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by Ron Hughes,
Reliability Center, Inc.
Click here for a printable 65k pdf version
Since lubricating oil
can oxidize, so can the base oil in grease.
When the grease
oxidizes, it usually darkens; there is a build-up of
acidic oxidation products, just as in other
lubrications. These products can have a destructive
effect on the thickener, causing softening, oil
bleeding, and leakage. Because grease does not conduct
heat easily, serious oxidation can begin at a hot point
and spread slowly through the grease. This produces
carbonization and progressive hardening or crust
formation. All things considered the effect of
oxidation is more harmful in grease than in oil.
The rate of oxidation is
mainly dependent on the temperature.
Here
it is good to understand that if a soap-based grease is
heated, its penetration increases only very slowly until
a certain critical temperature is reached. At this
point the gel structure breaks down, and the whole
grease becomes liquid. This critical temperature is
called the DROP POINT.
Grease, when
heated above its drop point and then allowed to cool it
usually does not fully regain its grease-like
consistency, and its performance subsequently will be
unsatisfactory. Therefore, it follows that at no time
should the drop point be exceeded. The problem with
multi-purpose greases is that at least one of the agents
will exceed its drop point thus adversely affecting the
soaponification process. Grease has a maximum
temperature at which it can safely be used. Therefore,
it follows that it must also have a minimum
temperature. This minimum temperature is the point
where the grease becomes too hard for the bearing, or
other greased component, to be used. Again, the base
oil of the grease determines the minimum temperature.
Obviously, the base oil of the grease for
low-temperature service must be made from oils having a
low viscosity at that temperature.
The temperature limits
for use of greases are therefore, determined by drop
point, oxidation, and stiffening at low temperatures.
Grease can be divided
into 6 general categories or types; i.e., mixtures of
mineral oils and solid materials, heavy, asphaltic-type
oils blended with lighter oils, extreme-pressure
greases, roll-neck greases, soap-thickened mineral oils
and multi-purpose grease.
MINERAL OILS MIXED WITH SOLIDS
These types
of greases are very heavy lubricants for specialized
applications. Such greases lubricate rough-fitting
machine parts operating under heavy pressures or loads
at relatively slow speeds. Examples of equipment that
will typically use this type of grease include concrete
mixers, bearings and rollers on conveyors and heavy
construction equipment.
HEAVY ASPHALTIC-TYPE OILS BLENDED
WITH LIGHTER OILS
These types of
lubricants are classified as greases but are actually
thick, heavy oils used to lubricate open-type gearing
and wire rope. A primary advantage of these oils is
that they form a heavy protective film when heated or
painted on surfaces and then allowed to cool. Lighter
oil is typically blended with the heavy oils in order to
improve the pour point of the oil.
EXTREME-PRESSURE GREASES
The unique
characteristic of this type of grease is that it
contains additives to improve firm strength under
various applications. In essence, film strength
provides the resistance of the lubricant to being torn
apart, thus preventing metal-to-metal contact of the
equipment being lubricated. A film is formed by a
chemical reaction of the metal to the additives in the
grease. The chemical reaction is usually brought about
(or accelerated) by pressure exerted on the grease,
creating heat.
A few of the additives
used in EP greases include compounds containing parts of
chlorine, phosphorus, active and/or passive sulfur,
chlorinated waxes and phosphates. Zinc and lead may
also be added, as well as asbestos in some lubricants as
a filler to cushion the shock loading on gear drives.
The specific additive being used will always depend on
the application for use. Factors to be considered for
types of additives include specific equipment operating
conditions such as load, speed, surface condition and
inherent design characteristics.
ROLL NECK GREASES
Roll neck
greases are specialized lubricants used almost
exclusively for lubricating plain bearings in rolling
equipment. For example, it’s fairly common to use a
block of NLGI No. 6 grease, which has the consistency of
a bar of soap, carved to mate with the shape needed to
accommodate the bearing of heavily loaded equipment.
SOAP THICKED MINERAL OILS
This is by far the most
widely used category of grease in industry today. This
type of grease varies by the additive that forms the
soap in the lubricants chemical makeup.
Sodium-base
greases are also general-purpose greases. Because they
have a higher dropping point (approximately 300o
to 350ºF), they are often used to lubricate machine
parts operating near heat. Sodium greases made with
lighter oils are used for ball and roller bearing
lubrications, as are combinations (mixed base) of
calcium and sodium grease
Sodium-soap greases have
a spongy or fibrous texture and are yellow or green in
color. Because of their working stability and
intermediate melting point, they are used for
lubricating wheel bearings (other than disc brakes) and
for general-purpose industrial applications. Typical
examples include rough, heavy bearings operating at low
speeds, as well as skids, track curves and heavy-duty
conveyors.
