Bearing Tips: What is grease? Why is grease used sometimes instead of oil?
Kuldeep Mistry: Grease is the thickened oil with additives added to enhance its property. The grease typically contains base oil, thickener, and additives. The thickener is like a sponge retaining the oil. Grease is used within 80% of bearings around the world.
Some of the reasons for using grease is that it requires less maintenance, it stays in the place, and it prevents the contaminants acting as a sealant. The downside to grease, compared to oil, is that grease has higher friction, poor heat dissipation, contaminant flush can be challenging and finally, oils are better suited for higher speeds compared to greases.
BT: Can you explain further what, specifically, grease is composed of?
KM: Grease is mainly composed of base oil, soaps, and additives. The base oil is around 80 to 90%, the thickening component is around 5 to 20% and the additive can be between 2 to 5%. The most commonly used base oil is petroleum-based mineral oil, it has around 90% global share because it is inexpensive and fit for use. Whereas, the synthetic oils, like polyalphaolefins, known as PAO, perfloropolyethers, known as PFPE, polyglycol, known as PAG and silicon oil are used, but they are used for getting wider operating temperatures and for longer grease life.
Thickeners can be of different types: soaps, complexes, and non-soap. The soaps have lower operating temperature, performance, and they can be soda or sodium, calcium, lithium, aluminum. These are all soap greases and they can have operating temperatures up to 121° C. Complex greases can increase operating temperatures and can be calcium-complex, aluminum-complex or lithium-complex. Some non-soap greases of very high operating temperatures are thickened with polyurea, clay and teflon.
After base oil, thickener, we have additives. There are several types of additives, but some of the commonly used are rust inhibitors. Rust inhibitors provide a barrier to water that might come in contact with the bearing contact surfaces. Then there is an oxidation inhibitor, which disrupts the oxidation process.
There is an antiwear additive that, as the name suggests, reduces wear. We also have an extreme pressure additive, it reduces the chance of scuffing wear or shock-loading, which is possible in bearings. There is also polymer and polymers are typically used to modify the viscosity as per operating conditions or make the greases tackier. Also some of the other solid lubricants like moly disulfide (MoS2) and graphite are used and they provide excellent extreme pressure properties. These are all different types of additives.
The properties that really need to be reviewed prior to the grease selection are the consistency of the grease; is it very thin, is it very thick? What is NLGI grade? The shear stability, the work stability, the roll stability, the oxidation properties, the corrosion properties, extreme pressure property, wear properties, water wash-out, water spray-off property, as well as viscosity (surface separation due to film formation).