Demand for a wide variety of grease formulations is expected to remain strong for the foreseeable future, as electric, fuel cell, hybrid and internal combustion engine vehicles share the road.
Electric vehicle (EV) lubricants must reduce friction and wear under high speeds, temperatures and voltages—as well as resisting oxidation and shear thinning and reducing noise.
OEMs develop their own specifications and testing methods for grease formulations, but they also collaborate with industry groups and academic researchers around the world.
“The field of electric vehicle (EV)
grease formulations is evolving,” says STLE member Anoop Kumar, senior staff scientist and subject matter expert on grease formulations at Chevron Technical Center in Richmond, Calif. Grease formulations for EVs are just one of the many changes shaking up today’s automotive industry, but getting the formulations right is essential to a vehicle’s performance overall.
“What we’re seeing is a progressive introduction of electrification with hybrid vehicles and battery electric vehicles (BEVs),” says Alberto Carlevaris, manager of the New Mobility Application Competence Center in Airasca, Italy, for bearing manufacturer SKF. He notes that the various EV designs are causing a “disruptive transformation” in vehicle powertrains.
Changes and constants in vehicle designs
Not everything will change, however. Although internal combustion engine (ICE) vehicles and EVs use different grease formulations, Kumar expects the total demand for vehicle greases to remain fairly constant. Despite declines in individual vehicle ownership, the total number of vehicles on the road is expected to increase.1
Kumar, who also is vice president of NLGI, notes that NLGI and ASTM have established standards for existing vehicle applications (including passenger and commercial vehicles) that will likely not change much for EVs. For example, NLGI’s GC-LB specification for greases suitable for wheel bearings and chassis lubrication applies to a variety of vehicle types. NLGI’s High-Performance Multiuse (HPM) designation, which is relevant to a wide range of applications, including some potential EV applications, has subcategories for enhanced water resistance, saltwater corrosion resistance, high-load-carrying capacity and low-temperature performance.
In ICE vehicles, grease is mainly used for bearings and constant-velocity (CV) joints. For EVs, grease lubricates bearings (and sometimes gears) in the motor(s) and drivetrain, as well as wheel bearings. Both types of vehicles require multiple types of grease for the various applications. Non-EVs require greases designed for high loads (e.g., wheel bearings), energy efficiency (friction reduction) and long life. EV greases must meet these requirements, as well as requirements for high-speed operations (especially for transmission gears), high torque and noise reduction, along with various requirements for resistivity or static electricity dissipation.
The powertrain is where the most obvious differences begin to appear. Electric powertrains are simpler and more compact than those for ICE vehicles. The electric motor is coupled to a gearbox with a fixed gear reduction, which transmits kinetic energy directly to the wheel axles. Some vehicle designs use two motors, one mounted on each wheel axle.
Challenges for e-axle vehicles at the application level, Carlevaris says, concern efficiency, compactness and power density. Meeting these challenges requires electric motors that can deliver in terms of speed and acceleration. This, in turn, requires lubricants that can perform under the high-speed, high-temperature conditions involved, while resisting oxidation and shear thinning and providing the right bleed and lubricity performance. These factors (individually and in combination) directly affect the performance characteristics required for bearing lubricants.
When it comes to connecting the electric motor with the reductor compartment (which contains the gears that reduce the motor speed down to the wheel axle speed), there are two main types of design. The “dry” e-axle configuration places a seal between the motor and the reductor compartment. Grease-lubricated bearings support the electric motor, while the bearings and gears in the reductor compartment use a low-viscosity oil. Water-based (emulsion) lubricants also are being developed for use in reductor compartments, Carlevaris adds.2
The second configuration has no seal between the motor and the reductor compartment, and the same low-viscosity liquid or water-based lubricant cools and lubricates both areas.
