This article was updated March 15, 2023. It was originally published Oct. 25, 2001.
Some engineers, as well as many non-engineers, might be unaware that electrical switches benefit from being lubricated. In fact, for many electrical switches, lubrication is required if they are going to meet their predicted life estimates while maintaining their operational specifications. To get the right lubricant, designers should understand the switch and its environment, as well as the lubricant.
Lubricants boost switch performance a couple of ways. They prevent environmental and galvanic corrosion on switch contacts, their most important function. Airborne contaminants attack metals, building up oxides in pores on electrical contacts until they reach the surface. Once the oxides make it to the surface, they impede current flow. Contact surfaces and switches made of dissimilar non-noble metals are especially susceptible to moisture, oxygen and aggressive gases. And even plates of protective noble-metals placed over contacts can corrode if they are worn or porous.
Lubricants also minimize wear, especially on sliding electrical contacts that undergo repetitive cycling or suffer arc damage. Lubricants might change or reduce arc patterns, but on sliding contacts, the more important task is to keep contacts separated when operating and keep debris from fouling the contact area. This stops or at least slows the microscopic wear particles from oxidizing on the contact and turning it into an insulator. Gritty oxide buildup also increase wear.
In general, hydrocarbon lubricants work best at preventing wear because they are stiffer and more rigid than other base oils. But the most appropriate lubricants strike a balance between wear prevention and maintaining electrical continuity.
Lubricants also reduce friction between switch components, the traditional role of lubricants. This lowers the amount of force needed to turn the switch on or off. Lubricants usually deliver 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 both low contact resistance and a stable signal or power path. Lubrication is mechanically important because it gives end-users smooth, uniform operation when using the switch.
Damping greases (high-viscosity lubricants) provide drag to give switches a “high-quality” feel. Silicones have historically been used as damping greases, but high-molecular-weight polymers offer an alternative with similar feel and no risk of silicone migration, which is more than an aesthetic problem. Under arcing, silicone turns into silicon dioxide (sand), an abrasive and insulating material that quickly destroys contacts.
Lubricants act as insulators in large quantities, so it’s not surprising that some people mistakenly believe lubricants used on switch contacts need to be electrically conductive. In fact, there is virtually no difference between lubricated and unlubricated contacts when it comes to contact resistance. To work on switches, lubricants must therefore not interfere with metal-to-metal contact.
Choosing the Right Lube
Petroleum, synthetic esters, polyalphaolefins and fluoroethers as straight oils or greases can serve as switch lubricants. Each has its advantages and disadvantages. Here are some tips for choosing the right one for an application.
Make sure the base oil will withstand expected operating temperatures. Generally, synthetic oils can handle a broader range of temperatures than petroleum. Synthetics are also less likely to evaporate or degrade at high temperatures and remain pliable at lower temperatures. This makes them widely used as switches’ operating environments become more severe. Synthetic oils cost more than petroleum, but the increase in life and performance usually justifies the cost, which is often below a penny per unit.
The base oil must be compatible with materials and additives used in the switch. Lubricants have no effect on most thermoplastics, but there are some oils that will 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 material. Compatibility charts supplied by manufacturers identify materials and lubricants that work well together. Testing, however, is the only sure way to guarantee a successful match between materials and lubricants.
Make sure the lubricant works at the switch’s current rating. Switches can be categorized as low- or high-current devices, with 1 A 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 important. For high-current switches, wear reduction is usually more critical because surface films get “burned” through.
High current levels also raise the risks of arcing. Under an arc, temperatures can hit 1,832°F (1,000°C). At that temperature, most metals melt and most hydrocarbons polymerize, becoming a tacky, viscous, insulating film that is not easily removed. No material can withstand this abuse, so eventually the switch fails and creates an open circuit.
To prevent arcing, go with the lubricant that has the longest life under such conditions. In theory, lubricants that vaporize instead of polymerizing—such as polyglycols and PFPEs—work better because they leave behind no insulating residue after polymerizing.
However, as a lubricant vaporizes, less remains to lubricate. It’s important to note that although ac arcs are self-extinguishing and shorter than dc arcs, whether the switch is in an ac or dc circuit does not dramatically alter lubricant life.
Choose your additives wisely. Adding surface passivators and oxide retardants to lubricants, for example, can enhance their performance. Base oils can also be mixed with thickening agents to create switch greases. The thickener’s efficiency depends on how much is needed to make a given grade (stiffness) of grease. The primary lubricating component is oil, so it’s best to have as much oil in the grease as possible. But lubricants should not be too viscous, which can lead to 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.
Kevin D. Akin was an application testing manager at Nye Lubricants Inc. when this article was originally published.