Getting the most from this lubrication workhorse requires a solid understanding of its composition, properties and applications.
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 introduced in the early 1940s. Lithium complex greases, 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. This is illustrated in Fig. 1.
As previously illustrated, grease consists of three components: thickener, base oil and additives.
The thickener defines the type of grease (see Fig. 2).
Greases are classified into two major families: soap and non-soap thickeners. More than 90% of the thickeners used worldwide are soap based.
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 acids: 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 three types of soap-based thickeners:
- Simple soap
Simple soap results from the reaction of one fatty acid, such as 12 hydroxy stearic acid (12 HSA), and a metallic hydroxide, such as lithium hydroxide. This produces a simple lithium soap that is the most common worldwide. The metallic hydroxide used defines the thickener type. If calcium hydroxide were used with a fatty acid, the grease would be called simple calcium soap.
- Mixed soap
The mixed soap grease type is not very common. It is produced by reaction of a fatty acid with two metallic hydroxides. For example, if 12 HSA reacted with lithium and calcium hydroxide, it would produce a mixed Ca/Li soap.
- Complex soap
Reaction of a fatty acid, such as 12 HSA, with a short chain complexing acid, such azelaic, produces a complex soap. If lithium hydroxide were used, the result would be a lithium complex grease-the most popular grease type in North America. The advantage of this thickener type over a simple soap type comes from it having much better high-temperature properties.
|NGLI Grade||Worked Penetration
Range @ 77 F, mm/10
|000||445 to 475|
|00||400 to 430|
|0||355 to 385|
|1||310 to 340|
|2||265 to 295|
|3||220 to 250|
|4||175 to 205|
|5||130 to 160|
|6||85 to 115|
Greases are classified according to their thickener composition, as previously discussed, as well as on their consistency, according to the NLGI system shown above in Table I.
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. Table II illustrates base stock types found in greases; Table III details the types of additives and their functions.
|Mineral Oils||Paraffinic & Naphthenic|
|Synthetic||PAO, Ester, PAG & Alkylbenzenes|
|High Performance||Silicones & Fluorinated Fluids|
|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
The basic properties of greases are noted below in Table IV.
|Consistency||NLGI grade is based on amount of thickener. Consistency describes the stiffness of the grease. NLGI 2 is the most common grade.|
|Dropping Point||This is the temperature of grease where the first drop of oil separates from thickener in 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 sheared 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%).|
Product data sheets are available for purchased greases—and they should be consulted to determine the correct grease for the application. Table V, on page 14, lists typical properties reported. This table is fairly complete; note that many suppliers do not report all this test data.
|Test Method||Expressed Value||ASTM #|
|Cone Penetration Unworked & 60 double strokes||Millimeters/10||D 217|
|Worked Penetration 10,000 & 100,000 double strokes||Millimeters/10||D 217|
|Dropping Point||Temperature in C & F||D 566|
|Corrosion Prevention||Pass/Fail||D 1743|
|Oil Separation||Percentage of oil separated||D 1742|
|Water Washout||% grease washed out||D 1264|
|Water Spray-Off Resistance||% grease sprayed off||D 4049|
|Timken OK Load||Maximum weight in Kg or Lbs||D 2509|
|Four Ball EP||Weld point in kilograms & load wear index as a number||D 2783|
|Four Ball Wear||Scar diameter wear reading in millimeters||D 2266|
Table VI summarizes key grease properties based on thickener types.
|Grease Thickener||Appearance||Shear Stability||Pumpability||Heat Resistance||Water Resistance|
|Sodium||Fibrous||Fair||Poor||Good to Excellent||Poor|
|Lithium 12 OH Stearate||Buttery||Excellent||Good to Excellent||Good to Excellent||Excellent|
|Lithium Complex||Buttery||Excellent||Good to Excellent||Excellent||Excellent|
|Calcium Complex||Buttery to Grainy||Good||Fair||Good||Good to Excellent|
|Aluminum Complex||Buttery to Grainy||Good to Excellent||Good||Excellent||Excellent|
|Calcium Sulfonate||Buttery to Grainy||Good||Good||Excellent||Excellent|
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 and 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
While grease is a very important part of every lubrication program, many people use it without fully recognizing the differences among various types and/or the guidelines for their proper selection and application. This article focused 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. These issues will be discussed in more detail in a future article on the proper selection and application based on equipment type and environment. LMT
Contributing Editor Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training in a number of industries. Telephone: (281) 257-1526.