Petroleum oils may always have a place in the world of gearing, but as mechanisms run hotter and faster, quality demands increase, and extended warranties push service life, interest in synthetic gear lubricants continues to grow.
Selecting the best lubricant for an application isn’t always easy. The American Gear Manufacturers Association (AGMA) has developed an Industrial Gear Lubrication standard (ANSI/AGMA 9005-D94) to help engineers select an oil viscosity based on the pitch line velocity of enclosed and open industrial gears. This standard references spur, helical, herringbone, straight bevel, spiral bevel, and cylindrical worm drives. While oil viscosity is important, choosing the right oil is the real key to getting the best lubricant for an application.
Unfortunately, there’s no handy guide for selecting the appropriate synthetic grease for gearing applications. To help with this task, this article describes synthetic base oils, discusses advances in synthetic gear greases, and provides guidelines for selecting synthetic gear lubricants.
Why Go Synthetic?
Synthetic base oils, unlike their petroleum counterparts, are built from the molecular level. The molecules can be manipulated to improve specific lubrication characteristics. With their consistent and predictable chemical structures, and exceptional performance profiles, synthetic oils are displacing petroleum oils that can no longer meet the needs of demanding applications.
The best known of the synthetic oils’ advantages is their ability to function at both very low and very high temperatures. In fact, engineers typically turn to synthetic oils because of extremes in operating temperatur e. They switch because petroleum begins to break down at temperatures of 100ƒC or less and to freeze up and become too viscous to flow at about -18ƒC. Sometimes these limitations can be overcome by lubricant additives, but many additives are expensive or contain undesirable components. In general, it is far more efficient to use an optimal base oil–and use additives to enhance specific performance features–than to try to make up for a base oil’s shortcomings with additives.
In contrast, most synthetic oils can tolerate a much wider range of temperatures. At higher temperatures, synthetic lubricants are less volatile than petroleum-based products, and they have lower pour points, meaning that they continue to flow at temperatures below those at which petroleum oils become intractable (see Figure 1).
Even when temperature is not a concern, design engineers often turn to synthetics for a performance edge. Synthetic oils offer better thermo-oxidative stability than petroleum oils, for instance. Oxidation not only depletes lubricant supply, permitting under-lubrication and excess wear, it creates abrasive oxides, which can speed gear failure. Synthetics also boast high “viscosity indices.” The viscosity index (VI) is a measure of the ability of an oil to maintain its viscosity at different temperatures: the higher the VI, the more consistent the viscosity as temperature changes. Synthetics also offer better film strength than petroleum, meaning that the protective film of lubricant that forms between mating gear surfaces will be less likely to weaken and rupture under load, which would accelerate wear.
Synthetic lubricants, though usually pricier ounce-for-ounce than petroleum lubricants, can be extremely cost effective. Typically, a synthetic will last at least three to five times longer than a petroleum oil of equal viscosity, and synthetics do not form carbon deposits as readily as petroleum lubricants. Because there is less evaporative loss, it generally takes less synthetic lubricant to protect a part. In addition, the higher protection afforded by the synthetic lube can help extend the lifetime of a part beyond its original specifications, lengthening service intervals, reducing warranty claims, and improving the cost performance of the total system.
Take the case of automotive rack and pinion steering components. Rack and pinion gears constantly change direction, and the potential for high shock loading puts a great deal of stress not only on the gears, but the lubricant as well. The design of this system relied on mixed-film or boundary lubrication to prevent gear wear and failure (in both mixed-film and boundary lubrication, a full fluid film cannot form, and there is contact between the lubricated surfaces). Additionally, the system had a spring-loaded, yoke-to-rack mechanism that kept the rack mated to the pinion. Originally, the system was tested with a petroleum-based grease. But under mechanical shock-load testing, simulating potholes and railroad tracks, the rack separated from the pinion, increasing wear and causing an annoying clunking sound. The design engineers needed a lubricant to reduce gear wear and the level of noise transmitted through the steering column, so they turned to a grease formulated from a high-viscosity, synthetic base oil with a lubricious thickening agent and extreme pressure and antiwear additives. This new grease was applied to the gear teeth, as well as the spring-loaded, yoke-and-rack interface. It passed both gear and yoke wear tests, while imparting a smooth, quiet, quality feel to the entire steering system.
Choosing the Right Synthetic
Deciding to use a synthetic lubricant is not enough to guarantee good performance. Though they all have advantages over petroleum, material compatibility, temperature range, ease of formulation, and other chemical and performance features need to be considered (see Figure 2).
Five basic types of synthetic lubricant base oils are important in gear applications. The most common type of synthetic oil used in gearing is the synthetic hydrocarbons, or polyalphaolefins (PAOs). The synthetic hydrocarbons offer excellent cold-temperature performance. They also have good oxidative stability, are compatible with many plastics, and are relatively inexpensive, on par with some of the newer, more highly refined petroleum oils.
Synthetic esters are ideal for cut-metal and powder-metal gearing, if proper seals are used. Because of their affinity for metal, especially steel and iron, esters provide maximum wear protection. They have become the clear choice for automotive supercharger gearing and other severe-duty applications because they withstand temperatures as high as 180ºC.
Polyglycols and polyethers have an affinity for brass and phosphate bronze. Accordingly, they are frequently used in worm gear applications to reduce friction and improve efficiency.
