Contributed by The Timken Company, manufacturer of engineered bearings and power transmission components.
Increasingly, countries and cities around the world are announcing bans on fossil fuel vehicles. And while these sanctions may be a decade or more down the road, the trend toward electrification is clear.
In 2017, more than 125,000 electric commercial vehicles (CVs) were produced globally with one Markets and Markets report suggesting that number will exceed 1.8 million units by 2025―more than fourteenfold growth in eight years. Driving the electric CV market is the need for emissions-free vehicles that minimize the additional liability of fuel expenses, the report notes.
Consider that every five weeks, 9,500 brand-new electric buses take to the roads in China (the equivalent of the entire London bus fleet). Growing populations, heightened concerns over urban pollution and increased greenhouse gas regulations are making electric city buses popular all over the world. Meanwhile in North America, the electric CV market is being driven by changing consumer buying habits. Online shopping, for instance, is creating higher demand for home deliveries that can be
As emissions concerns increase, electric commercial vehicle demand is escalating.
performed more sustainably by electric medium-duty trucks and delivery vans.
Ryan Fernandes, a principal application engineer with The Timken Company, explains how this major shift in commercial vehicle design will push the boundaries of electrification, causing engineers to think differently about the design of traditional mechanical power transmission solutions like bearings.
“Electric and hybrid vehicle powertrains are considerably different than traditional powertrains, which places our focus on modifying bearing dimensions and performance characteristics to accommodate these architectural changes,” he says. “Electric CV bearings must operate under extreme heat with lighter-viscosity lubricants, so the impact to bearing life and performance can be quite significant.
“Despite the differences, it’s still about offering power-dense, fuel-efficient bearing designs that allow manufacturers to compact product offerings to enable longer distances in between fuel-ups and battery charges.”
About Smaller, Lighter Bearings
Fernandes describes how Timken is leveraging its extensive engineering knowledge and customer relationships to become an early design influencer for electric CV powertrain solutions. Specifically, existing Timken fuel-efficient bearing technologies are being optimized to create “electric” power-dense, fuel-efficient bearings, or ePDFEs.
POWER-DENSE, FUEL-EFFICIENT POWERTRAIN SOLUTIONS ARE NEEDED FOR ELECTRIC BUSES, TRUCKS AND VANS.
“Smaller, lighter ePDFE bearings present automakers the opportunity to reduce the overall size of the powertrain and add weight back into more value-added areas, such as the battery,” says Fernandes. “Timken is very familiar with the application of power-dense, fuel-efficient solutions and is now working with customers globally to develop the next generation of eDrive systems.”
Fernandes adds that the basic architecture of conventional powertrains consists of an internal combustion engine, transmission, differential, axles and wheel ends.
“Hybrid vehicles have the complete mechanical powertrain, to which an electric motor is added as an alternate propulsion source,” Fernandes says. “A full electric powertrain, by comparison, still utilizes axles and a differential to supply power to the wheels. However, since the electric motor can supply power at various speed ranges, the transmission becomes a more compact gear system that operates at a wider and higher speed range. Notably, higher speeds increase power loss exponentially, driving the need for more efficient bearings.”
Overcoming Lubrication Challenges
Kuldeep Mistry, Ph.D., is a lubrication research specialist for Timken who also believes electric CVs present new challenges, particularly for bearings that operate under extreme conditions. Mistry points out that greases have varying torque properties, and through proper selection, this can further add to fuel efficiency for our customers.
“This involves the selection of better base oils, thickeners and additives that add up to long life, high efficiency and less noise as well as provide compatibility with various polymers and plastics, copper wires and other materials,” he says. “High-performance electric motors require a different approach to selecting a grease because the motors emit a lot of heat. It is desirable to have grease with a high oxidation resistance when operating under extreme conditions. A grease with a high dropping point―that is, the point at which it passes from a semi-solid to a liquid―is also preferable.”
“Fuel economy is a huge market driver,” Fernandes adds. “Heat generation typically comes from power loss. The energy that is increasing the transmission temperature is coming from the propulsion system, so decreasing this will help drive efficiency. There are many contributing factors to power loss, from bearings to seals to oil viscosity.
“A lot of energy is lost in the shear action of the oil, for instance. A thicker oil has higher shear resistance, meaning a thinner oil will generate less heat and, thus, less power loss. Over the years, Timken has seen the market pushing to use thinner oils, such as gear oil moving from a 140W to 85W or even lower.”
The trade-off, cautions Fernandes, is that low-viscosity oil or grease also reduces the film thickness between bearing contacts, which can increase operating temperatures. Figure 1 shows the bearing contacts for a tapered roller bearing that must be effectively separated to reduce friction and wear. These include the rib-roller large-end contact to guide the rollers (1), the rolling element and raceways contact to carry the application load (2 & 3), and the incidental cage-roller contact (4).
Each contact is separated by the lubricant where speed, load, oil viscosity and temperature all play a role in creating a sufficiently thick oil film to maintain surface separation. Should the oil film become too thin, then metal-to-metal contact and premature wear may occur. This can result in fine grain spalling (the pitting or flaking away of bearing surface material) and/or other common types of damage.
Engineering Efficiency
Around the world, Timken works with customers to leverage new materials, improved bearing designs and advances in surface topography to negate the negative impact of low-viscosity lubricants. By making electric CV bearings power dense and fuel efficient, Timken is making it possible to save more power at higher operating speeds while ensuring bearings can go the extra mile.
Figure 2 compares three industrial-use gear oils―75W-85, 75W-90 and 75W-140. Notably, the 75W-85 oil has a relative life of ~65% compared to the 75W-140 oil. However, by taking bearing materials, internal geometries and surface finishes into account, Timken can improve relative life to ~90% of the 75W-140 oil, achieving both higher efficiency and suitable life requirements.
SMALLER, LIGHTER BEARINGS THAT TOLERATE LOW-VISCOSITY LUBRICANTS CAN MEAN BIG BENEFITS FOR AUTOMAKERS.
Fernandes stresses that electric CV bearings must be considered early in the design stage to get the most out of today’s technologies.
“Designing from the ground up makes it much easier to tailor a bearing’s performance and life expectancy to the propulsion system,” he says. “Conversely, substituting into an existing architecture creates certain constraints that may not take full advantage of the bearing’s capabilities―for instance, where a power-dense, fuel-efficient bearing is operating in a housing with a smaller bearing envelope.”
Fernandes adds that moving forward, a fundamental knowledge of bearings will be essential for electric vehicle OEMs that utilize low-viscosity lubricants.
“Electrification is disrupting both the light and commercial vehicle markets and creating opportunities for companies to emerge as industry leaders,” he says. “As automakers adapt to EV trends by modifying existing powertrain designs, we believe our engineering knowledge can continue to push the boundaries of electric vehicle propulsion systems.”
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