greases are oxidatively stable because they contain a thickener that is composed of organic polymeric materials similar to rubber. As described above, soap-thickened greases contain metals, and metals can act as oxidation catalysts that can lead to fluid degradation. Because polymeric thickener systems do not contain any metals, they have the ability to provide good continuous service in applications that are subjected to high temperatures or are exposed to oxidizing environments.
The most common polymer-thickened grease is polyurea. Polyurea greases have excellent shear stability. This means they have the ability to stay in place in applications operating at fairly high speeds and temperatures without having to be re-applied. For this reason, a very common application for this type of grease is small to medium roller bearings used in electric motors. Typically, these are closed systems where it is nearly impossible to inject additional grease after the motor has started to run. One of the negative aspects of polymer-thickened greases is that they are expensive to produce, which can raise significantly the price of the finished grease. An additional drawback from a production standpoint is that many polyurea thickeners use raw materials that are classified as hazardous materials. While the finished greases are not hazardous, precautions still must be taken to protect the production personnel during the manufacturing process.
In addition to roller bearings on electric motors, other typical applications for polymer-thickened greases include: moderate- to high-temperature applications, anti-wear/EP, electric motors, seal-for-life bearings, rolling element bearings, and constant velocity joints, such as in an automobile.
Soap-Thickened Greases
Soap-thickened greases are the most common type and have been proven to be the most versatile. Common soap types include lithium, lithium complex, aluminum, aluminum complex, calcium and calcium complex. Others include sodium and barium. Worldwide, lithium soap is by far the most common grease thickener system used. Soap-thickened greases can be formulated for almost any application. Depending on which metallic base is used, soap-thickened greases have been found to be very easy to work with, safe and versatile. Certain types of soap-thickened greases naturally possess properties such as rust protection, water resistance, shear stability, and good thickening efficiency with lower amounts of thickener than the other categories. With the help of additives, most soap-thickened greases can have acceptable to excellent EP performance as well as and oxidative stability.
Typical applications for soap-thickened greases include: electric motors, gearboxes, chassis, wire rope, chain, cable, food grade applications, high-temperature applications, EP, non-EP, and all purpose.
