Oil is the hands-down lubricant of choice, allowing the generation of a fluid film under elastohydrodynamic lubrication. At high speeds, this film separates mating parts, minimizing frictional effects and prolonging component life.
However, due to difficult and expensive arrangements often needed for containment and circulation, liquid lubricant (oil) may be impractical. The next best thing is grease. Grease is essentially an oil-delivery system, used to bind lubricant to working mechanisms.
Sometimes it is unavoidable that different greases will get mixed together. Ideally, we do not want this – there is never an upside. If a machine is working acceptably, pumping in a different form of grease will likely contribute to problems. Nevertheless, greases get changed and mixed, for one reason or another. Perhaps the old grease is unknown or in some ways not wholly effective, and its complete removal uneconomical due to unacceptable downtime and the direct expense of tearing machinery apart.
If we get the mixture wrong, then, what is the danger? The mixture may become soft and run out of the mechanism. At the very least, this bleeding or purging of the grease is unsightly, messy, and can get onto the product.
The results can be a bit more painful, though. As the thinning grease leaves bearing contact areas, and the bearings get increasingly hotter, total lubrication failure sets in, with full metal-to-metal contact and extreme heat. The component eventually, mercifully, seizes up.
Bearing temperatures can exceed 2,000°F, where steel turns to putty. Yet, many industrial machines will continue turning with the incredible amount of horsepower driving the system. Ultimately, the bearing melts enough to give the shaft so much play that other portions of the machine come into contact and force a stall. Or, the horrendous noise alerts the operator to shut it down. What the bearing analysts often see returned are shapeless hunks of melted metal.
This can be avoided by handling greases properly.
To better grasp incompatibility issues, we should understand the constitution of grease. It consists of three main components, in varying proportions:
• Base oil, 70-95%
• Thickening agent, 4-20%
• Additives, 0-10%
Oil is the primary ingredient, as it should be – it’s what actually lubricates the mating parts. While there are hundreds of bearing greases available, they can be broadly categorized according to their base oil and thickener.
The base oil is either mineral or synthetic. Mineral oils include paraffinic, napthenic, and aromatic varieties. Synthetic oils are sub-categorized as synthetic hydrocarbons, esters, silicones, ethers, and fluorinated.
Each type of oil has its advantages and disadvantages. Mineral oil is inexpensive and therefore the most commonly used. Its primary shortcoming is poor performance outside the “normal” grease operating temperature range of -10 to 250°F. Synthetic oils perform better at extreme temperatures and speed, and cost considerably more.
Thickeners are either soap or nonsoap. Soap thickeners are divided into lithium, sodium, aluminum, barium, and calcium types. Non-soap thickeners are polyurea, carbon black, microgel (clay), and fluorinated.
Soap thickeners are actually the chemical soaps of various metals. Each type has its own unique characteristics, and therefore no broad generalizations are tied to the soap category. However, lithium thickeners are by far the most common, being relatively inexpensive and versatile.
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Polyurea, the most widely used non-soap thickener, is a synthetic organic substance made from the same highly toxic components as foam peanuts used in packing. This thickener combined with mineral oil forms the basis for some of the longest-lasting bearing greases available. The best known are Shell Dolium R (discontinued as of February 2000) and Chevron SRI-2.
Carbon black has a niche where electrical conductivity is required. Microgels were popular in the past, but have fallen out of favor due to noise issues.
“Fluorinated” refers to carbon chains with fluorine replacing hydrogen (PTFE is the best known such substance). Fluorinated greases, while extremely expensive, are the only kind able to survive above 350°F. They also have the advantage of being chemically inert. Fluorinated thickeners are most commonly combined with fluorinated oils. Krytox, made by Du Pont, is the most familiar brand.
Additives, the least ingredient of any grease, are put in to enhance certain lubricant properties. Among the most common additives are antioxidant, anti-wear, anti-corrosion, dye, viscosity index, extreme pressure, and filler. All greases have at least a small amount of additive, even if next-to-nothing on a percentage scale. The additive package can impact grease performance as much as base oil and thickener. Every manufacturer has its own highly guarded, tradesecret formulations. The actual compounds used are well known, but the art is in knowing how to combine them in the right ratios. Each concoction is made to fill particular requirements, and there is no one best package.
Consistency and incompatibility
The exact definition of “grease incompatibility” is open to debate. One source says, “Incompatibility occurs when a mixture of two greases shows properties or performance significantly inferior to those of either grease before mixing.” This is a questionable definition, mainly because it is not precise enough. What property or performance parameters are being discussed?
Incompatibility assessment is often narrowed down to specifically deal with grease consistency. In fact, this is the standard guide. Consistency is one of several ASTM-tested critical grease properties that are easily obtained, usually provided in manufacturers’ product literature.
