Simple liquids are said to be Newtonian when the applied shear stress is directly proportional to the shear rate. In other words, the viscosity (applied shear stress/shear rate) of a Newtonian fluid is constant and does not depend on shear stress or shear rate at constant temperature.
A numerical scale for classifying the consistency of lubricating greases, based on the ASTM D217 worked penetration at 25Â°C (77Â°F). NLGI Grades are in order of increasing consistency (hardness) as follows:
|NLGI Consistency Number||Worked Penetration Range, 25Â°C (77Â°F)|
Some grease suppliers use descriptions such as NLGI Number 1.5, which indicates that the grease is between NLGI Numbers 1 and 2.
Some fluids and many plastic solids, including lubricating grease, exhibit non-Newtonian behavior. In other words, the viscosity (applied shear stress/ shear rate) is not constant, but depends on shear stress and shear rate at a given temperature. Thus, non-Newtonian fluids are described by their apparent viscosity, which may vary widely with the shear rate. Conventional types of viscometers with uncontrolled shear rates are not suitable for measuring Non-Newtonian materials.
Any of several specially treated naturally occurring or synthetic materials, excepting the metallic soaps, which can be either thermally or mechanically dispersed in liquid lubricants to form lubricating grease. Sometimes called Synthetic Thickener, Inorganic Thickener, or Organic Thickener.