Viscosity Defined
By Dave Wooton

Viscosity is one of the most important properties of lubricating oil. It is one factor responsible for the formation of lubricating films under both thick (hydrodynamic lubrication) and thin film (boundary lubrication) conditions. Viscosity affects heat generation in bearings, cylinders and gears due to internal fluid friction. It affects the sealing properties of oils and the rate of oil consumption. It determines the ease with which machines can be started at various temperatures, particularly cold temperatures. The satisfactory operation of any given piece of equipment depends on using oil with the proper viscosity at the expected operating conditions.

Viscosity describes how a liquid resists the laminar movement of two neighboring layers. This resistance to flow can be seen with gases, liquids and even solids.

The dynamic or absolute viscosity “η” is defined as the ratio of the shear stress to the velocity gradient vertical to the direction of flow. Absolute viscosity is defined as liquid's resistance to flow and shear. It is expressed as centipoise (cP) or pascal-seconds (Pa.S)

The quotient   (from the dynamic viscosity and density) is given as the kinematic viscosity.

Kinematic viscosity (KV) is liquid's resistance to flow and shear by the forces of gravity and is typically expressed in centistokes (cSt) or (mm 2/sec). Used oil analyses of kinematic viscosity at 40 oC and 100 oC are the standard for determination.

Absolute viscosity typically uses rotary viscometers to measure the torque on rotating spindle to measure a fluid's shear resistance. The Cold Cranking Simulator (CCS), Mini-Rotary Viscometer (MRV), Brookfield Viscometer and Tapered Bearing Simulator are all rotary viscometers. These rotational techniques are designed for low temperatures (below zero) or high viscosity ranges (2000 cP and up).

Kinematic viscosity is a capillary-low shearing viscometry technique. An ASTM Standard Test Method (1),(2) exists which details the kinematic viscosity test procedures for analysis of liquid petroleum products. D445 specifically says that it is "intended for application to liquids for which primarily the shear stress and shear rates are proportional (Newtonian flow behavior)." It requires the use of calibrated glass capillary viscometers, where the flow over a given volume is typically measured. A number of commercially available viscometric tube types for this measurement are cited in the ASTM standards, all of which fall under the classification of capillary viscometry. These tube types are determined by application. For example, the Cannon ®-Fenske Routine viscometer tube is the most often used for transparent liquids with kinematic viscosity up to 20,000cSt. While the Cannon ®-Fenske opaque viscometer is a common one used for reverse flow of opaque liquids.

1. ASTM D-445 “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity).”

2. ASTM D-446 “Standard Specifications and Operating Instructions for Glass Capillary Kinematic Viscometers.”

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