Thin film of lubricant can cause positioning errors. All-helical planetary gearheads, from Bayside Motion Group, Port Washington, N.Y., use a light, thixotropic, synthetic grease and handle input speeds to 10,000 rpm. The grease helps the gear teeth retain their original profile after full-load, 300-hr, 3,000-rpm life tests without positioning errors.

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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