A variety of vehicle types, including hybrid vehicles, fuel cell vehicles and ICE vehicles, will be sharing the road with BEVs for the foreseeable future, says Sharbel Luzuriaga, energy project manager for the Energy Practice Market Research Division of the Kline Group, a market research organization, in Prague, Czech Republic. His group recently published a study (see Figure 1)
covering the forecast for the passenger car motor oil (PCMO) market to 2040,3
and a similar analysis for commercial vehicles is in progress.
Figure 1. Internal combustion engine vehicles, battery electric vehicles, hybrids and plug-in hybrids will likely share the roads for the next two decades. Figure courtesy of The Kline Group.
Despite market disruptions caused by recent vehicle battery and semiconductor chip shortages,4
Luzuriaga cites a double-digit drop in worldwide demand for ICE vehicles in 2020, coupled with a similar rise in demand for EVs, driven in part by generous government incentives. Demand for passenger EVs is strongest in China and the European Union (EU), he says, with Japan and the U.S. also emerging as major players. Germany, France and the UK are leading EV demand in Europe, and Germany is aiming for carbon neutrality by 2050. The European Commission has proposed a ban on ICE vehicles by 2035, driven by societal pressure and trade unions. The Chinese government, although committed to the transition to EVs, has seen some weakening of support, in part because of the COVID-19 pandemic. The country remains a major market, however, he adds. Demand in the U.S. is lagging behind the other major players—there are some 500,000 EV charging stations in China, but only about 50,000 in the U.S. (mostly in California). However, U.S. demand could grow, he says, especially with the current administration’s focus on infrastructure building.
Although passenger vehicles currently dominate the BEV market, there is a significant demand for light- to medium-weight commercial on-road vehicles. Heavy-duty vehicles still require other sources of power, because batteries capable of handling these loads take up too much space at today’s power densities, Luzuriaga says. For these vehicles, alternate-fuel internal combustion engines, hybrid designs, liquified natural gas (LNG) and various forms of hydrogen fuel remain better options. One exception, he notes, is vehicles used off road for mining or construction in environmentally sensitive areas. Companies can maintain their own onsite rechargers for these vehicles, but power density remains a challenge.
Demand for the range of base oils, additives and finished-lubricant products for all these types of vehicles will continue, Luzuriaga explains. Even though most grease-lubricated parts on existing vehicles are fill for life, there are at least 25 grease points on a typical EV or ICE vehicle, and they don’t all use the same grease formulations. He expects original equipment manufacturers (OEMs) to continue to require a variety of greases, formulated for the specific demands of each application.
At present, EV greases makes up a very small fraction of the total grease market volume, Kumar says, and the EV market is dominated by passenger cars. Some 60% of all greases go toward industrial applications, including industrial vehicles, he adds. Overall, however, the automotive sector (all types of vehicles) represents about one-third of the grease market, and this sector is expected to be a high-growth area in the coming years.5
One area where electrification is changing things, Luzuriaga says, is with regard to the operating conditions under which greases perform. High-operating temperatures and voltages are key concerns. As current densities increase, so does the risk of stray currents that can pit bearings. Also, he adds, typical electric motor speeds, currently around 15,000 rpm, are expected to exceed 20,000 rpm in the near future.
For mechanical components that operate in much the same way they always did—windshield wipers, door hinges and window winder mechanisms, for example—conventional greases will still be suitable, says STLE member Gareth Fish, Technical Fellow at the Lubrizol Corp., Wickliffe, Ohio. Even here, however, Fish says, OEMs may be shifting the balance from greases with a longer life to greases with greater energy efficiency.
Long-life greases require quality base fluids (generally synthetic base oils) and additives that retard wear, fretting and oxidation. Energy-efficient greases need friction-reducing additives—this is especially important for tapered bearings, where sliding can reduce efficiency. For all these reasons, Fish says, grease additive concentrations are expected to increase overall. Efficiency also depends on the type of thickener in a grease formulation. A thicker grease may separate the solid surfaces better, but it also may produce higher churning losses, which increases friction.
