Lubrication
and Oil Analysis
Most
lubricants used in industry are mineral based and obtained
from petroleum by refining processes and further purification and
blending.
Function
of a Lubricant
Lubricants
have three major functions: limit friction, minimize wear and
dissipate heat.
Limit
Friction
Friction
is defined as the resistance to motion of contracting surfaces. Even
smooth metal surfaces have microscopic rough spots called asperities.
Friction is increased by the presense of asperities on surfaces.
Attempts to overcome the force of friction will increase the
localized heat generated by the contracting surfaces. This heat can
actually create temperatures high enough to weld two surfaces
together.
Lubrication
prevents peaks of asperities from touching each other through what is
called film strength. Molecules of lubricants are naturally
bonded together, often in chains. Any attempt to break the chain
creates an opposite tension that prevents separation.
Minimize
Wear
Wear is
the removal of material from one or more moving surfaces in contact
with each other. The material removed becomes the source of
additional friction and increased wear on the surfaces involved.
A
quality lubricant will fill the valleys of the asperities and
provide an additional film over the peaks of the asperities. The
asperities of the two surfaces are prevented from contracting each
other and wear will be minimized.
Dissipate
Heat
Even
well-lubricated parts will heat up as a result of friction and
external heat. One advantage of liquid lubricants is their
ability to absorb and dissipate point sources of heat.
Types
of Industrial Lubricants
Lubricants
can be divided into two types: solid and liquid.
Solid
Lubricants
Solid
lubricants are materials such as graphite, molybdenum
disulfide and PTFE ( polytetrafluoroethylene ); they are used in
smaller equipment or on surfaces where just a minor amount of
movement is expected. Lead, babbitt, silver, gold and some metallic
oxides are solid lubricants that can provide for more movement
or pressure between surfaces. Some machines designers use ceramics or
inter-metallic alloys to coat the surfaces of moving parts; they can
also be considered lubricants.
Liquid
Lubricants
Liquid
lubricants in industry fall into two categories: greases and
oils. We will look at both of these types of lubricants
in the following sections.
Measuring
the Properties of Grease and Oils
Criteria
for measuring the properties of grease are these:
Hardness.
Greases range from hard to soft. The NLGI is the National Lubricating
Grease Institute.
Hardness
of Grease
NLGI Number
|
Consistency
|
ASTM Worked
Penetration
at 25°C ( 77°F )
10-1 mm
|
000
|
Fluid
|
445-475
|
00
|
Semi-fluid
|
400-430
|
0
|
Very Soft
|
355-385
|
1
|
Soft
|
310-340
|
2
|
Common Grease
|
265-295
|
3
|
Firm
|
220-250
|
4
|
Very Firm
|
175-205
|
5
|
Hard
|
130-160
|
6
|
Very Hard
|
85-115
|
Grease
with an NLGI #000 are like liquid, whereas #6 greases are almost
solid. The most frequently used grease are #0, #1 and #2.
Dropping
Point
This
is the temperature at which the grease will change from semisolid to
liquid, basically the melting point.
Water
Resistance
This
determines whether a grease will dissolve in water. This is very
important quality if there is a chance that water will come in
contact with the lubricant.
Stability
This
property determines the ability of a grease to retain its
characteristics with time.
Criteria
for measuring the properties of oil are these:
Viscosity
This
is the most important characteristics; it refers to the thickness of
the fluid and can also be described as the resistance to flow.
Viscosity is affected by temperature and decreases as temperature
increases. There are many ways of measuring viscosity but they are
also based on the time taken for a fixed volume of oil to pass
through a standard orifice under laboratory conditions.
There
are three commonly used terms for viscosity: Saybolt Universal
Seconds ( SUS ), centipoise ( cP ) and centistokes ( cSt ).
Saybolt
Universal Seconds is an indication of the time it takes 60
millimeters of fluid to flow through a calibrated Saybolt Universal
Tube ( also called Viscosimeter ). This is an old method used to
describe viscosity.
Centipoise
is an absolute viscosity unit in the metric system. A centistoke is
0.01 stoke. A 1 stoke fluid has an absolute viscosity of 1 poise and
a density of 1 gram per cubic centimeter. In other words, centistokes
differ from centipoise by a density factor.
Viscosity
Index
This
is the rate of change of viscosity with temperature. A high viscosity
index shows that the oil will remain the same over a wide
range of temperatures, whereas a low index indicates that the oil
will thin rapidly with an increase temperature.
Flash
Point
This
is the temperature at which the vapor of a lubricant will
ignite.
Fire
Point
This
is the temperature ( higher than flash point ) that is required to
form enough vapor from the lubricant to cause it to burn
steadily.
Pour
Point
This
is the low temperature at which the lubricant becomes so thick
and it is not capable of flowing.
Oxidation
Resistance
If
oil is exposed to the atmosphere, especially at a higher
temperature, oxygen is absorbed into the oil. A chemical
change takes place in the oil that drastically reduces its
lubricating properties.
Emulsification
This
is the measure of the tendency for oil and water to mix
together.
Grease
Applications:
Oil
Analysis
The
most commonly used technique to monitor oil and use the
information to indicate both the serviceability of the oil and
the internal wear of components is spectrographic oil analysis.
Often this service is provided free by the oil vendor to the
plant. Large hydraulic oil reservoirs, gearbox sumps and
internal combustion equipment are prime candidates for lubrication
testing.
Taking
a sample is not hard but needs some thinking. The correct sample
desired is a representative sample of the oil in the
reservoir. The best time to take a sample is with the machinery
running, if it is possible. The oil port is opened and a clean
tube connected to a small portable hand-powered vacuum pump is
inserted into oil port. The tube is lowered into the oil
reservoir and a sample is pulled using the vacuum pump. The bottle is
capped, identified, labeled and sent out for testing. The tube and
the pump are cleaned with acetone or any fast drying solvent and made
ready for the next sample to be taken. In many cases, however, taking
the sample while the machine is running is uneasy. Inserting the
sample tube into the gearbox and catching the tube in the gear mesh
is a possibility. In these cases, the machine needs to be shut down
and the sample taken is as soon as possible to ensure that the oil
still mixed well and particles suspended in the oil captured
in the sample.
Imagine
that the oil being sampled is from automobile engine or diesel
engine that might be attached to the emergency generator. The result
back from the analytical lab show the oil is serviceable but
loaded with chrome. The report shows what would be expected as a
normal chrome plated. In this case, ring wear might be suspected from
this simple test. A compression test on the cylinders could be run to
confirm this analysis of the condition. Obviously, it takes a lot of
time to run a compression test on all cylinders as opposed to a few
minutes to get an oil sample and send it to the laboratory.
Oil analysis makes more sense as a screening type of tool. If
a problem is found, then the additional check of equipment
components makes good economic sense.
Other
good candidates for oil analysis are large gearboxes and large
hydraulic reservoirs. Spending a few dollars on an oil analysis
versus draining all the oil and removing inspection plates or
covers to physically inspect the critical components is money well
spent.
Oil
Applications:
