Lubrication is of two general types based on the operating environment;
that is, load and speed of the equipment and viscosity of the lubricant.
Smooth surfaces separated by a layer of lubricant do not come into
contact and, hence, do not contribute to frictional forces. This
condition is called hydrodynamic lubrication. Boundary lubrication, on
the other hand, arises when there is intermittent contact between
surfaces, resulting in significant frictional forces.

Dynamic viscosity is usually reported in poise (P) or centipoise (cP,
where 1 cP = 0.01 P), or in SI units as pascal-seconds (Pa-s, where 1
Pa-s = 10 P). Dynamic viscosity, which is a function of only the
internal friction of a fluid, is the quantity used most frequently in
bearing design and oil flow calculations.

Because it is more convenient to measure viscosity in a manner such that
the measurement is affected by oil density, kinematic viscosity is
normally used to characterize lubricants. Kinematic viscosity of a fluid
equals its dynamic viscosity divided by its density, both measured at
the same temperature and in consistent units. The most common units for
reporting kinematic viscosity are stokes (St) or centistokes (cSt, where
1 cSt = 0.01 St), or in SI units as square millimeters per second
(mm2/s, where 1 mm2/s = 1 cSt).

Dynamic viscosity in centipoise can be converted to kinematic viscosity
in centistokes by dividing by the fluid density in grams per cubic
centimeter (g/cm3) at the same temperature. Kinematic viscosity in
square millimeters per second can be converted to dynamic viscosity in
pascal-seconds by multiplying by the density in grams per cubic
centimeter and dividing the result by 1000.

Other viscosity systems, including Saybolt, Redwood, and Engler, have
also been used because of their familiarity to many people. The
instruments developed to measure viscosity in these systems are rarely
used. Most viscosity determinations are made in centistokes and
converted to values in other systems.

The viscosity of any fluid changes with temperature, increasing as
temperature decreases, and decreasing as temperature rises. Viscosity
may also change with a change in shear stress or shear rate.

To compare petroleum base oils with respect to viscosity variations with
temperature, ASTM Method D 2270 provides a means to calculate a
viscosity index (VI). This is an arbitrary number used to characterize
the variation of kinematic viscosity of a petroleum product with
temperature. The calculation is based on kinematic viscosity
measurements at 40 and 100°C. For oils of similar kinematic viscosity,
the higher the viscosity index, the smaller the effect of temperature.

The benefits of higher VI are: 1. Higher viscosity at high temperature,
which results in lower engine oil consumption and less wear. 2. Lower
viscosity at low temperature, which for an engine oil may result in
better starting capability and lower fuel consumption during warm-up.

The measurement of absolute viscosity under realistic conditions has
replaced the conventional viscosity index concept in evaluating
lubricants under operating conditions.

Another factor in viscosity measurements is the effect of shear stress
or shear rate. For certain fluids, referred to as Newtonian fluids,
viscosity is independent of shear stress or shear rate. When viscosity
is affected by changes in shear stress/shear rate, the fluid is
considered non-Newtonian.

Kinematic viscosity measurements are made at a low shear rate (100 s-1).
Other methods are available to measure viscosity at shear rates that
simulate the lubricant environment under actual operating conditions.
Different instruments used to measure kinematic viscosity are:

1. Capillary Viscometers measure the flow rate of a fixed volume of
fluid through a small orifice at a controlled temperature. The rate of
shear can be varied from near zero to 106 s-1 by changing capillary
diameter and applied pressure. Types of capillary viscometers and their
mode of operation are:

Glass Capillary Viscometer — Fluid passes through a fixed-diameter
orifice under the influence of gravity. The rate of shear is less than
10 s-1. All kinematic viscosities of automotive fluids are measured by
capillary viscometers.

High-Pressure Capillary Viscometer — Applied gas pressure forces a fixed
volume of fluid through a small-diameter glass capillary. The rate of
shear can be varied up to 106 s-1. This technique is commonly used to
simulate the viscosity of motor oils in operating crankshaft bearings.
This viscosity is called high-temperature high-shear (HTHS) viscosity
and is measured at 150°C and 106 s-1. HTHS viscosity is also measured by
the Tapered Bearing Simulator (see below).

2. Rotary Viscometers use the torque on a rotating shaft to measure a
fluid's resistance to flow. The Cold Cranking Simulator (CCS),
Mini-Rotary Viscometer (MRV), Brookfield Viscometer and Tapered Bearing
Simulator (TBS) are all rotary viscometers. Rate of shear can be changed
by changing rotor dimensions, the gap between rotor and stator wall, and
the speed of rotation.

Cold Cranking Simulator — The CCS measures an apparent viscosity in the
range of 500 to 200,000 cP. Shear rate ranges between 104 and 105 s-1.
Normal operating temperature range is 0 to -40°C. The CCS has
demonstrated excellent correlation with engine cranking data at low
temperatures. The SAE J300 viscosity classification specifies the
low-temperature viscometric performance of motor oils by CCS limits and
MRV requirements.

Mini-Rotary Viscometer (ASTM D 4684) — The MRV test, which is related to
the mechanism of pumpability, is a low shear rate measurement. Slow
sample cooling rate is the method's key feature. A sample is pretreated
to have a specified thermal history which includes warming, slow
cooling, and soaking cycles. The MRV measures an apparent yield stress,
which, if greater than a threshold value, indicates a potential
air-binding pumping failure problem. Above a certain viscosity
(currently defined as 60,000 cP by SAE J 300), the oil may be subject to
pumpability failure by a mechanism called "flow limited" behavior. An
SAE 10W oil, for example, is required to have a maximum viscosity of
60,000 cP at -30°C with no yield stress. This method also measures an
apparent viscosity under shear rates of 1 to 50 s-1.

Brookfield Viscometer — Determines a wide range of viscosities (1 to 105
P) under a low rate of shear (up to 102 s-1). It is used primarily to
determine the low-temperature viscosity of automotive gear oils,
automatic transmission fluids, torque converter and tractor fluids, and
industrial and automotive hydraulic fluids. Test temperature is held
constant in the range -5 to -40°C.

The Scanning Brookfield technique measures the Brookfield viscosity of a
sample as it is cooled at a constant rate of 1°C/hour. Like the MRV,
this method is intended to relate to an oil's pumpability at low
temperatures. The test reports the gelation point, defined as the
temperature at which the sample reaches 30,000 cP. The gelation index is
also reported, and is defined as the largest rate of change of viscosity
increase from -5°C to the lowest test temperature. This method is
finding application in engine oils, and is required by ILSAC GF-2.

Tapered Bearing Simulator — This technique also measures
high-temperature high-shear rate viscosity of motor oils (see High
Pressure Capillary Viscometer). Very high shear rates are obtained by
using an extremely small gap between the rotor and stator wall.