Oil Or Grease, Natural Or Synthetic

May 17, 2000
BRAD RICHARDSONRegional Engineering Manager Nye Lubricant Inc.

Regional Engineering
Manager Nye Lubricant Inc.
Fairhaven, Mass.

Generally, there are few if any government regulations about lubricants for nonimplant medical devices. Nonetheless, if a device ever touches the patient, it is wise to stick with base oils and additives that are "generally recognized as safe." Typically, synthetic hydrocarbon oils are preferred. At worst, they would act as a fast-working laxative if they were ingested. (Don't try this at home). Beyond this caveat, the best lubricant for a medical or any carefully engineered device is one designed for the application and the operating environment.

When choosing a lubricant, the first decision is oil or grease. Generally, low starting torques require oil. The consistency of a grease may overpower available motive force and slow or impair any movement. Oils, on the other hand, provide minimal drag, making them more suitable for small, delicate mechanisms in precision instruments. For high speeds, where oil migration is not an issue, oils also dissipate heat better than greases.

Oils do have one major limitation: they tend to creep. Lubricant migration can often be prevented with a barrier film, a stable, nonwettable, fluorocarbon polymer with a surface energy well below the surface tension of most lubricating fluids. The film acts as a dam around the lubricated area to prevent oil migration and potential contamination of nearby, sensitive components.

Greases are created by combining an oil with a gelling agent, usually a soap or a clay. It's the oil in the mixture that lubricates. The force created by two mating parts — slide-on-slide, gear-on-gear, bearing-ball-against-raceway, for example — squeezes oil out of the gellant and onto the parts to reduce friction and prevent wear. Experts generally recom-mend greases for heavier loads, especially where vibration or shock loading is likely. They also resist water wash-out better than oils, and effectively seal out contaminants and moisture.

Even with low-power devices, don't quickly rule out a grease. Greases stay where you put them, and that can reduce costs associated with manufacturing air-tight oil reservoirs. Light, low-shear greases — formulated with light oils, special gelling techniques, and adherence modifiers — can almost become semi-fluid under shear. So for all except low-powered devices, a custom-formulated, extra-light grease may be more economical than an oil.

There's also a choice between natural or synthetic lubrication. Petroleum, a hydrocarbon, is nature's most popular lubricating oil. Synthetic lubricants, which include hydrocarbons, esters, polyglycols, silicones, and perfluoropolyethers (PFPEs), are man-made through controlled chemical processes. Usually, the operating temperature is the arbiter. Petroleum becomes virtually intractable at subzero temperatures and begins to degrade at or before 100°C. In either case, it won't lubricate.

By contrast, synthetic hydrocarbons perform down to -40°C; synthetic esters to -60°C; and some PFPEs all the way down to -90°C. At the other end of the thermometer, synthetic hydrocarbons keep working up to 130°C, silicone to 200°C; and PFPEs to 250°C or higher.

Even when temperatures don't mandate it, design engineers often choose synthetic lubricants. Compared to petroleum, synthetic oils are more chemically homogeneous and offer better thermooxidative stability. Oxidation depletes the lubricant and often leaves abrasive oxides in its wake — which hastens component failure. At elevated temperatures, even without the presence of oxygen, synthetic lubricants are also less volatile than petroleum-based products of equivalent viscosity.

Synthetic lubricants offer better film strength than petroleum products. The "film" of lubricant on a sliding or rotating part is what reduces friction and prevents wear. If the film is weak and ruptures under load, wear accelerates. In general, under similar operating conditions and viscosities, a synthetic lubricant will perform more consistently and for longer periods of time than a petroleum product.

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