What Is Grease?
According to the Practical Handbook of Lubrication, grease is a lubricant composed of a fluid lubricant thickened with a material that contributes a degree of plasticity.
Greases are typically used in areas where a continuous supply of oil cannot be retained, such as open bearings or chassis components.
Greases are comprised of three essential components: a base fluid,a thickening system and an additive system. Different types and combinations of thickeners and base fluids, along with supplemental structure modifiers and performance additives, combine to give the final product its special lubricating properties. I like to use the analogy of a sponge saturated with oil.
Base Oil – Many different types of base oils may be used in the manufacture of a grease, including petroleum (napthenic, parafinic) and synthetic (PAO’s, esters, silicones, glycols). Just as with motor oils and transmission fluids, the viscosity of the base oil is the most significant property. Lighter, lower viscosity base oil is used to formulate low temperature greases or greases suitable for high rotational speeds, while heavier, higher viscosity base oils are used to formulate greases used in applications where high loading is encountered, high temperature, and or low rotational speeds are seen.
Thickener- Thickener is the term describing the ingredients added to a base oil in order to thicken it to a grease structure. The two basic types of thickeners are organic thickeners and inorganic thickeners. Organic thickeners can be either soap-based or non-soap based, while inorganic thickeners are non-soap based.
Simple soaps are formed with the combination of a fatty acid or ester (of either animal or vegetable origin) with an alkali earth metal, reacted with the application of heat, pressure or agitation through a process known as saponification. The vessel most commonly used to “cook” greases is referred to as a contactor. Think of a large pressure cooker with rotating blades like that found in a blender. The fiber structure provided by the metal soap or other thickener system determines the mechanical stability and physical properties of the finished grease as well as other factors.
In order to take on enhanced performance characteristics, including higher dropping points, a complex agent is added to the soap thickener to convert it to a soap salt complex thickener. The greases are then referred to as “complexes” and include the most popular thickener system, lithium complex, as well as aluminum complex and others.
Many factors combine to determine the performance characteristics of the finished lubricant. The time spent blending and cooking the grease, the temperature at which it is blended and even the timing of additive blending and the milling which occurs after blending all contribute to the high levels of performance provided by superior products in the marketplace.
Additives- Chemical and metallic additives are added to grease in order to enhance their performance, much like the additives added to lubricating oils. Performance requirements, compatibility, environmental considerations, color and cost all factors of additive selection.
Greases may be categorized according to their thickener system, for example lithium complex or bentone, or polyurea, their metallic or solid additive constituent, for example moly or Teflon, their performance characteristics, as an example, high temp, low temp, impact resistant or by application, ie.water pump, wheel bearing, chassis etc.
Grease consistency correlates to the firmness of the grease. Depending on the applications they’re designed for, greases can range from semi fluid consistencies to almost solid. Care must be taken to select the correct consistency for the application. If the grease is too hard, it may not adequately flow to the areas in need of lubrication. If it is too soft, it may leak away from the desired area. Since consistency directly correlates to pumpability, equipment greased through a dispensing system may require a grease representing a compromise between what is required for lubrication and what can be adequately pumped by the hardware used.
Consistency is measured with the ASTM Cone Penetration Test D 217. Under prescribed conditions, a standardized cone is allowed to drop into the grease for 5 seconds. The level of penetration is measured to determine its NLGI consistency number, ranging from 000 to 6. The higher the penetration number, the lower the consistency number.
Oxidation stability has to do with the performance of a grease after being exposed to oxygen. Both the ASTM D 942 Bomb Oxidation Test and the ASTM D 3527 High Temperature Bearing Life Test are used to measure oxidation.
Exposure to water may impact grease performance in several ways. First, it can cause grease to change consistency, becoming softer or firmer. Second, it may change the grease’s texture, perhaps becoming less adhesive. Third, it may form an emulsion with the grease, losing its lubricating effectiveness or washing away.
The Water Washout (ASTM D 1264) Test and Water Spray 0ff: (ASTM D 4049) Test measure the amount of grease washed away under specific water washing and spraying conditions. The Rust Test ASTM D 1743, measures rust inhibiting characteristics and the ASTM D 4048 test measures copper corrosion.
A lubricant’s main job is to separate bearing surfaces to prevent wear. If the amount of lubricant is inadequate, the lubricant film becomes so thin that asperity contact may occur. Known as boundary lubrication, it may cause a modest level of wear on one or both bearing surfaces, which may be addressed with specific additives.
Lubricants differ in their load-carrying abilities, at times maintaining a thicker film and/or acting chemically on the surfaces to prevent them from welding.
Three separate tests for load-carrying capability are commonly used. The Four-Ball Wear ASTM D 2266 measures wear at light loads, while the Four-Ball EP ASTM D2596 and Timken EP ASTM D 2509 indicate more severe wear or welding. Keep in mind that Four ball tests effectively measure performance under point contact conditions while the Timken test measures performance under line contact conditions. These performance characteristics may not necessarily correlate with each other.
One should always be aware that the use of various tests such as these may be hyped to show decidedly superior results that the typical layman will be impressed with. There are many threads on various boards alluding to the claims that various manufacturers or additive marketers make stating their products have superior load carrying characteristics or wear reducing capabilities. This is an area that is often exploited. The important aspect of this is that one must recognize that a reputable manufacturer of grease (as well as a blended oil) must achieve a balanced approach to the formulation of their products. There are many products and additive marketers who hype one characteristic of their product that may have no correlation whatsoever to the actual conditions that exist between the metal surfaces the product is designed to protect.