Grease characteristics in the running-in period also affect efficiency and wear, Fish says. As a rolling element passes through a polyurea-thickened grease with a “rice pudding”-type thickener microstructure, the grease flows around the rolling element, and the energy losses are low. In contrast, a grease with a solid soap structure has to be shear-softened to allow the rolling element to move through it (see Figure 2
). Thus, soap-thickened greases tend to be less efficient than urea-thickened greases. Overall, a low coefficient of friction is desirable, but in systems prone to sliding, using a grease with too low a coefficient of friction may lead to skidding and bearing failure.
Figure 2. Lithium and lithium complex grease thickeners have fibrous microstructures. Figure courtesy of STLE.
Even though many mechanical components in EVs do not require changes in grease formulations, the increase in EV production, and the resulting demand for lithium batteries, may drive change of a different sort. The battery industry’s demand for lithium has driven some grease manufacturers to look for alternatives to lithium-based thickeners to prepare for potential lithium price increases or shortages.6
Even with the small amounts of grease used in each part and fill-for-life parts that don’t require replenishments, small increases in lithium prices can add up to a major concern for large OEMs, says STLE member Paul Shiller, owner of the tribological research consultancy AARDCO LLC, in Youngstown, Ohio. Polyurea-thickened greases run very quietly, and they tend to bleed less than lithium greases, Shiller says. Calcium sulfonate greases, another potential alternative, have inherent antiwear properties that lithium greases lack, he adds, which reduces or does away with the need to use separate antiwear additives.
Lithium complex greases are prevalent in North America, says Fish. However, some 80% of Japanese-made passenger cars use polyurea-thickened greases, which also are popular with Korean carmakers. The EU is currently considering classifying certain lithium salts (including those used in grease manufacturing) as hazardous to fertility and fetal development, so non-lithium greases may become more prevalent there as well.7
Shiller notes that the main drivers for change in grease formulations of all types may be environmentally conscious ingredients and friction reduction to improve energy efficiency. However, because EVs are quieter than ICE vehicles, there also is a push toward “quieter” greases (i.e., greases that reduce noise, vibration and harshness [NVH]) by replacing lithium stearate with polymeric thickeners. Coupling the right mechanical component designs with the right grease, Carlevaris says, can reduce the “hooting” noise (a steady, low-frequency sound) caused by a periodic pulsing in the grease that electric motors sometimes make at low temperatures.
Buzz, squeak and rattle (BSR) noise can be more noticeable in EVs than in ICE vehicles, Kumar says, because it is not masked by noise from other sources. Grease formulations that are well matched to the size and speed of the parts they lubricate can reduce noise levels by reducing friction. Polyurea thickeners are especially good at noise reduction, Kumar explains, because they are thixotropic: they become less viscous under applied stress. Moving bearings cause these greases to soften, which dampens vibrations and noise. Another advantage to polyurea thickeners, he says, is that they are better at resisting oxidation than greases with thickeners containing metal ions like lithium.
STLE member Piet Lugt, senior scientist at SKF Research and Technology Development in Houten, the Netherlands, and professor of tribology-based maintenance at the University of Twente in the Netherlands, adds that the right match between components and greases not only extends component life, but it increases grease life as well. For example, hybrid bearings that couple ceramic rolling elements with steel cages and raceways can increase grease life, he says.8
Carlevaris notes that the higher initial investment in hybrid bearings pays off in the long run in terms of parasitic current management, low friction and longer grease life.
Battery weight continues to be a consideration in all EVs because of the load that batteries place on wheel bearings, Fish says. What this means for bearing greases is a tradeoff between load-bearing capability and speed (which is related to grease viscosity). More viscous base oils and higher amounts of extreme pressure (EP) and antiwear additives increase a grease’s load-carrying capacity. Bearings under heavier loading run hotter, so extra antioxidants are needed to extend the grease’s life.
