Machine Design

It’s Not Your Grandfather’s Grease

Choosing synthetic lubricants for automotive components.

Brian Holley
Nye Lubricants Inc.
Detroit, Mich.

Tinkering under the hood is no longer part of the car owner’s unwritten contract with automakers. Instead, it’s warranties and 100,000 mile tune-ups, and automotive manufacturers have to make their components work harder and last longer, or fix them free of charge. At first, engineers turned to better metals, plastics, and rubbers to extend the life of products, but that wasn’t enough. The petroleum-based lubricants, which maintain fluidity from about –20 to 100°C, could not keep up with broader temperature requirements. They’d bake or turn to sludge, and hardly last the tens of thousands of cycles required to pass today’s life tests.

In the early 1980s, automotive component engineers began to leave their grandfathers’ greases behind and turn to synthetic lubricants to ensure performance and reliability. Synthetic oils had already been around for quite some time. Esters were developed in the 1940s and 1950s for the fast-growing aviation industry, where lubricants had to withstand freezing, high-altitude temperatures and the heat from jet engines. Over the next two decades, temperature requirements expanded even further in the aerospace industry, which gave rise to new classes of synthetic lubricants: polyphenylethers and perfluoropolyethers.

Not counting noncommercial experimental synthetic oils, there are currently six basic families of synthetic lubricants: synthetic hydrocarbons, polyglycols, esters, silicones, fluoroethers, and polyphenylethers. Together, they extend the temperature capability of lubricants from –90 to 250°C, a quantum improvement over what was once known as black gold.

Sensors were among the first types of automotive components that required something better than petroleum grease. The 1980s heralded the introduction of throttle-position sensors and exhaust gas recirculating sensors to monitor electronic fuel injection and exhaust emissions. These potentiometers, mounted on the air-intake and exhaust valves, send valve-position data to optimize performance. The signals have to be accurate, despite the fact sensors operate in wide temperature ranges, exposed to fuel and exhaust vapors. Greases were applied to the resistive elements to prevent wear, since any wear may alter the signal sent to the computer. If the grease dried, varnished, froze, or was dissolved by fumes, the potentiometer was doomed — and so was engine performance. Synthetic esters, silicones, and fluorinated oils help ensure the reliability of these sensors.

Today’s cars and trucks rely on dozens of sensors to monitor functions under the hood, beneath the chassis, and inside the passenger compartment. Positioning sensors are found in fuel tanks, steering columns, seats, exterior mirrors, accelerator pedals, as well as in active-suspension packages. The right lubricant for these sensors means the difference between years of uneventful operation and repeated trips to the dealership.

Connection protection
Synthetic greases are also being called upon to protect against corrosion on electrical contacts. In the winter, engine temperatures can quickly climb from subzero to over 100°C, and back again. Over time, this thermal cycling causes oxides to build up on the metal surfaces of mated connectors and degrade electrical signals. Vibrations from components such as motors or fans can cause the same type of problem — fretting corrosion.

Using gold or other nonoxidizing metals, or increasing the contact pressures on terminals, can minimize fretting corrosion. But those are costly solutions, generally reserved for only very critical circuits, like those in air bags. Synthetic hydrocarbon and silicone greases, however, can achieve similar results at a significantly lower cost. They also serve two other functions. They reduce the force needed for mating and unmating connectors (some have as many as 100 terminals) by as much as 80%. And if the grease is designed to be salt water and chemical resistant, it also serves as backup protection, virtually eliminating environmental corrosion for the life of the component. Even with rubber seals, this is an important consideration because heating and cooling of the connector housing will still let moisture get inside and cause problems.

“You want to put grease in my what?”
Most people are comfortable using grease on bearings, gears, slides, and other mechanical devices. Many don’t realize that grease also extends the life of electrical switches. Years ago, electrical relays with butt contacts were the norm. However, the need to reduce costs and the number of parts in a car pushed electrical engineers to come up with a better device. Today, most control is handled by switches.

