Kevin D. Akin
Application Testing Manager
Nye Lubricants Inc.
Edited by Stephen Mraz
Lubricants are critical in getting electrical switches to meet their life and operating specifications. Choosing the right one requires a full understanding of the switch and its environment.
Lubricants improve switch performance in three ways. Primarily, they prevent environmental and galvanic corrosion on switch contacts. Airborne contaminants attack metals, causing oxides to gradually build up in pores until they reach the surface, where they impede current flow. Nonnoble contact surfaces and switches made of dissimilar metals are especially susceptible to moisture, oxygen, and aggressive gases. Even noble-metal plating is at risk if it's worn or porous.
Lubricants also minimize wear, especially on sliding electrical contacts which see repetitive cycling or arc damage, two common causes of failures. Though evidence suggests lubricants change or reduce arc patterns, the lubricant's real job on sliding contacts is to separate the surfaces during operation and keep debris out of the contact area. Otherwise, the microscopic wear particles oxidize quickly, turning into insulators. Buildup of this oxide grit also accelerates wear. In general, hydrocarbon lubricants work best at wear prevention because their molecular structure is more rigid than other base oils. Proper lubricants strike a balance between preventing wear and maintaining electrical continuity.
And finally, lubricants reduce the friction between switch components, thus reducing the amount of force needed to activate a switch. Lubricants usually ensure a coefficient of friction of 0.1 or less, which means it takes little force to operate a device with a high preload. This can be important in switches where high normal forces ensure low contact resistance and a stable signal or power path. Lubrication is also mechanically important because it gives the end user smooth, uniform operation.
Damping greases (high-viscosity lubricants) are used to provide drag and give switches a "high-quality" feel. Although silicones historically have been used as damping greases, new high-molecularweight polymers offer a similar feel without fear of silicone migration, which is more than an aesthetic problem. Under arcing, silicone degrades to silicon dioxide (sand), an abrasive and insulating by-product that destroys contacts quickly.
Choosing the right lube
Petroleum, synthetic esters, polyalphaolefins, and fluoroethers — either as straight oils or greases — can all serve as switch lubricants. Each comes with its strong points and drawbacks. Here are some tips for choosing the proper one.
• Make sure the base oil can withstand expected operating temperatures. Generally, synthetic oils handle a broader temperature range than petroleum. Synthetics are also less likely to evaporate or degrade at high temperatures and they remain pliable at lower temperatures. This is making them more widely used as operating environments for switches become more severe. Synthetic oils cost more than petroleum, but the increased life and performance can justify the cost, which is often less than a penny per unit.
• The base oil must be compatible with materials and additives used in the switch. Lubricants have no affect on most thermoplastics, but some oils can craze, crack, or embrittle a variety of plastic and elastomer components. Esters, diesters, and polyesters, for example, are incompatible with polycarbonate, PVC, polystyrene, and ABS resins. Only fluoroethers are inert enough to be considered safe with practically every component and seal. Compatibility charts from manufacturers identify which materials and lubricants work well together. Testing, however, is the only way to guarantee a successful material match.
• Make sure the lubricant will work at the switch's current rating. Switches can be categorized as low or high-current devices, with 1 A being the dividing line between the two. For low-current switches, temperatures at the contacts are not high enough to displace oxides, so surface protection is most critical. For high-current switches, wear reduction is usually more important because surface films become "burned" through.
High current levels also raise the issue of arcing. Under an arc, temperatures can reach 1,000C. At that temperature, most metals become molten and most hydrocarbons polymerize, becoming a tacky, viscous, insulating film that is not easily displaced. No material can withstand this abuse, and eventually the switch fails, causing an open circuit. To prevent arcing, choose a lubricant with the longest life under such conditions. In theory, lubricants that vaporize instead of polymerize — such as polyglycols and PFPEs — work better because they leave no insulating residue. However, as a lubricant vaporizes, less remains to lubricate. It's important to note that although ac arcs are self-extinguishing and shorter in duration than dc arcs, whether the switch is in an ac or dc circuit does not dramatically affect lubricant life.
• Choose your additives wisely. Surface passivators and oxide retardants, for example, can enhance lubricant performance. Base oils can also be mixed with a variety of thickening agents to formulate switch greases. A thickener's efficiency depends on how much of it is needed to make a given grade (stiffness) of grease. The primary lubricating component is oil, so it's beneficial to have as much oil in the formulation as possible. But a lubricant should not be too viscous, otherwise it causes hydroplaning (an open-circuit condition), especially at low temperature. A thickener's ability to resist water is also an important consideration. Switches are generally protected from the environment, but humidity can condense inside them and displace the grease or become entrained and accelerate corrosion. Lithium-soap greases have good freshwater resistance, but poor saltwater resistance. Clay and PTFE generally perform well in wet applications. PTFE also lowers friction, especially on plastic components.
HOW CONTACT LUBRICANTS WORK
Lubricants in large quantities act as insulators. So it's not surprising that some people mistakenly think that lubricants used on switch contacts need to be electrically conductive. Curiously, there is virtually no difference in contact resistance between lubricated and unlubricated contacts.
To work on switches, lubricants must not interfere with metal-tometal contact. A switch's contacts may appear smooth, but under a microscope, they would resemble a landscape of tiny peaks (asperities) and valleys. Current flows only where the peaks touch. So, the actual contact area is considerably smaller than the apparent contact area, sometimes less than 1% of the apparent area. The normal force on the contact is distributed across these asperities, so the pressure is high, typically hundreds of pounds per square inch. This pressure easily forces lubricants out of the contact zone and lets the contacts make metal-to-metal connections.