Not rocket science, maybe, but the technology behind lubricating grease is far from simple, and using a grease not suitable for the application, or mixing incompatible formulations, can cause serious problems when machine pivot points, sliding surfaces, bushings, and bearings are inadequately lubricated, wear rapidly, fail prematurely, and require expensive repair.
Umut Ürkün, global grease marketing advisor, ExxonMobil Corp., touches on the very fundamental purpose of grease, which, he says, is to provide lubrication between mating surfaces that move relative to each other in applications where a liquid lubricant would not remain in place.
How grease works
Grease is formulated with three ingredients—base oil, thickener (sometimes called a gelling agent or filler), and additives. ExxonMobil’s Ürkün says that the proportions of these constituents can vary with the specific formulation, but typically are in the range (by volume) of 80-90 percent base oil, 5-15 percent thickener, and 2-5 percent additives.
With a nod to Wikipedia, “grease is a semi-solid lubricant that has a high initial viscosity, but when shear forces are applied [such as those forces in the front-linkage pivots of a wheel loader working a stockpile] the viscosity of the grease drops to give the effect of lubricating the pivot with a fluid oil having the approximate viscosity of the base oil. This change in viscosity is known as ‘shear thinning.’”
Ürkün uses layman’s terms. “Think of the thickener as a sponge that holds the base oil and additives,” he says. “When pressure is applied at the point of contact, the thickener, like squeezing a sponge, releases some of its oil and additives to perform the lubrication task, that is, building an oil film between mating surfaces. When the force is removed, a well-formulated grease can reabsorb the fluid oil. The grease also serves as a barrier or seal against contaminants.”
For the more technically minded, CITGO’s Turner explains that grease behavior, that is, thinning with shear and then recovering its pre-shear state, typically is described as “thixotropic.”
Grease thickeners
According to Ürkün, controlling the release of oil from the thickener requires the grease formulation to “balance the cohesive forces between the lubricating oil and the thickener matrix [structure].” The thickener must be able to withstand the shear forces applied, he says, and if it can’t, then the thickener can be damaged and the grease can soften and not remain in place.
Grease thickeners generally are classified as simple metal soaps, complex metal soaps, and non-metal types, the latter category including such materials as clay and polyurea. With apologies for a bit of a chemistry lesson, Turner explains simple and complex metal soaps.
“A simple soap,” says Turner, “is a reaction product of a fatty acid—for example, 12 hydroxy stearic acid, a derivative of castor oil—with an alkaline-type metal, such as calcium, sodium, lithium, or barium—usually in the form of a hydroxide. Other soaps take it a step further. You still have the same basic ingredients, but you’d use additional amounts of the metal hydroxide, say, lithium hydroxide, and then a smaller amount of what’s called a ‘difunctional fatty acid.’ Mixing the two types of soaps together results in what we call a ‘complex soap’ thickener.”
Len Badal, global Delo Brand manager, Chevron, adds that having two types of soap thickeners—simple and complex—can be beneficial.
“Simple soap thickeners tend to have lower-temperature performance capabilities, where as complex thickeners typically provide better performance at higher temperatures,” says Badal. “But that can be of benefit, because not all operating conditions require high-temperature performance, but might instead require other performance capabilities that simple-soap thickeners can provide at better cost levels.”
According to the 2016 Annual Grease Production Survey by the National Lubricating Grease Institute (NLGI), the most common soap thickeners (by percent) are lithium (55), lithium complex (20), calcium (10), and aluminum (4). The most-used non-metal thickener is polyurea (6). Polyurea is a generic term for non-metal thickeners, says Turner, and is used, for example, in grease that must be highly resistant to oxidation (aging) and have long life, such as that used in constant-velocity joints of front-wheel-drive vehicles. An NLGI “Others” category accounts for the remaining 5 percent of thickeners.
“The different thickener technologies tend to have different cost profiles, depending on the amount needed for the formulation, so this has to be taken into account,” says Chevron’s Badal.
Performance characteristics of grease
Why so many thickeners? “Because the various thickeners provide different performance properties,” says Turner. “Calcium soaps, for example, have excellent water resistance and help keep grease from being washed out in applications where it might be continually exposed to water. But the dropping point of calcium-based grease [its melting point] is only moderate. A lithium complex would be more resistant to heat, but has only good resistance to water. It’s a trade-off of properties.”
(Watch a dropping-point test below. The maximum useable temperature for a formulation is usually 75 F to 100 F below the dropping point—the experimental temperature at which the grease becomes so fluid that it does not remain in place.)
The type and amount of thickener used in a grease formulation also determines its “consistency”—it’s ability to resist deformation when force is applied. Practically, consistency is a measure of how well grease stays in place.
Nine NLGI classifications for consistency range from 000 to 6 and are based on laboratory tests that measure the depth to which a weighted cone penetrates the grease when dropped into the product under specific test conditions. According to ExxonMobil’s Ürkün, class 0, 1, and 2 greases are commonly used for construction machines. (Watch a consistency test below.)
Base oil and additives
As with thickeners, base oils are chosen for the specific performance properties they bring to the grease formulation. And like engine oils, the oils used in grease are of various qualities and viscosities—the latter a measure of the oil’s resistance to flow.
Jeremy Wright, director of product management, Advanced Technology Services (ATS), a contract-maintenance company, explains that base oils are classified by the American Petroleum Institute into five Groups.
