MGLS SYNERGY

GREASE was first used by the Egyptians on their chariot axles more than 3000 years ago. Today, over 80% of the world's bearings are lubricated with grease. Lithium soap greases - the most common worldwide - were introducted in the early 1940s.

Lithium complex grease, which are becoming the most popular in North America, were introduced in the early 1960s. The National Lubricating Grease Institute (NLGI), defines grease as:

" A solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included."

Some people describe grease as a sponge. This is not entirely a correct analogy, but liquid lubricant is dispersed in a fibrous thickener network resembling the pores in a sponge.

Most people think grease is primarily thickener but, in actuality, it is mostly oil - which is what does the lubricating.

COMPOSITION:
As previously illustrated, grease consists of 3 components: Thickener, Base Oil and Additives.

THICKENER:
The thickener defines the type of grease (Soap/Non-Soap). More than 90% of the thickeners used worldwide are soap baed.

Soap-based thickeners are produced from an acid base reaction. The acid is a fatty, along with, in some cases, a short-chain organic complexing acid.

Saponification, the process for producing a soap-based thickener, is a follows:

Acid + Base = Soap + Water

* Common acids
High molecular weight fatty acides: Stearic and 12 Hydroxy Stearic Acid; Short chain complexing acids: Tallow, Azelaic and Sebacic Acid

* Common bases
Lithium Hydroxide, Calcium Hydroxide, Sodium Hydroxide, Barium Hydroxide and Aluminum Hydroxide

There are 3 types of soap-based thickeners:

* Simple soap
* Mixed soap
* Complex soap


The consistency of grase is determined by placing a funnel called a penetrometer on a smooth cup of grease that has a tempetrature of 77 F and measuring the penetration in tenths of a millimeter after five seconds. The greater the penetration the softer the grease and the lower the NLGI Grade number. Most grease used today falled under the classification of NLGI 1, 2 and 3, with the most common being NLGI 2 grade. High penetration grease such as 00 and 0, are used in centralized lubrication systems in colder temperatures.


Thickener Classification...
Grease are classified according to their thickener composition, as previously discussed, as well as on their consistency, according to the NLGI system.


Base stock and additives...
Most of our discussion up until now has focused on the thickener. The base oil and additives are also key components of grease formulations. For example, a high-temperature thickener grease will not be effective if the base stock does not have good oxidative stability.


Base Stocks of Greases:

* Mineral Oils (Paraffinic & Naphthenic)
* Synthetic (PAO, Ester, PAG & Alkylbenzenes)
* Natural (Vegetable Oils)
* High Performance (Silicones & Fluorinated Fluids)


Gease Additives and Functions:

* Antioxidant
Retard oxidation of base stock for longer lubricant life

* Rust Inhibitor
Protect ferrous surfaces from rusting

* Antiwear
Provide wear protection during boundary lubrication

* Extreme Pressure
Provide protection during high load and shock loading conditions

* Tackifiers/Polymers
Enhance water resistance and metal adhesiveness

* Molybdenum Disulfide/Graphite
Solid lubricants providing protection and friction reduction under high load/sliding conditions at low speeds


KEY GREASE PROPERTIES

* Consistency
NLGI grade is based on amount of thickener. Consistency describes the stiffness of the grase. NLGI 2 is the most common grade.

* Dropping Point
This is the temperature of grease where the first drop of oil separates from thickener i a perforated cup. It is the point when the thickener breaks down. Grease should be operated no higher than 100-150 F below the dropping point. Complex soaps and polyureas have dropping points around 500 F.

* Water Resistance
Water washout test measures ability of a thickener to remain intact in bearing when submerged in water. Water spray-off measures ability of a thickener to remain in bearing in presence of water spray. Both of these tests measure percent grease removed.

* Base Oil Viscosity
Because oil does the lubricating in a grease, and viscosity is the most important property of the lubricant, the viscosity of the base oil needs to be designed correctly for the application.

* Load Carrying Ability
Under high-load conditions, high-viscosity base stock is required and usually with an EP additive or solid additive like molybdenum disulfide.

* Shear Stability
Grease needs to maintain its consistency under high shear conditions. The shear stability test measures the softening of grease when sheard for 10,000 or 100,000 double strokes with a grease worker. Loss of less than one NLGI grease grade signifies a stable thickener under high shear conditions.

* Compatibility
This is one of the most important grease properties. Whenever two incompatible thickeners are mixed, grease usually becomes soft and runs out of the bearing. When mixing different thickener types, consult supplier on compatibility. Some incompatible thickeners are Aluminum and Barium Soaps, Clay and some Polyureas.

* Pumpability
This is an important property when pumping grease in centralized systems at low temperatures. Most common test is Lincoln Ventmeter.

* Oil Separation
For a grease to be effective, a small amount of oil must separate from the thickener (usually less than 3%).


APPLICATIONS:
Based on the properties of grease, the following list describes situations where grease is the lubricant of choice:

* Where leakage and drippage is present
* In hard-to-reach places where lubricant circulation is impractical
* Where sealing is required in a high-contaminant environment (i.e. water & particles)
* To protect metal surfaces from rust and corrosion
* To lubricate machines that are operated intermittently
* To suspend solid additives such as moly during slow-speed, high-load sliding conditions
* For use in sealed-for-life applications such as electric motors
* To lubricate under extreme or special operating conditions
* To lubricate badly worn machines
* Where noise reduction is important


CONCLUSION
While grease is a very important part of every lubrication program, many people use it wihtout fully recognizing the differences among various types and/or the guidelines for their proper selection and application.

This article forcused primarily on various greases and their compositions, and only touched on their key properties. Those properties, however, need to be understood so that the correct selection can be optimized.