Silicones and fluoroethers, such as perfluropolyether (PFPE), are compatible with nearly all gearing plastics. Both silicones and fluoroethers are suitable for broad temperature applications and have shown exceptional low-temperature torque characteristics. PFPE is also resistant to aggressive chemicals. In addition, some PFPEs have very low vapor pressure, which is essential when out-gassing is a concern.
Oil vs. Grease
In addition to choosing an appropriate base oil chemistry, the engineer must decide whether an oil or grease is called for. Gear greases are formulated with the same base oils as gear oils, but a powdered soap, salt, clay, or synthetic material is added to act as a carrier for the oil. The thickener functions something like a sponge. When a force or load is applied, such as when gear teeth mesh, the oil is squeezed out of the grease, forming the lubricating film that protects the moving parts.
Gear oils have been the norm in most applications, and their light consistency has made them particularly attractive in such assemblies as small, low-torque, and low-horsepower gear motors, or where purging or cooling performance has also been required. However, the use of an oil requires proper sealing, and seal material compatibility and leakage are persistent concerns.
Greases, conversely, tend to obviate sealing problems and leakage concerns. But while grease is often thought of as a necessarily thick and goopy substance, it can take many light forms, as well. Increasingly, gearing operations are taking advantage of soft, even pourable, thixotropic synthetic greases. These greases slump into the gear-teeth mesh after operation, resuming their gel-like consistency, but flow like thick oils during gear operation. Special additives called “tackifiers” can improve adherence of even light, thixotropic greases without impairing their flow. Tackifiers are particularly necessary with plastic gears.
Soft, adherent synthetic greases can tackle a variety of problems. For example, in a reciprocating saw design, both the wobble (or reciprocating) mechanism and the gearbox needed lubricants. The operating speeds, heat, and sliding motion of the wobble mechanism called for a grease that was soft enough to accommodate tight tolerance between the parts, yet not so soft that it would sling or drip off during operation. The gearbox called for a lubricant with slumping capability. One grease — a synthetic hydrocarbon formulated with a lithium soap thickener and a mild tackifier — handled both tasks. Because of its good performance in the reciprocating saw, it is also being tested in hammer drills, which have similar design concerns.
The National Lubricating Grease Institute (NLGI), which establishes standards for grease performance, has developed a classification system for greases that relies on a measurement of their consistency or stiffness during a specific laboratory test (ASTM D217). These classifications and their physical analogs are shown in Figure 3. Typically, gear applications would call for the lighter greases that have slumping ability (NLGI 1 and lower), although in certain heavy-duty applications, a heavier grease might be required.
Synthetics & “No-Lube” Gears
While the quality of plastic gears has improved, that shouldn’t rule out the use of a lubricant. Without exception, even lightly loaded, low-speed plastic gearing will last longer and run more quietly with a lubricant than without one.
Materials compatibility issues are as important for plastic gears as they are for metals and sealing materials. Esters, diesters, and polyesters, for instance, are noted for their incompatibility with polycarbonate, polyvinylchloride, polystyrene, and acrylonitrile-butadiene-styrene resins. The fluoroethers, conversely, are markedly inert and are therefore compatible with almost all plastic gear materials.
The same is true for gears containing internal lubricants, such as polytetrafluoroethylene or silicone; because such compounds can impair the performance of gear greases — if the base oil in the gear grease is not compatible with the gear’s internal lubricant.
Because additives included in a grease can also cause compatibility issues, and because impurities in gear materials can cause them to react differently than those manufactured to more stringent requirements, the only way to truly establish lubricant-gear compatibility is by testing the total lubricant-gearing system under conditions similar to those that will be encountered in service.
Improving the Total Quality Experience
Damping greases — those specifically designed to control noise or prevent unwanted motion in machinery — are finding special favor among manufacturers looking to improve the total-quality experience for their products. Damping greases are increasingly finding their way into applications that demand a certain “touch” or “feel” for the user, such as control knobs on appliances or automobiles, or in the threads used to set the focus of camera and microscope lenses.
Synthetic damping greases are typically formulated with synthetic hydrocarbon or silicone base oils. What sets them apart from other greases is their high internal shear resistance. This resistance to shear means a highly stable oil film exists between the parts the lubricant is protecting, translating into smooth, quiet motion when a force is applied and virtually no motion when the force is removed. Because of their resistance to thinning induced by shear stress, wear is typically very low when damping greases are used. In addition, most damping greases have a highly viscous consistency that imparts a self-sealing quality, meaning that moisture, dust, and other contaminants will be kept out of the mechanism longer, lengthening service life.
Although damping greases are resistant to shear and tend to be viscous, they can also be surprisingly light. A special pourable damping grease, for example, was used to prevent squealing of a worm gear in a household stand mixer. In this case, the grease was able to slump between the gears when they were not moving, but had a high enough internal shear resistance to prevent metal-to-metal contact while the mixer was running — clearly a benefit to mixer and user alike.
Design and Synthetic Lubes
While the performance of a lubricant depends on many variables, early evaluation of key lubricant selection criteria can help avoid design pitfalls and shorten product development time.
As noted above, operating temperature alone will often persuade the engineer to specify a synthetic gear lubricant. But many other factors must also come into play. By gathering the information specified in Table 3, the design engineer and lubricant specialist will have a good starting point to specify a lubricant for the gear or gearbox assembly.
Traditional petroleum gear lubricants have served the industry long and well, but the high demands placed on today’s products mean that today’s gear lubricants must do better. Along almost any metric — versatility, material compatibility, durability, and even cost competitiveness — synthetic gear lubricants are going further than their petroleum counterparts ever could. And today’s manufacturers are clearly taking notice.