Low-current switches, which send signals to relays and computers, don’t require much metal to do the job, so they don’t have much contact force. The proper contact grease can make all the difference. It has to be light enough to allow good electrical contact, not freeze in subzero environments, which causes the contacts to separate by hydroplaning, and it can’t dry or varnish, causing poor conductivity. Light viscosity synthetic hydrocarbons and esters, which perform well at and below –40°C, have been successful lubricants for low-current switches.

High-current switches carry 20 to 60-A loads to starters, headlights and turn signals. Large metal contacts carry the electrical load, and when the contacts mate and break there may be a fair amount of arcing. This arcing super-heats the grease, burning many and heating up the metal contacts. So heat resistance is the target when selecting high-current switch lubricants.

Fortunately, synthetics can take the heat, and in different ways. Glycols, for example, are relatively clean burning, which minimizes carbonizing residue on contacts. Esters are slow burning. They withstand higher temperatures longer than synthetic hydrocarbons, and do not polymerize as radically. PFPEs, on the other hand, are nonburning, an attractive feature to OEMs and switch manufacturers for critical applications.

Switches that carry both high and low current often require different contact greases, one for low current and one for high. Many switch designs don’t allow for two greases, but the flexibility of synthetics often allows one grease to do double-duty.

Finally, the ideal mechanical lubricant would never allow two surfaces to make contact, thereby eliminating friction and wear. However, that lubricant would also prevent electricity from flowing from one switch surface to the other. Since contact must be made, contact wear debris will be created. Too much debris causes problems. So care must be taken when choosing switch lubricants. Synthetic hydrocarbons, esters, and fluoroethers, for example, help minimize debris problems.

Keeping cables working
More than a dozen cables can be found on most cars. They might vary in length, load-carrying capability, and duty cycle, but they have a few things in common. They are hand operated, have long stranded bundles of wire inside a plastic sleeve, and if they don’t work properly, the car owner quickly gets annoyed.

The beauty of a cable is its flexibility. It can be wrapped through and around all kinds of obstructions in the car. But this erratic path causes a great deal of friction between the stranded wire and the plastic sleeve. The right lubricant is critical to minimize friction. The natural sliding action tends to squeeze the lubricant out of the space between the wires and the plastic sleeve, making cables difficult to lubricate properly. The right lubricant should get in between those parts quickly, particularly on frequently used accelerator, brake, clutch, and shifter control cables which often have to pass 1,000,000 stroking cycles before designs are accepted.

Cables must also operate efficiently at subzero conditions, and those routed to the engine compartment must tolerate the high temperatures near exhaust manifolds. Other cables, like parking brake and hood release cables, can sit for weeks or months without being cycled. They require long-idle lubricants to work the first time, every time.

The near-universal solution for cable lubrication is silicone. Silicone oils and greases offer wide temperature and good surface-wetting characteristics. While cable lubricants are not exclusively silicone based, it is rare not to find some type of synthetic lubricant on these popular parts.

The need for more power
Synthetic lubricants play significant roles in improving the performance and life of electrical motors. Some of today’s cars have over 60 electric motors to power both essential and luxury components. Over the years, these motors have gotten smaller to reduce weight, costs, and the amount of electricity needed to power them.

Starter motors are a good example of high-output motors that have undergone significant size reduction in recent years. Yet, they still need to start six, eight or even 10 cylinder engines. In addition, they are exposed to road splash, grime, and 150°C heat from nearby exhaust pipes. Despite the miserable operating environment, these motors must last at least 10 years. Other underhood motors strapped with the same environment and reliability issues include those for exhaust pumps, cooling fans, ABS and traction controls, and the windshield wipers. An ordinary grease won’t cut it, but ester and fluorinated greases and oils usually do the trick.

While extreme temperatures and road grime aren’t issues inside the passenger compartment, weight, power output, long life, and low noise are. Windowlift motors are a good example. The trend over the years has been to make the inside of the car quieter. One measure was to make the rubber window seals fit tighter against the glass to reduce outside noise. But this means windowlift motors needed higher outputs in the same sized packages. Synthetic greases and oils based on synthetic hydrocarbons and esters, combined with the proper low-friction and noise-reducing additives, have done a good job with power output efficiency and low noise on all interior motors.

