Viscosity -- Viscosity Units
Viscosity is a measure of a fluid's resistance to flow when subjected to external forces. Viscosity expressed in this brochure is kinematic viscositythe ratio of dynamic or absolute viscosity to density-at the same temperatures. Kinematic viscosity is the value usually measured, but is easily converted to dynamic viscosity as follows:
centistoke x density = centipoise cSt x glcc = cP Because of the high density of Krytox® fluids, the dynamic viscosity is almost double the kinematic. Grease Viscosity
Typically, greases are compared based on the viscosity of their base oil. Because greases are thickened, they have a viscosity that is much higher than the base oil. The grease viscosity is affected by temperature, hardness grades, and base oil viscosity. Values for Krytox® GPL 205 are about 9000 cSt at 25°e (77°P) and 6000 cSt at lOOoe (212°P). The viscosity varies with shear rate. Speed Factor
The speed factor (DN) indicates the permissible speed range for a grease in a rolling bearing. The DN value is the inner race ID in mm x rpm. DN values of 100,000-400,000 and higher have been achieved at temperatures of 204-260°C (400- 5000P) in actual field service using Krytox® greases. The speed factor is affected by the base oil type, in addition to viscosity and thickener type, and is a measure of the lubricant's internal friction. The limiting speed for grease-lubricated rolling bearings is dependent on the type of bearing, its load, speed, precision, and lubrication system. Electrical Properties
Krytox® oils are good insulators. The electrical properties of additive-free Krytox® greases approach those of the oils; however, incorporation of some additives may significantly alter these values. In fact, the conductivity of the grease may be increased, if desired, by incorporating a conductive additive, such as powdered copper metal.
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