Motor oil enjoys a certain automatic respect over grease thanks to its various multinumber grades. Grease is often seen as just grease. It's sometimes red, green, or blue. As it turns out, grease has grades, too.
Grease even has its own institute. The National Lubricating Grease Institute (NLGI) grades grease by its thickness, or fluidity. Essentially, the difference between grades boils down to how much soap, or thickener, is added to the base oil. The higher the grade number, the greater the amount of soap used and, thus, the greater the tendency for the grease to stay put in components.
“The thickener acts like a spider's web,” explains Mark Betner of Citgo. “It interlaces and locks in the oil until you need it to be released.”
But there are different types of thickener bases used in grease, and they don't all get along.
“There's family feuding between thickening agents,” Betner warns. “Lithium greases are like type O blood; they get along with everyone. Other thickeners, such as aluminum-type greases, offer performance advantages, but they don't like other thickeners.”
Grades of grease range from Grade 000 (the most fluid of grease with a consistency of molasses) to Grade 6 (which resembles a bar of soap). Grade 0 grease is most commonly used in automotive lubrication systems. Grade 1 grease can be a good choice for cold climates in winter because they maintain pumpability when temperatures plummet. Grade 2 is typically used in the summer in colder climates and in warmer climates year-round. It provides a balance between pumpability and clingability.
A common mistake is to confuse grease consistency with its base oil viscosity. Viscosity determines a grease's load-carrying ability and how well it will perform under extreme pressure.
Load ratings are reported on data sheets as the four-ball weld load and the Timken OK load. “The higher the numbers, the greater the load-carrying capacity and the greater the grease's ability to withstand heavy loads without breaking down,” explains Corey Taylor of BP Lubricants.
Additives at work
Greases are also formulated with a wide range of additives that enhance their performance like corrosion and rust inhibitors, washout resistance (to water), and temperature resistance (to prevent melt and leak-out at high temps).
Differences between greases can be confusing. That's why you need to ask your lubricant provider what works best for your equipment.
“Ask what grease selection would give you the best return to protect your costliest investment,” Betner advises. “Don't always go with the cheapest or what's easiest.”