Selecting The Right Grease
The application of grease can be at times very complex, although a little knowledge goes a long way. For example, in construction equipment the environment is harsh with water, dirt, poor seals and heavy loads being common. In this type of situation, you need a grease that is highly water resistant, tenacious, with good rust protection and film strength. Barium greases work well along with the lithium 12‑hydroxy greases (usually with polymers). Some specific formulations using an aluminum-complex thickener also find use here.
Bearings (ball, roller, needle, etc.) normally see much less contamination but experience wide variations in both speed and temperature. Here, you need a grease that has outstanding mechanical stability (very minor softening or hardening), excellent oxidation stability (long life) and performance over a wide temperature range. Where temperatures do not exceed 250 degrees F, the lithium 12‑hydroxy greases find the widest use. When temperatures increase or long life is required, either a lithium-complex or polyurea can be used.
Sliding or rubbing contact such as with automotive ball joints, sleeve bearings, some gears, etc., requires both extreme pressure and anti‑wear qualities along with a tenacious film to prevent squeeze-out, washout or both. In this type of application, the use of solid lubricants to aid in the prevention of wear is common, with molybdenum disulfide being the dominant solid. Here, the lithium 12‑hydroxy, lithium complex, barium and aluminum-complex greases would be good choices.
Improper selection and application of the proper type of grease to perform in the intended application is one of the biggest reasons for failure. Also at the top of the list is grease compatibility or the lack of it. As a general rule, keep similar thickeners in the same application (e.g., lithium with lithium complex). If possible, do not mix greases of different thickener types.
Especially try to avoid situations where aluminum complex, clay or silica-thickened greases may become mixed with other greases. Excessive softening is usually the result. These thickeners, while producing excellent lubricating greases, do not tolerate other greases well.
Failure of almost any grease‑lubricated part generally falls into four categories:
- The selection and application of the wrong type of grease
- Incompatibility resulting in excessive softening
- Contamination resulting in excessive wear
- Too much or too little grease applied
Remember, the lack of grease can cause failure, but too much grease is often the primary cause of failure because the excessive lubricant causes high temperatures, which in turn oxidizes the grease.
Ref: Machinery Lubrication