There is a churning or running-in period for one to 24 hours after bearings are first filled, Lugt says, followed by a bleed phase. During the initial running-in period, the grease migrates between the balls of the bearing. However, if the grease stays there during operation, it increases drag, he adds. It’s important to optimize the grease’s churning properties to ensure good clearing and channeling performance.
EV greases also face issues in chemical compatibility that differ from those in ICE vehicles. Because of the prevalence of copper wiring in EVs, grease and oil formulations must not contain additives that promote yellow-metal corrosion. Any grease that gets out of the bearing could deposit on the motor windings, Fish says, where it could attack the copper or the insulating sheathing around the wires.
Chemical compatibility with polymer vehicle components could force a move away from mineral oil-based greases to those using fully synthetic base oils like PAOs, Fish says. However, because PAOs can cause shrinkage of seal materials, the formulation should be balanced with some other base oil component (e.g., esters or alkylated naphthalenes) to reduce the risk of seal failure. Not only must base oils not draw components out of polymer parts, Shiller says, which could cause the polymers to become brittle, but lubricant additives also must not seep into polymers, which causes them to swell.
Conductivity is another issue for EV greases. Until recently, the general assumption was that greases were insulating, and there was no need to consider conductivity, Fish says. For ICE vehicles where greases see a few volts at most, that assumption was sufficient. However, some of today’s EVs run on 12-volt electronics, and 24- or 48-volt systems may become common in the near future.
Industrial equipment is easy to ground, Fish says, so grease conductivity (or the lack of it) is less of an issue. But EVs have tires against the road, so stray currents find other pathways. Because the motor shaft rotates in a magnetic field, it builds up a charge. If the charge is not dissipated, it can damage bearing raceways by arcing from one raceway, through the rolling elements and cage, and out through the other raceway. Single high discharges can burn holes in the surface of the components, and steady current flow can soften the hardened raceways. Some OEMs that have tried to correct the problem using hardware design changes have found that steering electrical charges away from one component (e.g., the motor) can cause charge buildup and damage to other components (e.g., wheel bearings).
Oils tend to have more additives than greases, says Shiller, but they are generally not good electrical conductors. Any time a large voltage potential exists between two vehicle components, stray currents will be a concern. Not only that, he adds, but because of the high temperatures and currents associated with EVs, lubricant flammability also is a concern.
Antioxidant grease additives may have a minor effect on conductivity, Fish says, but the real issue is ionic additives, often used as rust inhibitors. Intentionally conducting greases, currently used in specialized applications like the rotating anode bearings used in X-ray instruments, may incorporate higher levels of ionic species or conducting solids like carbon black, carbon nanotubes or powdered silver. Conducting greases require higher concentrations of antioxidant additives to prevent the conducting species from degrading the base fluid. Thin separating films that allow some metal-metal contact can lead to ohmic currents being transmitted, Fish adds. Here, a sufficiently thick grease film is needed to prevent wear damage from the solid particulates, but the film cannot be so thick that it causes churning losses in the bearing.
In addition to dielectric breakdown, greases in EVs also experience joule heating. Because greases are thermal insulators, they aren’t much help in conducting heat away from the bearings, Fish says. Enough heat may be generated to soften steel components to the point of plastic flow, which can cause a component to deform. Reducing the amount of steel in EV bearings can help with this; rolling elements can be made from ceramics, and cages can be made from polymers. These nonconductive components can stop bearing surface damage due to electrical discharge, but they add significant cost, Fish says.
Specifications and standards
Grease formulators face the challenge of anticipating EV manufacturers’ needs in an evolving, unconsolidated market, Kumar says. Various industry organizations are collaborating on performance requirements, but these efforts are in an early stage, he says, adding, “We don’t have standard specifications for EV greases.” He notes that various OEMs and grease manufacturers and distributors have set up their own testing methods, and grease suppliers may develop standard test limits instead.
There is a method for measuring dielectric breakdown for oils, Fish says, but it must be adapted for use with greases. He adds that there is no standard method for measuring the conductivity of grease. One test method involves putting a layer of grease at a specific thickness between two conductive plates and measuring the conductivity or voltage breakdown between the two plates, but this method has not been standardized. This lack of testing standards has led to a lack of data on repeatability or reproducibility, he says.
