Equipment manufacturers often make recommendations for a grease and use such terminology such as lithium complex soap, dropping point 230°C minimum, NLGI No. 2 consistency, etc.

What do these grease terms mean? While there are many specifications and performance requirements expected on greases, we will just cover the basic but important ones.

By definition, grease is a fine dispersion of an oil-insoluble thickening agent (usually a soap) in a fluid lubricant (generally a mineral oil). Additives are used in greases to improve a property already present in the lubricant or give it properties not naturally possessed.

Each grease thickener has it's own set of traits and benefits. There is no right or wrong thickener and it's choice is usually based on the grease appli- cation. The most common greases are currently thick- ened with lithium simple and lithium complex soaps. However polyurea, aluminum com- plex, calcium and bentone are examples of others used in the market place.

While thickening agents impart many properties to a grease, the main one is dropping point. The dropping point of a grease, is that temperature at which the grease passes from a semi-solid to a liquid state. The dropping point is a qualitative indication of the heat resistance of grease on applications where a semi-solid lubricant is required. The dropping point is not the recommended operating temperature which is usually much lower. For example, a lithium simple soap thickened grease may have a dropping point of 195°C but its recommended maximum service temperature is 160°C.

The NLGI Grade of a grease is the measure of its consistency on a scale from 0 to 6. Penetration is a measure of consistency of a grease to determine its plasticity. Penetration, with respect to a lubricity grease is the depth (in tenths of a millimeter) that a standard cone penetrates a sample of the grease under prescribed conditions of weight, time, and temperature.

With the issuance of ASTM D 4950 Standard Classification and Specification for Automotive Service Grease, it became not only possible but convenient to offer the products needed to provide proper service of automotive equipment. ASTM D 4950 includes specifications for two Category Groups: chassis lubricants (letter designation L) and wheel bearing lubricants (letter designation G). Performance Classifications within these Groups result in two letter designations for chassis greases (LA and LB), and three for wheel bearing greases (GA, GB, and GC). The automotive industry is in general agreement that the highest performance classification, to date, in each group (LB and GC) is suitable for service relubrication. Any greases meeting these requirements will carry the NLGI certification mark on the tube.

Another important characteristic of a grease is base oil viscosity. When pressure is applied to a grease, oil is released to lubricate and give the desired performance. For very high speeds, a low viscosity oil is required in the grease with a high oil release. For low speeds, a high viscosity oil is needed because it will support high loads, low oil release characteristics. For example, electric motor bearings would use a grease with a base oil viscosity of 100 cSt @ 40°C (ie ISO VG 100 viscosity grade). Large paper mill bearings would use a grease with a base oil viscosity of 460 cSt @ 40°C (ie ISO VG 460 viscosity grade).

Grease additive technology is a sophisticated science that the type of additives used can influence the structure of the grease (ie consistency, oil separation, dropping point, etc). Additive types and amounts will vary according to the application to which the grease will be subjected.

Some of the more common types of additives are anti-wear, extreme pressure, corrosion inhibitors, anti-oxidants, tackifiers, and dyes.

The effectiveness of additives in greases is confirmed by the laboratory tests outlined in the physical characteristics of the grease. For example, extreme-pressure (EP) properties can be measured by the Timken Tester, or the Four Ball EP Tester which can also measure anti-wear properties. Other important laboratory tests that are conducted on greases are Oil Separation, Wheel Bearing Leakage, Rust Prevention Rating, Copper Strip Corrosion, Roll Stability, Water Washout Tendency and Oxidation Stability.

A special thank-you to Roland Parker, Technical Manager, Wynn's Australia Pty Ltd for the use of excerpts of his article published in the December-January 2002 issue of Australian Turf grass Management Volume 3.6.