|
The Thick and Thin of
Viscosity
Of all the characteristics a lubricant may
possess, the most important is its viscosity. The viscosity of a fluid and how
that fluid reacts to certain variables will determine how well the fluid can
perform the basic functions of a lubricant. What is viscosity? When a fluid is
subjected to external forces, it resists flow due to internal molecular
friction. Viscosity is a measure of that internal friction. Viscosity can be
referred to as the measurement of a fluid’s resistance to flow.
Viscosity can be viewed in two different ways.
The first is a fluid’s tendency to flow as is visually indicated. One can think
of this as the time it takes to watch a fluid pour out of a container. The term
used to describe this is Kinematic Viscosity and it is expressed in units
indicating flow volume over a period of time. The most commonly
used unit of Kinematic Viscosity is the centistoke (cSt).
A fluid’s viscosity can also be indicated by
measured resistance. You can think of this as the energy required to move an
object through the fluid. It takes little energy to stir water with a spoon.
However, significantly more energy is required to stir honey with that same
spoon. The term used to describe this is Apparent Viscosity and it is expressed
in units known as centipoise (cP). There are other ways to refer to a fluid’s
viscosity. Some of the more common generic terms are thin, light and low. These
terms suggest the fluid flows readily. Water would be an example of such a
fluid. Terms such as thick, heavy and high suggest the fluid demonstrates a
strong resistance to flow. An example of such a liquid would be honey.
Viscosity is very important because it is directly related to a fluid’s load-carrying ability. The greater a fluid’s viscosity, the greater the loads that it can withstand. The viscosity of a fluid
must be adequate to separate moving parts at the operating temperature of the
equipment. Knowing that a fluid’s viscosity is directly related to its ability
to carry a load, one would think that the more viscous a fluid is, the better it
can lubricate and protect. The fact is, the use of a high viscosity fluid can be
just as detrimental as using too light an oil. If you use too low of a viscosity
oil, you get metal-to-metal contact, poor sealing and increased oil consumption.
If you use too high of a viscosity oil, you get increased fluid friction,
reduced energy efficiency, higher operating temperatures, and hard starting –
particularly at cold temperatures. The key is to select a fluid that is not too
light and also not too heavy.
Viscosity is a measurement of a lubricant’s
flow properties.
AMSOIL chemical engineers measure every batch of all
AMSOIL lubricants to ensure proper viscometric standards
are met.
Fluids thicken as their temperatures decrease
and thin as their temperatures increase (like candle wax). The extent that they
change is indicated by their viscosity index (VI). A viscosity index number
indicates the degree of change in viscosity of an oil within a given temperature
range, currenty 40-100°C. An oil with a high viscosity index, say 160, would
look and behave similarly at these two temperatures. However, a low viscosity
index oil, say 90, would be quite different. It would become very fluid, and
thin and pour easily at high temperatures. Honey will do the same if you heat it
up on a stove. This is why multi-grade oils exist.
Multi-grade oils are concerned with
performance at both high and low temperatures and are designed for use when
operating temperatures vary widely. Somebody driving from Fargo to Dallas in
January would want a multi-grade oil in his crankcase because it allows for
equipment operation in changing climates without the need for changing the oil.
A straight SAE 5W oil is suitable for low temperature applications, but thins
readily as the temperature increases. On the other hand, a straight SAE 30 is
suitable for high temperature applications, but thickens when the temperature
decreases. A 5W-30 would thicken and thin less in temperature extremes and would
thus be more suitable to the average motorist because it remains in a useable
viscosity range for a greater ambient temperature range.
AMSOIL
chemical engineers used this basic
principle to design the new XL- 7500 5W-20 Motor Oil required by 2001 Ford and
Honda vehicles.
Recommended for applications requiring GF-3
and Ford specifications, AMSOIL XL-7500 5W-20 Motor Oil offers superior engine
protection and unsurpassed fuel economy in engines calling for 5W-20. Its
synthetic formulation ensures low volatility, reducing oil consumption and
improving fuel economy throughout its entire service life of 7500 miles or six
months (whichever comes first). In below-freezing weather, XL-7500 remains fluid
and is pumped to sensitive engine parts easily and quickly, offering protection
at the critical moments following cold engine start-up. In warm weather, XL-7500
5W-20 doesn’t thin beyond an SAE 20 grade, so it keeps moving parts separated at
high speeds and temperatures.
|