Earlier this year, NLGI implemented a major update of its standards for HPM greases (see Figure 3
), prompted by a need to address new formulations currently on the market.9
Shiller explains that the new standards will be more focused on uses and performance than on specific formulations. Any formulation that meets the requirements for oxidation resistance, base oil viscosity, flow and other properties will meet the new standards.
Figure 3. NLGI High-Performance Multiuse Grease Classifications. Figure courtesy of NLGI.
The existing ASTM D4950 standard for automotive service greases was reapproved in 2019 and will remain valid until at least 2024, but work is in progress to modernize the tests associated with the specifications of this standard. Shiller notes that new standard tests are similar to those used in the past, but that there are more stringent tests for new formulations. The newer tests also take advantage of newer available technologies (thermocouples rather than mercury thermometers, for example). He adds that new standards represent an attempt to balance the needs of industrial and automotive applications.
“The grease industry has always been very collaborative,” Shiller says. “Of course, different companies have their trade secrets and their patents. But they’ve always been, in my opinion, very open to collaborating.” He recalls his work doing pre-competitive research, sponsored by various lubrication companies, at the University of Akron’s Center for Surface Engineering and Lubrication Research (CSELR). “This was testing that the industry thought was important, and important enough that everybody who was working in that industry should know about this.”
He adds that the grease industry in Europe has collaborated to make it possible to register materials as a group, rather than registering each individual product. For example, the Europe’s REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulatory program registers various product components (e.g., lithium stearate for greases) rather than finished formulations.10
Grease manufacturers then register their products by referring to registered components.
Bearing manufacturers also collaborate with university research groups, lubricant suppliers, as well as companies that supply bearings to OEMs. Bearing manufacturing companies develop grease specifications for their products, says Lugt. The university’s SKF University Technology Centre for grease lubrication, which opened in September 2020, promotes collaboration among industry and academic researchers from various institutions. The focus is on fundamental research and modeling, specifically for grease lubrication. “We have to understand how the grease works in the bearings,” he explains.
Lugt notes that SKF works directly with grease manufacturers as well as performing in-house testing to evaluate grease performance with specific bearing designs, materials and operating conditions. Standard-setting organizations around the world also are collaborating, he says. “It’s a global thing,” he says, adding that it also is common for individual experts in the field to participate in several organizations in various countries. Equipment and parts manufacturers also have their own specifications for particular applications, above and beyond specifications set by existing standards, he says.
Developments down the road
Right now, when we think about EVs, we’re thinking about battery vehicles, Shiller says. However, some people are beginning to talk about fuel cell vehicles (powered by hydrogen), and these might require different lubricants.11
“There are other technologies that may be out there for vehicles that we haven’t really looked into yet,” he says.
One difference between greases and oils in EV applications is in the area of thermal management, Shiller says. Because greases don’t migrate through a system as freely as oils do, greases are used mainly in areas that don’t require cooling fluids. Thermal management in a battery vehicle is focused on keeping the batteries within a certain temperature range. Fuel cell vehicles, however, could operate in the vicinity of 200 C, and the focus could be on high-temperature operation more than the lower temperature extremes. Heat from the fuel cells could affect other components not directly in contact with the motor, so the lubricants for those components (including greases) will need to maintain adequate viscosity at higher temperatures.
Shiller adds that rather than torque varying with motor speed, as is the case with ICE vehicles, EV motors generate instantaneous torque. So, rather than a gradual running-in time, in which grease has time to work its way into gears or bearings, these components experience a great deal of force, even at low speeds, before a fluid film can form. Thus, solid coatings may be needed to protect surfaces at low speeds.