Barium-soap
greases are general-purpose types, valued for their
ability to work over a wide temperature range. Their
dropping point is approximately 350ºF or higher,
although they are not intended to be used in continuous
operation at temperatures above 275ºF.
Barium-soap greases are
chosen for a variety of jobs, especially for nearly all
types of bearing lubrications. They have a high-soap
content. However, this makes this type of lubricant
less suitable for use at low temperatures and in very
high-speed applications. They have a buttery of fibrous
texture and are reddish-yellow or green in color.
Lithium-soap
grease handles extremes of temperature quite well, which
makes them highly suitable for both high and low
temperature application. They have a dropping point of
approximately 350ºF, and can be used in continuous
temperatures of 300ºF. One reason for their successful
low-temperature performance is that they are made with
oil having a low pour point. In fact, lithium-soap
greases have been used successfully at temperatures of
-60ºF. Use of a lithium-soap grease at higher
temperatures requires a different formula, however, the
same grease can't be used at both extremes of high and
low temperatures because the change would be in the
viscosity of the oil used in the grease.
Basically, lithium-soap
greases have very good stability; good water resistance,
and are also readily pumpable. They have a buttery
texture and a brownish-red color.
Calcium-soap,
also
called lime-soap greases, are probably the best known
and most often used of all greases. Depending on the
method of manufacture they are usually relatively
inexpensive. Uses include axle grease, water pump
grease and general machinery applications.
Because its water
content begins to dry out, and the soap and oil
separate, calcium-soap grease isn’t suited to
applications where the temperature will get above 160ºF.
The major advantage of
calcium-soap grease is that they don’t dissolve in
water. However, it is not suited to use in lubricating
high-speed bearings. Ordinary general-purpose
calcium-soap greases have a dropping point of
approximately 175o to 200ºF.
Calcium-complex grease
has unusually high heat resistance making it of
considerable value in extreme-pressure applications.
The dropping point of this type of grease is 500ºF or
even higher. This means that this type of lubricant
will maintain its stability while running continuously
at high temperatures. However, this type of grease has
not replaced lithium-soap greases because they are not
as mechanically stable.
Calcium-soap greases are
yellow or reddish in color, and have a smooth buttery
texture.
Aluminum-soap
greases are special-purpose lubricants. Their
particular advantage is that they are very sticky making
them perfect for applications requiring surface
lubrication.
MULTI-PURPOSE GREASE
Multi-purpose greases
combine the properties of two or more specialized
greases. This permits the use of a single type of
grease for a variety of applications. It is possible to
replace as many as six specialized greases with single
multi-purpose grease and get better results all at the
same time. Most of the multi-purpose greases have a
soap base of barium, lithium, or calcium complex. For
example, the lithium-soap greases discussed earlier.
They are not only water-resistant and corrosion
inhibiting, but they have very good mechanical and
oxidation stability as well.
By reducing the number
of lubricants, which a company keeps in stock, the
lubricator's job becomes much easier. Another advantage
is that it helps reduce the chances of error in
application.
Good multi-purpose
lubricating grease has to perform well in a number of
applications. It should have a high melting point, and
operate well at continuous temperatures of 250ºF or
more. Such grease should also have good resistance to
water, and exceptional stability.
SUMMARY
Of the
numerous applications for grease, most are for
lubricating bearings of various types. Bearings fall
into two main categories; i.e., plain or anti friction.
In addition, these types of lubricants are often used
for the lubrication of ways and guides.
As a general rule,
grease used for lubricating ways and slides are
sodium-base greases. Plain bearings on the other hand
use grease for limited speeds, typically below 300 RPM
with a practical maximum of about 400 RPM. On the other
hand, greases for anti-friction (high-speed) bearings
include those used for plain bearings with the exception
of barium greases. Barium should never be used for
high-speed applications.
Extreme pressure greases
are commonly used in heavy-duty ball and roller
bearings, as well as plain bearing applications that are
subjected to high-loading conditions. A gear set is a
perfect example where EP grease is used to overcome
high-load conditions. However, operating temperatures
are typically limited to a range of 150o to
200ºF for this type of lubricate.
About the author
Ron Hughes, a Mechanical Engineer, has spent 27 years as
an engineer, supervisor, instructional designer and
trainer. The bulk of Mr. Hughes’ experience has been in
the power industry. Mr. Hughes has conducted front end
job/task analysis, systematic design and development of
a vast array of training seminars. He has provided
training in maintenance, engineering in the mechanical,
electrical and civil arenas as well as various
management topics. He is a certified nuclear instructor.
In addition he is also experienced in defining
performance standards and auditing their outcome. Mr.
Hughes is employed with Reliability Center, Inc. as a
Reliability consultant, trainer and instructional
technologist -
rhughes@reliability.com.
Related site:
Lubrication-FluidPower.com
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