“The first three Groups [I, II, and III] are refined from petroleum crude oil (mineral oil) by various methods,” says Wright. “The way oil is refined determines its properties. Solvent refining used for Group I is the easiest, cheapest way to refine, but the oil’s properties are hindered—for example, it oxidizes quickly. Group II oils are hydro-treated to break apart unwanted molecules and realign them; the resulting oil costs more, but has better properties.
“Group III oils are hydro-cracked and are of such high quality that, from a lab-testing perspective, they’re difficult to distinguish from Group IV synthetic polyalphaolefins, PAOs [a man-made oil based on natural constituents]. Group V includes all other base oils.”
Base-oil selection, both the quality and the viscosity, says ExxonMobil’s Ürkün, depends on the application and operating conditions the grease will encounter. Synthetics, for example, he says, offer the best performance in temperature extremes, and those with a high viscosity index are better at maintaining their stated viscosity across a wide temperature range. But the base oil alone, he says, is usually not capable of handling “all the challenging tasks of lubrication” and must be supplemented with additives.
Additives have three functions, says ATS’s Wright: enhancing the base oil’s properties; suppressing its undesirable properties; and adding properties the base does not have. Additives can include those that help the base oil better handle oxidation, improve anti-wear properties, inhibit rust and corrosion, or improve anti-friction properties, perhaps with the addition of molybdenum disulfide.
One other aspect of grease behavior related to base-oil viscosity and thickener properties is the product’s “apparent viscosity,” which is a measure of a grease’s ability to flow through an automatic lubrication system at different temperatures, orifice sizes, and line lengths. By measuring flow characteristics, with an instrument such as the Lincoln Ventmeter, for example, a grease’s suitability for an application can be determined. This subject is a science of its own, however, and best discussed with a knowledgeable lubricants specialist.
Compatibility of greases
When two grease formulations are mixed together, the result might or might not provide the lubrication properties required by the application. Grease compatibility charts attempt to give guidance in this regard by classifying mixtures as compatible, borderline (“moderately compatible”), or incompatible. A web search for these charts will turn up more than you’ll want to read.
Most charts base recommendations solely on how two thickeners get along, and to that extent are useful. But be advised; all charts do not agree. Although incompatibility is usually thickener related, it also can be a function of additive incompatibility—or in rare instances, base-oil incompatibility. Nor do compatibility charts address the effect of grease mixtures on performance aspects, such as resistance to extreme pressures or water washout. Nor do they address possible effects on seals or yellow metals.
Typical manifestations of grease incompatibility include significant thinning of the grease, which might increase as temperature and shear forces increase, and hardening of the grease, a function of base oils bleeding from the mixture at elevated temperatures.
In the lab, says CITGO’s Turner, compatibility tests usually include those for consistency and dropping point—for each of the neat (unmixed) greases, for a 90/10 mix of the greases, both ways, and a 50/50 mix. If consistency, compared with the neat greases, drops to the next lowest NLGI class, then the greases are incompatible—as they are if the mixture’s dropping point falls out of the range of either neat grease. If changes in consistency or dropping point still fall within the ranges of the neat greases, the mixture might be considered “borderline.”
Chevron’s Badal adds another dimension to compatibility testing when proposing to replace one grease with another. The comparisons, he says, also should include mixing the new neat grease with a sample of the current grease pulled from the application. The reason, he says, is that the current grease might be working in an environment where it picks up contaminants that could affect compatibility, or the application itself might render the mixture incompatible under specific operating conditions.
The practical side
In the day-to-day task of maintaining machines, ATS’s Wright says that the fundamentals of grease lubrication entail attention to the type of grease used, the volume of grease used, and frequency of application. Operating conditions and duty cycles affect these fundamentals, he says, and the machine owner must adjust accordingly. A machine digging 12 hours a day, in wet muck, and in cold temperatures, says Wright, might well require a different greasing strategy than a machine used four hours per day in a hot climate digging abrasive sand.
“Eighty percent of the time, the manufacturer’s recommendations will serve the machine owner well,” says Wright, “but the owner must be aware of conditions that require deviation from the recommendations.”
Chevron’s Badal makes the further observation that if fleet managers are working toward extending engine-oil drain intervals in various vehicles, then they must be sure that greases presently in use can go the extra distance and still perform acceptably.
With a mixed fleet, in which an equipment manager must see that proper greasing fundamentals are practiced in applications as varied as truck U-joints, excavator swing bearings, jaw-crusher thrust plates, and trailer wheel bearings—policing the use of grease is a daunting task. And a conscientious effort to comply with every manufacturer’s grease recommendation can lead to a long list of grease types.
To keep things straight in these situations, some have suggested color-coding grease guns and grease zerks; others suggest using different types of grease fittings that only mate with the appropriate gun. But what about consolidating grease types without jeopardizing lubrication quality on the various machines?
Complicating that endeavor—at least according to a few fleet managers who have voiced this opinion—is the absence of guidance from some equipment dealers who have a surprising lack of lubrication knowledge regarding the products they sell. In other instances, goes the complaint, equipment manufacturers provide so little information about grease specifications, that fleet managers can’t intelligently choose an alternative product to that branded and recommended by the manufacturer.
For those who face this task, we can, at best, cautiously direct them to lubrication specialists who have demonstrated a thorough grasp of grease formulations and their applications. Consolidating grease types can be done, we’re told, but must be approached with due diligence—which might include consulting an oil company’s product data sheets and a methodic compilation of grease requirements across varied machines in the fleet.