A wide variety of power-train parts also benefit from synthetic lubricants. Greases in underhood components such as alternators, condensers, and water pumps, minimize frictional drag, thus optimizing fuel economy. Clutch and brake systems, fuel and air controls, and even superchargers and turbochargers — all tested at 150°C and higher for long periods of time — can handle the heat much longer with synthetic lubricants. Generally, esters, silicones, and fluorinated greases are used to meet the performance targets. In addition, synthetic hydrocarbon greases are significantly extending the life and improving the efficiency of CV joints, U joints in rear axles, and wheel bearings.

A bumpy road ahead
Steering and suspension systems have become much more complex in recent years. Active suspension systems that accurately adjust the flow of shock absorber fluid to change the response and feel of the car must use oils that remain fluid below –40°C. Traditional power steering systems that use hydraulic fluid are being replaced with electric motor-driven systems that must actuate quickly, responsively, and quietly. Synthetic hydrocarbon lubricants support active suspensions and new power-steering systems.

The ball bearings and gears in these systems place high demands on lubricants. Ball joints in front-end suspensions once had grease fittings. Today, those fittings are gone and parts are “lubed for life,” 10 years or longer, with synthetic hydrocarbons, esters, silicones, and fluorinated lubricants.

What’s left?
Light-duty, hand-operated knobs, buttons, and various other automotive controls and devices might not have extreme temperature requirements, but they all must feel and sound good — and operate at subzero temperatures for three times their expected lives. For mass fabrication and low cost, many are made of plastic. To boost perceived quality, greases based on high-viscosity synthetic oils lubricate these parts. The oils behave like molasses, reducing the lash of loose-fitting parts, absorbing audible sound, and giving parts a controlled tactile response. In a nutshell, they improve perceived quality as well as reduce friction and wear. Although most high-viscosity lubricants become more viscous when cooled, and become almost solid at –40°C, specially formulated “damping greases” can operate under those conditions. Typically, they are formulated using synthetic hydrocarbon and silicone oils.

What’s next?
Cars of the next millennium will certainly need to use electricity more efficiently, especially battery-powered vehicles. There will be more sensors, more wire harnesses, more weight reductions and increased fuel economy. Designers will add more computers and screens, more creature comforts that must feel and sound good, all with longer life requirements and better warranties. Underhood temperatures will get hotter as the engine compartment gets smaller. Cold temperature requirements have already plunged lower than –40°C. Fiber optics, which are replacing signal-carrying wiring, also use synthetic oil-based gels to improve signal transmission at the end of each fiber. With all these changes, synthetic lubricants will continue to play an ever more important role. They’ll help engineers meet performance requirements for every moving part, and some nonmoving parts. And as they have for the last decade, they will play a critical role in keeping the auto industry, and the vehicles they produce, moving.

A word to the wise
Choosing the right synthetic oil is the key to choosing the right synthetic lubricant. All oils — synthetic, animal, vegetable, and mineral — are liquids and subject to freezing and evaporation. In either state, the oil can’t lubricate and the component fails. So matching the temperature range of an oil to the temperature extremes in which a component must operate is essential to selecting the right lubricant for an application.

Many people are comfortable when told to use a lithium grease. However, knowing you have a “lithium grease” really tells little about the lubricant’s temperature capabilities. All greases are made by mixing a powdered material, called a thickener, with a base oil, but it’s still the oil that lubricates and it’s still the oil that determines the grease’s temperature capabilities. The grease can be thought of as a “sponge of oil.” Moving parts “squeeze” oil out of the grease to prevent friction and wear. Since the lithium is only the “sponge,” behavior will vary with the type of oil in it. Always consider the temperature range of the base oil, not the thickening agent, when specifying lubricant for components.

© 2010 Penton Media, Inc.

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