Passenger vehicles can have multiple electric motors, located near the drive wheels. In contrast, large tractor-trailer rigs will probably continue to use a main motor near the front wheels. To get the requisite power and range, they will probably rely on hybrid ICE-EV configurations, Shiller says. The grease used in the trailer components will remain the same or be improved for efficiency.
Today’s polyurea-thickened greases change viscosity in response to applied stress, but other types of responsive fluids are now on the market as well. In between greases and oils are magnetorheological fluids, which increase their yield shear stress in response to an applied magnetic field.12
These fluids, Shiller says, are currently used in automotive applications, like shock absorbers that adapt the stiffness of the ride in response to road conditions. Luzuriaga notes that “liquid greases”—low-viscosity greases that polymerize and bond to a bearing’s surfaces to reduce drag—are used for certain steel and ceramic cartridge bearings.
Luzuriaga foresees continuing government support for various types of EVs and their infrastructure, which will support continuing demand. Battery development will focus on power density, efficiency, service life, range and battery recycling (driven by economic and environmental factors). In addition, he expects to see continuing development in powertrains and transmissions, as well as better integration between the mechanical and electrical systems.
At present, there is no consensus on the best form of powertrain technology, Luzuriaga says. However, many OEM companies are forming strategic partnerships to share benefits and minimize risks as they move ahead. Some companies are going at it alone. Other OEMs, especially those that currently make ICE vehicles, are looking to adapt their existing facilities in such a way as to make a smooth transition to EVs. China has a reputation for being open to startup companies, he says, while companies in the EU are focused on approaches that help them compete with China and the U.S.
Referring to bearing designs, Carlevaris says, “In general, there is nothing that is standard—everything needs to be customized.” In any industrial sector undergoing disruptive change, it’s common to have an “explosion” of solutions for designs, he says. Typically, this period is followed by a period of consolidation, where designs converge to more standardized solutions, and companies undergo mergers and acquisitions. Consolidation and standardization reduce development costs and overall risks, he says.
The EV industry is in a growth phase now, Carlevaris says. E-axle layouts are going toward simplicity and cost effectiveness, he says, but some of the more complex designs are aimed at optimizing performance. OEMs are working to achieve the maximum possible efficiency and engine speed. Minimizing friction is becoming increasingly important as OEMs endeavor to increase the mileage and range of their EVs, he adds, noting that some 25%-30% of EV efficiency losses are attributable to friction.
Manufacturers also are working on the problem of brinelling, indentation damage to wheel bearings that can occur during vehicle transport from the factory to the showroom, Lugt says. Small vibrations during transport push grease and base oils out of the bearing interfaces and produce this damage. Thus, good contact replenishment and good antiwear and extreme pressure (AW/EP) grease additives are needed to protect bearings in the absence of a hydrodynamic film.
As hybrid vehicles and BEVs become more common, bearing manufacturers are not likely to see great changes in the number of bearings in a powertrain, Lugt says. However, the value of each bearing is likely to increase because of the greater demands on performance that come with electric motors. EV bearings require protection against parasitic currents, enhanced noise reduction and greater durability (moving toward an application life of some 300,000 to 500,000 kilometers, or even more). These demands are prompting manufacturers to invest in innovation, Lugt says, with the aim of differentiating themselves from their competitors.
Small, but vital
Making mechanical and electrical components function together “is where tribology meets electronics,” Luzuriaga says. Thermal management, friction reduction, wear protection and electrical conductivity all come into play in EVs. He adds that advances in the EV market are happening so rapidly that “we’re having to publish [reports] by the semester rather than annually.”
Regardless of the type of vehicle, Shiller says, “the function of lubricant really stays the same—to reduce friction, keep out dirt from the system, manage thermal properties and prevent corrosion.” A typical passenger vehicle can weigh about 3,000 pounds.13
Grease represents 4 to 5 pounds of this, says Kumar. However, greases play an outsized role in keeping any type of vehicle running. “You can’t get rid of grease,” he says.
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Nancy McGuire is a freelance writer based in Silver Spring, Md. You can contact her at email@example.com