Ode to Grease

After Roy conned me into the job of Tech Editor, he responded to my several questions regarding content of my articles with the response, “You can write about anything”. The following Ode to Grease will test Roy’s commitment to that representation.

We take grease for granted. It lubricates our wheel bearings and CV joints and is sometimes called upon to hold a screw to the tip of a screwdriver. It is only fitting, therefore, that we understand a bit about it. Grease is essentially a semi-solid to solid product of the dispersion of a thickening agent, typically referred to as “soap”, in a liquid lubricant. Other ingredients are added to the mix which imparts special properties. Soap-thickened greases are classified by their alkali component: lithium, calcium, sodium, barium, and aluminum. Soaps are either simple or normal soaps, mixed alkali soaps, or complex soaps. Fluid types include mineral oils, silicones, poly glycols, and polyalphaolefin.

Additives include those which impart extreme pressure, corrosion, anti-wear, and anti-oxidant qualities to the mix. Fillers include zinc oxide, molly, graphite, Teflon, and titanium oxide. Greases are classified pursuant to standards which have been developed by the National Lubricating Grease Institute (NLGI).

The original classification scheme was comprised of Grade 0 through 6 and was published in 1938. Two additional grades were added in 1962. Thus, you will see listed on the side of a container of grease a reference to an NLGI number. The NLGI grades, from number 000 through 6 are based upon a penetration testing method. Penetration is the consistency or hardness of a grease which is measured by the distance a standard cone will penetrate the surface by a free fall at a standard temperature (77°).

Another method by which grease types are differentiated is referred to as the “dropping point”. A grease’s dropping point is that temperature at which grease will pass from a semi-solid to a liquid state. Needless to say, when it passes to the liquid state it is no longer adhering to the components to be lubricated, such as your precious wheel bearings. Dropping points can range from 160° F for calcium greases, to 375° F for barium greases, to 575° for lithium complex. Thus, the temperature of the application is critical to the selection of the type of grease which is to be applied.

Grease is also evaluated based upon shear stability. Shear or mechanical stability of a grease is its ability to withstand repeated working with minimum change in its structure or consistency. The additives which are put into grease by the manufacturer have a significant effect. Additives can allow it to resist oxidation, act as a metal deactivator, be corrosion or rust resistant, be water repellant, have color stability, or have film strength. It can also be improved to enhance pumpability, the ability the prevent wear, or resist a condition known as “stringiness”.

The application of different types of greases is rather interesting. Aluminum-soap based greases are typically used as thread lubricants. Conventional calcium greases are used in general applications and are an economical choice. Lithium-based greases, the leading type of greases, are seen in multi-service automotive and industrial applications. These greases were typically found in automotive wheel bearings and like applications for many years. Lithium complex greases, however, are increasing in popularity. They have excellent automotive-related properties such as a very high drop point (in excess of 500° F), have fair to excellent water resistance, good to excellent properties against rust, and are available in extreme pressure grades.

So the next time you’re staring at a shelf full of different types of greases at the local automotive store, remember the various components of which the grease is comprised and consider the application. The choice of a lithium complex-based grease with extreme pressure additives is probably the best bet for your wheel bearings.

(Thanks to Paul P. Converso of Battenfeld Grease & Oil Corp. of Tonawanda, New York.)

© W. S. Cline/Rose Lane Garage 1999

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