What is a lubricating grease?
The American Society for Testing and Materials (ASTM) defined lubricating grease as: "A solid or semi-fluid lubricant consisting of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may be included".
This definition describes the major components in lubricating grease and has been used since 1955, but nothing is mentioned about the required properties or how it will work. In 1952 Vold and Vold suggested a different and more practical definition: "A grease is a lubricant which has been thickened in order that it remain in contact with the moving surfaces and not leak out under gravity or centrifugal action, or be squeezed out under pressure. Thus, a major practical problem is the provision of a structure that will stand up under shear and at all temperatures to which it may be subjected during use. At the same time the grease must be able to flow into the bearing through grease guns and from spot to spot in the lubricated machine as needed, and must not of itself add significantly to the power required to operate the machine, particularly at the start. This is an exacting set of rheological requirements".
Already in the 1950's people were aware of the need to describe lubricating grease by their rheological properties. Even in 1937 Klemgard wrote "Incidentally, flow studies of grease at various shear rates and temperatures are of considerable value in predicting service performance". More recently, in 1974, Sinitsyn suggested a new definition of lubricating grease incorporating the rheological properties. "A grease is a lubricant which under certain loads and within its range of temperature application exhibits the properties of a solid body, undergoes plastic strain and starts to flow like a fluid should the load reach the critical point, and regains solid-body properties after the removal of the stress".
Altogether these attempts to define lubricating greases, among many others, try to give a picture describing lubricating grease as a multiphase product with a complex rheological behaviour. Since there are no clear limits differentiating today's fluids (for example, many motor oils contain dispersed polymers for improvement of viscosity behaviour), an exact and clear definition might be impossible to give.
Lubricating grease is maybe best described as a thickener or gellant diluted by a lubricating fluid. To improve certain properties and functions, additional components are sometimes included (often called "additives"). This controversial way to define the thickener as being diluted by a lubricating fluid, rather than the more common way to describe it as thickening the fluid, comes from recent research. This work shows the active part, the gellant, at least when being a metal soap, takes in the actual lubrication contact zone (see next chapter for more information).
Lubricating grease properties depend on both its composition and the manufacturing process used. The actual possible compositions and the different available processes are too large a subject to be covered within the scope of this book, but the generalised principles are given below.
