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