Jeffrey Lay, Nye Lubricants Inc.Petroleum and mineral oils may always have a place in the world of gearing, but synthetics are rapidly gaining acceptance. In a marketplace where consumers demand more from their products, synthetics deliver performance advantages that give manufacturers an edge on the competition.
As components get smaller and faster, they also get hotter — sometimes too hot for petroleum. In fact, extreme operating temperature is why most OEMs turn to synthetic lubricants. Petroleum begins to degrade at or before 100°C and becomes intractable at -18°C. In contrast, synthetic lubricants function from -90 to 260°C.
Even when temperatures don’t call for synthetic lubricants, more designers are selecting them for a performance edge. That’s because unlike with petroleum, designers can manipulate synthetic oils at the molecular level to improve their lubrication characteristics. The resulting molecules are even more homogeneous than molecules of super-refined petroleum.
Assuming similar operating environments and viscosities, this molecular homogeneity gives synthetics several advantages over petroleum lubricants. For example, synthetics offer better thermooxidative stability. Oxidation not only depletes lubricants, abrasive oxides hasten component failure. Synthetic lubricants are also less volatile at elevated temperatures than petroleum-based products even without the presence of oxygen.
Synthetics also offer better film strength than petroleum. The film of lubricant provides a barrier between moving parts, which reduces friction and wear. If the film is weak and ruptures under load, wear accelerates.
Synthetics also score higher than petroleums when it comes to viscosity indices. A higher viscosity index means the base-oil viscosity remains more constant as temperatures change. These lubricants generally last three to five times longer than petroleum products of equal viscosity.
Evaporative loss presents fewer problems for synthetics as well. Because there is less evaporative loss with synthetic lubricants than with petroleum, geared mechanisms use less synthetic lubricant per part.
Oil meets gear
Synthetic lubricants make gears run smoother and last longer. Mechanically, the lubricants form a protective film between mating gear teeth, effectively broadening the line of contact between teeth. This increases the area that supports the load, reduces pressure on gear teeth, and slows wear.
Choosing the right synthetic oil is key to designing smooth-running geared mechanisms. Each of the six families of synthetic oils has its own exclusive attributes. Synthetic hydrocarbons are the most widely used synthetic lubricant for gears and gearboxes. They offer good low temperature performance to -60°C and good oxidative stability. Synthetic hydrocarbons are compatible with many plastics and relatively inexpensive compared to other synthetic fluids.
Some synthetic lubricants are compatible with specific types of metals. For example, synthetic esters work especially well with steel and iron and are ideal for cut-metal and powdered-metal gearing. They have become a common choice for automotive supercharger gearing and other severe-duty applications because they provide high wear protection and withstand temperatures to 180°C. Polyglycols offer good compatibility with brass and phosphate bronze. Because of this, they are often used in worm gear applications.
Silicones and PFPEs are compatible with nearly all plastic gear materials. Both are suitable for broad temperature applications and have exceptional low-temperature torque characteristics. PFPEs also resist harsh chemicals, such as sulfuric acid, hydrochloric acid, alkalis, halogens, and petroleum solvents. In addition, some PFPEs have very low vapor pressure, which is essential for vacuum chamber and aerospace applications where outgassing can be a problem.
Polyphenylethers (PPEs) are used in a few niche gear applications. For instance, because PPEs resist radiation, they are good candidates for gearing in medical and dental applications.
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Although oils have been the traditional lubrication choice for gears, they raise design concerns, such as leakage and increased cost. Grease offers a solution. By choosing grease, engineers can often save money through elimination of oil seals.
Greases are a mixture of powdered material, or thickener, and base oil. Thickeners include sodiums, clays, and soaps, such as lithium, as well as synthetic materials, such as PTFE.
Choosing the right oil is essential when specifying grease for gears because the grease’s behavior depends on the type of oil. In most gear applications soft greases designed specifically for gears offer the best qualities of oil and grease. Soft greases flow back into the gearteeth mesh like oil but remain gellike to reduce leakage common with oil lubrication.
With their stay-in-place qualities, light, thixotropic, synthetic greases are viable alternatives to conventional gear oils, which are often mistakenly considered a requirement for low-torque applications. Greases can be formulated light enough to accommodate even small gear motors. In fact, gear greases can be engineered soft enough to flow under shear and return to gel consistency when static.
Although greases can be used in high and low-speed gearboxes, the housings must be designed with minimal open spaces, where grease can get trapped away from moving parts that need lubrication. In existing oil-lubricated gearboxes, grease retrofits use plastic baffles to reduce the amount of grease required to fill the box and keep the grease in between moving parts.
The plastic question
Plastic gears are often designed to operate without lubrication. But, in the struggle to maximize performance and life, many engineers are finding that external lubrication improves plastic-gear designs. In fact, even lightly loaded, lowspeed, plastic gearing lasts longer and runs quieter with lubrication than without it.
When selecting grease for plastic gears, the base oil must be compatible with component materials. Additives and material densities affect compatibility. Although lubricants do not affect most thermoplastics, some oils can craze, crack, or embrittle various plastics and elastomers. Esters, diesters, and polyesters, for example, are noted for their incompatibility with polycarbonate, PVC, polystyrene, and ABS resins. Only fluoroethers are inert enough to be safe with most components. Many manufacturers provide compatibility charts, but testing is the only way to guarantee a successful material match.
Engineers should also consider how well lubricants will adhere to gears. Tackifiers, which are additives that help grease adhere to gear teeth, are usually recommended for plastic gears because they reduce lubricant sling-off.
Internal lubricants, such as PTFE or silicone, may hamper the external lubricant’s ability to provide an adequate film of oil between gear teeth. Therefore, when selecting an external lubricant for plastic gearing, either choose gears without internal lubricants or make sure the internal lubricants can function with the base oil in the external lubricant. Typically, however, no internal lubricants are required when external lubricants are used.
Grease beats the heat
When Autotrol Corp., Crystal Lake, Ill., prepared to introduce its Model 150 Class N fractional hp gearmotor to the appliance marketplace, the company selected a high-temperature, engineered plastic for the gearing. Class N motors automatically lock oven doors during self-cleaning cycles when temperatures hit 232°C and unlock the doors during cool-down cycles. But the gears were failing before the customer’s 6,000-cycle requirements.
The problem was solved with an external lubricant. Engineers selected a synthetic grease composed of fluorinated oil (PFPE) and fluorinated thickener (PTFE). PFPEs survive 250°C for extended periods and are compatible with plastics, elastomers, and metals.
Tests of the motors with the grease revealed minimal wear after 6,000 cycles, surpassing UL requirements and the typical life of ovens. Although at roughly $100/lb, PFPE/PTFE is one of the most expensive synthetic greases, a little goes a long way. Each Model 150 gearmotor only uses about four cents worth of grease.
Jeffrey Lay, Gear Industry Director at Nye Lubricants, Inc., Fairhaven, Mass., is a member of the American Gear Manufacturers Assn., Technical Assn. of Pulp & Paper Industries, and the Society of Mechanical Engineers.