Multiple bearing types in EV propulsion systems, with increased ceramics use, is driving new grease developments. (Lindsay Brooke)

‘Retuning’ lubricants for EV duty

Lubrizol’s top grease expert talks about how the electric vehicle trend is driving innovations and creating opportunities.

“My focus is lubricating grease; I’ve been doing it for 32 years,” explained Dr. Gareth Fish with a high degree of pride. Lubrizol’s top expert on the subject, globally recognized in the lubricants field, Fish has authored more than 70 technical papers on grease and tribology and has four U.S. patents. He finds the industry’s shift to electrified vehicles and its impact on automotive greases “an extremely interesting and challenging time.”

Broad challenges include strategic supply – the lubricants industry competes with the battery industry for lithium – and sustainability by reducing grease’s environmental footprint, Fish noted. Removing the internal combustion engine and its ancillary systems from vehicles is an obvious trend with major impact; Lubrizol provides the industry with significant technology development and R&D support.

“Today we have 50-60 greases, about 2 kilograms (4.4 lb.) in total, on a standard, mid-size passenger vehicle,” Fish (below) noted. “On a hybrid-electric vehicle you’ll have a similar number, because the vast majority of componentry will not change. Moving to electric vehicles [EVs] there will be some significant changes, such as elimination of crankcase oil. But the overall volume of greases will stay the same as today, about 2 kilograms.”

While EVs retain traditional grease-intensive chassis components such as CV joints, wheel hubs and bearings, their propulsion systems’ grease requirements are anything but traditional. “Drive motors in electric vehicles must operate at high rpm to be efficient – 18,000 to 20,000 rpm,” Fish explained. “When you get to those high speeds, noise becomes critical, so you need low-noise greases. You also have to have durability, and these motors can get hot, which requires wider temperature-range greases. You need a special premium grease for those electric drive motors. Add to that it may be electrically-conductive as well – all of these create challenges.”

The EV movement is creating the need for new grease formulations, he said. “For wheel bearings alone, there are additional loads caused by the heavy battery packs. Then there is electrical conductivity. If your electric drive motor has ceramic rolling elements in its bearings, it has different behavior and so needs different lubrication than bearings with caged steel balls and steel races. The requirement is basically hardware-specific,” Fish explained.

He noted that as an AC induction motor rotates, it will create stray currents within the system. Those stray currents build up charge on the motor shaft. “At some point, that current has to go somewhere; it can be grounded away by some means (such as slip rings),” he observed. “If you don’t handle this properly, you can basically build up charge that will discharge through the bearing, which ends up damaging the bearing. Or you get current flowing through the bearing which can cause a softening of the rolling elements. So, you need to guard against that.”

He explained that Lubrizol is working with industry to help implement greases for future technologies. “We’ve talked with a couple of motor manufacturers; they’re still unsure where everything is going to go. We’ve also talked to bearing manufacturers and some of them believe the best way to insulate electric-motor shafts is to use ceramic rolling elements, but they’re high-cost. So they’re looking for other solutions, one of which is conducting grease. But the technology of conducting grease today makes this a non-trivial problem.”
 
Lubricant ‘tuning’
Lubrizol develops additive packages of components for greases to meet industry specifications, then works with grease manufacturers to take the product to market. The EV market increasingly is focused on reducing the noise of propulsion systems, which are already inherently quiet compared with their combustion-engine predecessors. Fish explains: “If you need a low-noise grease, you typically filter the base oils and filter the additives. Make sure the thickener was nicely milled to a very smooth, uniform consistence. If you can do that, it goes a long way to reducing noise generation in the bearing.”

To make a grease conducting, conducting solids are added. Fish has seen many solutions and proposals, including using carbon nanotubes (which are expensive); powdered metal; silver particles and even carbon black (which can generate noise). “If you add the wrong solids, you can get abrasive wear, but you need to make sure the particle sizes are controlled. If the particle size is smaller than the lubricant film thickness, that shouldn’t cause any damage.”

On the market today there are low-noise damping greases, high-performance/high-speed greases, extended-life greases and conducting greases. “The industry wants all four of those in one! That’s the challenge – and energy efficiency,” Fish said. “So one of the changes we’re likely to see is a focus on where throughout an EV can the grease absorb significant amounts of energy and reduce range, calling for more energy-efficient greases.”

“One of the challenges is how do you measure the energy-efficiency of a grease in a bearing?” Fish noted. “You have to ensure that the film thickness is sufficient to separate the surfaces, but not too thick that it causes churning losses. There’s a zone where you need to be, lubrication-wise.” In the world of grease, “it’s a matter of balancing and adjustment of the base oil, thickener, additives and manufacturing. You have to ‘tune’ those four elements to meet requirements,” he said.

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