To anyone working with equipment having moving parts, grease is a familiar substance, commonly used as a lubricant. The conditions of its use - speed, load, nature of moving parts, moisture and chemical environment, temperature - dictate grease composition.
All grease is a dispersion of some thickening agent in a liquid lubricant (generally a mineral or vegetable oil). Most thickeners are derived from organic fats. Treating such fats with a strong alkali, such as a compound of sodium, lithium, or barium, produces a metallic salt of the fatty acid involved - in chemical terms, a "soap", capable of absorbing and holding liquid within a semi-solid matrix.
In lubrication terminology, that soap is called a "base". A common grease formulation involves a lithium base. Temperature extremes or other special conditions may dictate synthetic bases such as bentonite clay, with a diester or silicone oil.
To suit unusually high (10,000 rpm) or low (5-10 rpm) speeds, grease may contain graphite additives. In heavily-loaded bearings, extreme pressure (EP) additives are used. These include compounds of sulfur, chlorine, or phosphorous that react chemically with metal surfaces to enhance load-bearing capability. Still other grease additives reduce wear by filling in microscopic surface variations within bearings.
Greases are classified by various characteristics. One is the National Lubricating Grease Institute grade number, derived from measurement of the depth into a grease sample to which a standard weight drops within a specified time.
Another significant property is "dropping point" - the temperature at which the grease reverts to liquid. "Bleeding rate" measures how rapidly oil tends to separate out. "Shear stability", determined by observing how penetration rate varies as the grease is churned by a moving piston, is a measure of service life.
Though not widely available, electrically conductive grease containing a metallic filler can mitigate the problem of arcing within ball bearings subject to current flow when motors operate from transistorized variable-frequency inverters. However, bearing wear may occur more rapidly when such grease is used.
On this page is a summary of the Electrical Apparatus April 2004 featured technical article, by Richard L. Nailen, P.E. , "Grease: What it is; How it Works" Schmierfett: Was es ist und wie es funktioniert ... La graisse: qu'est-ce que c'est; comment fonctionne ... La Grasa: Lo que es, como funciona...
To order backissues which contain the foreign language summaries and the full article, call 312-321-9440 or visit our online webstore. See also links to additional technical summaries, below.
For our international readers, summaries of the technical articles are provided in German, French and Spanish. (Summaries do not appear when the article is U.S.-specific.)