Grease Basics

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:

Fig. 1. Contrary to popular belief, grease is mostly oil, which is what does the lubricating.

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.

Fig. 2. Thickeners define grease types.

Composition
As previously illustrated, grease consists of three components: thickener, base oil and additives.

Thickener…
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:

• 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.

Table I. NLGI Grease Classification by Consistency

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

 

The consistency of grease is determined by placing a funnel called a penetrometer (shown in the accompanying diagram) on a smooth cup of grease that has a temperature 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 falls under the classification of NLGI 1, 2 and 3, with the most common being NLGI 2 grade. High penetration greases, such as 00 and 0, are used in centralized lubrication systems in colder temperatures.

Thickener classification…
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.

Key grease properties
The basic properties of greases are noted below in Table IV.

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.

Table VI summarizes key grease properties based on thickener types.

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 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

Conclusion
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.