What is viscosity?

What is viscosity?

As we saw in yesterdays’ blog posting we need to be concerned about the viscosity of the lubes we employ in our WW2 jeeps (of course this is true in any vehicle).  More from EM 1110-2-1424 28 Feb 99:

Effect of viscosity on flow of light and heavy oils.

Note–The spout in each container is of the same size, and each grade of oil has flowed for the same time.  Graphic from TM 10-540, December 26, 1940

Viscosity. Technically, the viscosity of an oil is a measure of the oil’s resistance to shear.  Viscosity is more commonly known as resistance to flow. If a lubricating oil is considered as a series of fluid layers superimposed on each other, the viscosity of the oil is a measure of the resistance to flow between the individual layers. A high viscosity implies a high resistance to flow while a low viscosity indicates a low resistance to flow. Viscosity varies inversely with temperature. Viscosity is also affected by pressure; higher pressure causes the viscosity to increase, and subsequently the load-carrying capacity of the oil also increases. This property enables use of thin oils to lubricate heavy machinery. The load-carrying capacity also increases as operating speed of the lubricated machinery is increased. Two methods for measuring viscosity are commonly employed: shear and time.

(1) Shear. When viscosity is determined by directly measuring shear stress and shear rate, it is
expressed in centipoise (cP) and is referred to as the absolute or dynamic viscosity. In the oil industry, it is more common to use kinematic viscosity, which is the absolute viscosity divided by the density of the oil being tested. Kinematic viscosity is expressed in centistokes (cSt). Viscosity in centistokes is conventionally given at two standard temperatures: 40 EC and 100 EC (104 EF and 212 EF ).

(2) Time. Another method used to determine oil viscosity measures the time required for an oil sample to flow through a standard orifice at a standard temperature. Viscosity is then expressed in SUS (Saybolt Universal Seconds). SUS viscosities are also conventionally given at two standard temperatures: 37 EC and 98 EC (100 EF and 210 EF). As previously noted, the units of viscosity can be expressed as centipoise (cP), centistokes (cST), or Saybolt Universal Seconds (SUS), depending on the actual test method used to measure the viscosity.