Putting the simple back into viscosity
Written by John Sander
Viscosity Defined
As mentioned above, viscosity is a physical measurement of a fluid's internal resistance to flow. Assume that a lubricating fluid is compressed between two flat plates, creating a film between the plates. Force is required to make the plates move, or overcome the fluid's film friction. This force is known as dynamic viscosity. Dynamic viscosity is a measurement of a lubricant's internal friction and it is usually reported in units called poise (P) or centipoise (1 P = 0.01 cP). A common tool used to measure dynamic viscosity is the Brookfield viscometer, which employs a rotating spindle that experiences torque as it rotate against fluid friction. This test will be discussed in more detail later.
A more familiar viscosity term is kinematic viscosity, which takes into account the fluid density as a quotient of the fluid's dynamic viscosity and is usually reported in stokes (St) or centistokes (1 St = 0.01 cSt). The kinematic viscosity is determined by using a capillary viscometer in which a fixed volume of fluid is passed through a small orifice at a controlled temperature under the influence of gravity.
Grease viscosity, traditionally called consistency, cannot be measured using the tests noted above. However, it is still relevant for selection of the correct grease for a specific application. Greases are fluid lubricants enhanced with a thickener to make them semi-solid. They usually are used in applications where a liquid lubricant would run out. Greases are sold by consistency grade, which in this case will be used synonymously to viscosity grade. Grease consistency is measured using the cone penetration test. The National Lubricating Grease Institute (NLGI) created grade ranges for greases that have become the industry standard. These ranges characterize the flow properties of greases.