Consistency is a measure of the hardness or stiffness of grease. It can be measured both before and after working, which is the shear that takes place when one layer of grease moves relative to another. Whenever grease is stirred or mixed such as in the operation of a bearing, it is worked, and this usually changes the consistency. The grease most commonly gets softer or less consistent.
Grease consistency, measured as penetration, is determined by the ASTM D 217 test using a device called a penetrometer. Grease is worked in a standardized grease worker – 60 double strokes within one minute at 77°F is the standard exercise. The grease sample is then positioned under the penetrometer cone. The tip of the cone is held just above the sample surface. The 150-g cone is released to fall and sink under its own weight. Penetration is measured after five seconds of this, in units of 0.1 mm. The deeper the penetration, the lower the consistency.
A substantial quantity of grease is required for penetrometer testing (the traditional cone penetration test takes several ounces of grease) and this method is therefore impractical in the field or as a failure analysis tool. Most returned bearings hold no more than a few milligrams of grease. In most cases, then, it is impossible to measure the consistency change of greases mixed in a machine. Incompatibility tests are generally confined to controlled laboratory experiments.
Tying the base oil viscosity to the overall grease consistency is a common fallacy. In fact, the oil has nothing to do with it; thickener is the ingredient that promotes consistency. Grease with a high-viscosity mineral oil and little thickener in the formula may have a very low consistency. On the other hand, a lot of thickener added to low-viscosity synthetic oil can lead to a tar-like consistency.
A reaction between thickeners is often at the root of incompatibility. Although different oil types may not mix well, and the mixture may not lubricate as effectively as each oil by itself, oils generally don’t catastrophically react or break down. Hence, a common approach to judging grease compatibility is to compare thickening agents and give general statements on how different combinations have worked. It’s generally assumed that two greases with the same type of thickener are compatible even if they have different base oils. This isn’t always a safe assumption, however, as there are examples of similar thickeners not agreeing with each other. Plus, there can also be reactions with additive packages. The way to make absolutely sure two greases are going to agree is to test ahead of time, although in many cases this may not be feasible.
Dealing with it
Unwary maintenance personnel mix greases thousands of times a day in rotating machines around the world. Improperly handled bearings are the rule. The amazing thing is that most of these machines continue to turn.
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There are several reasons why grease incompatibility problems are not more evident. Even a soupy mix provides oil to the contacting surfaces, particularly if loads are light and the seals are in good shape.
Furthermore, most instances of incompatibility are brief. A small amount of old grease gets blended with some new and different grease for a short time, and as things continue to run and new grease is added, the effects become negligible. Many applications are actually overgreased, particularly those with zerk fittings, and the too-frequent pumping-in of new grease forces out the old stuff that much faster.
Without stripping down machinery to ensure all old grease is removed, there are some safeguards that can be taken:
•Whenever practical, stop the machine to drain and clean the system before changing over. This is the only sure way to eliminate the chance of incompatibility.
•If mixing is inevitable, try to determine the thickener types of the two greases about to be introduced. If they are the same, proceed. If they are different, consult a compatibility chart.
•After adding, increase the grease consumption temporarily. This will quickly move the mixture interface through the system and out.
•If you must mix incompatible greases, or if you are unsure, greatly accelerate the grease consumption (temporarily) and watch closely, even expectantly, for signs of failure. Monitoring bearing temperature is also advisable.
How the test was done
Within the last year, a major oil company introduced a new mineral oil and polyurea grease intended for electric motor and general-application bearing markets. This grease’s primary advantages, instilled by improving the shear stability of the polyurea thickener, are a much longer service life and lower operating noise than similar formulations that have been around for years.
Before going to market, the oil company undertook an extensive program to test compatibility with existing mineral oil and polyurea blends. (It’s an easier sale when a manufacturer can tell customers to simply pump the new grease in on top of a competitor’s formula.)
They evaluated compatibility by mixing two greases in various ratios and comparing the 1/2-scale penetrations (ASTM D 1403) of the mixtures. Penetration tests were performed after the standard 60-stroke worked condition and again after two hours in a roll-shearing apparatus (ASTM D 1831). The ratios were 100/0, 75/25, 50/50, 25/75, and 0/100.
The charts illustrate compatible and incompatible greases. Compatibility is decided by comparing the penetration of each grease mixture to the line that joins the penetrations of each of the pure samples. This is done for both rolled and unrolled samples. In general, if the difference (Æh) between any penetration and the straight line joining the pure samples’ penetrations is 30 points (3 mm) or less, the greases are considered to be compatible. If the difference is 60 points or more, they are considered incompatible. Obviously, there is a transitional area.
For determining compatibility under more severe conditions, the roll stability is run at a higher temperature such as 80°C (176°F). Other testing can be employed to determine the compatibility of greases under various extremes, for example, ASTM D 2265 (dropping point) and ASTM D 1263 or D 4290 (wheel bearing leakage).
Steve Witkowski is an Engineering Supervisor with NSK Corp., Ann Arbor, Mich.