Greases are semi-fluid lubricants that are made with base oils such as mineral, synthetic, vegetable, or a blend of mineral and synthetic. Soap (thickener) is then added. Lithium is by far the most common thickener because it is inexpensive. Others thickeners are sodium, aluminum and calcium. Finally additives are blended in to provide extreme pressure properties, corrosion protection, anti-oxidation, etc. Greases are generally used where a fluid lubricant is not practical. Roller bearings, pins and bushings and open gears are typical applications.
Higher quality greases generally have additional solid lubricants added to help offer protection between sliding surfaces to prevent direct contact even at high loads. Graphite and moly (molybdenum disulfide (MoS2 )) are common solid lubricants. Boron nitride, tungsten disulfide and PTFE (aka Teflon) are also used as solid lubricants.
The type of thickener in your grease will determine some of the characteristics that are offered. Each thickener has different qualities. It’s very important to note that some thickeners are not compatible with others. You should always check for compatibility before switching to a new type of grease. Mixing incompatible greases can cause a drastic change in properties of the grease which can cause severe damage or failure to your equipment. The following paragraph lists some of the most common thickeners and some of the pros and cons of each.
-Lithium- Most common. Economical and readily available. Has good mechanical stability, some water resistance and oil separation resistance. Dropping point 385F.
- Bentone Clay - Has no dropping point. However it can cake up at high temperatures and is not compatible with other greases.
-Calcium Sulfonate- Good EP properties. Expensive. Good water resistance.
- Polyurea - Recommended for sealed for life electric bearings. Not compatible with other greases.
- Aluminum complex - Best for water washout. Good low temperature pumpability. Dropping point 470F.
Choose your NLGI# based on the application. The higher the NLGI number, the thicker the grease. It’s important to consider the RPMs of the bearings. The NLGI (National Lubricating Grease Institute) uses a measurement system to specify the thickness of a grease. For example, NLGI #2 grease (most common) is suitable for bearings between 500 – 2500 RPMs. The table below lists some other ratings.
RPMs NLGI Consistency #
500-2,500 #2 Most common.
Dropping points are the maximum temperature before the oil separates from the thickener. This is important to consider in severe duty applications. Bentone thickened greases have no dropping point and can be recommended for applications over 350F and up to 600F. Some grease manufactures have designed specialty greases designed to work at even higher temperatures.
Finally, you should consider pumpability. If you live in a warm climate this may not be as important, but if you live in colder areas you want to make sure that you can pump the grease, even on the coldest days. Some mechanics choose one NLGI grade lower in the winter to help with this.