Traditional beam-type drive axles as used in most pickups and full-size SUVs are simple, robust, relatively inexpensive—and a hefty chunk of cast-iron and steel. Benchmark axles for a typical ½-ton pickup weigh 180 lb (82 kg) each. They’ve evolved slowly in the nearly 120 years since Louis Renault first employed a shaft-driven “live” axle on his car. Significant mass reduction has eluded them until three years ago, when a brainstorming session at a Detroit-based supplier kicked off a thorough investigation of what’s possible in terms of axle mass, internal efficiency, NVH reduction, durability and package volume.
American Axle & Manufacturing CTO Phil Guys and his advanced engineering team were looking at refinements for their company’s incumbent axles. “New bearings, lighter weight lubes, superfinished hypoid gears…a very iterative exercise and a good business case to provide those technologies,” Guys (pronounced geez) told Automotive Engineering. But it wasn’t enough. The team concluded that further finessing to eke out tenths-of-a-percent more efficiency and reduce mass by mere ounces would not be enough of a stretch. Nor would it be unique to AAM.
“There’s a typical way you make hypoid gears and there’s a narrow source for bearings,” Guys said. “We [axle suppliers] all go to the same seal guy and casting guy. The value of your creation is in the execution from a manufacturing perspective. But from a product-functional perspective there’s not a lot of differentiation.”
Their investigation led AAM into arguably its boldest product-development program since the company’s 1994 founding. Compared with the 180-lb benchmark beam axle for 1500-series trucks noted above the new AAM family of axles, called Quantum, weighs 125 lb (57 kg)—a 30% mass reduction. Similar gains are offered for ¾-ton truck drive axles; where a benchmark in that segment is 317 lb (144 kg), the Quantum weighs 216 lb (98 kg).
“We haven’t even tried to quantify the savings on the vehicle side that can come from the ‘cascade’ benefits we bring; we’ll let the customers do that,” Guys said. Along with the lightweighting is a 30-40% reduction in parasitic losses, claimed to be good for approximately 1% improvement in label-type fuel economy to the vehicle.
“Our customers’ order of priority is Cost, Efficiency and Light Weight,” Guys said. “And with global vehicle platforms, we’re doing well in designing a core ‘module’ that will fit in tight spaces. So it behooves us to be much more focused on the power density because it leverages our technology in more and more applications. We’ve been focused on making those individual axles as efficient and lightweight as possible—that’s what generated the Quantum technology.”
AAM has had Quantum axles in customer demonstrator vehicles for about a year and is “addressing multiple Request for Quotes for 2021-2022 opportunities,” Guys said. New driveline programs typically must align with all-new vehicle platforms or significant refreshes. The tech demos “have proven the technology to be quiet, efficient and durable,” he said.
Quantum represents a significant re-architecting of traditional rear-drive axle design. Guys recalls that the first iterations appeared so “radical” that AAM leadership was a bit unsure of demand.
“It doesn’t look like a traditional axle [inside the case] so there was a bit of skepticism from some of our customers,” he explained. Because of this, the development team collected “a tremendous amount of validation, development, efficiency, NVH and durability data in support of the technology,” Guys said. “We also built models to prove that it’s a quiet axle when installed in a vehicle.”
The Quantum family encompasses RWD and AWD modules. The design is scalable and available in open, mechanical limited-slip, electronic limited-slip and torque-vectoring configurations. It also can be equipped with AAM’s EcoTrac axle-disconnect system pioneered on the 2014 Jeep Cherokee, Guys noted. Beam-axle development was pursued first due to the regulatory pressure on the U.S. full-size truck segment.
So, what’s the differentiator that makes the AAM unit, as Guys describes it, “radical?” A unique approach to the gear-support structure inside the A356-T6 center section. The case design and bearing saddles are “totally different,” said Guys, who would not show AE details of the internals despite a recent media drive of Quantum-equipped test vehicles.
Aerospace ball bearings
All of the new axle’s efficiencies are found in the ring-and-pinion support, which uses an aerospace-type ball bearing that’s not pre-loaded, which cuts parasitic losses. Guys claims the new bearing “does not come with aerospace-industry costs.”
Ball bearings are commonly used to support the pinion gear in today’s drive axles; some applications use double-row for the head bearing and single- or double-row for the tail bearing. An AAM production axle also uses ball bearings in the differential unit.
“I can tell you that for Quantum we kept tapered rollers for the differential; we could clearly go to ball bearings for those for an incremental improvement,” Guys said. Double-row bearings are not used in the ring and pinion support; it is not preloaded and thus does not use shims.
Pinion bearing shims have been part of drive-axle architecture for decades. Two designs are used in the industry to set pinion bearing preload. One is the collapsible spacer method, in which the pinion nut is tightened until the spacer collapses and applies a specific preload to the bearings. The other is the non-collapsible spacer method that uses selective shims to set the proper preload. Shims are used by AAM in its current axle families as well as by competitor Dana in its Spicer AdvanTEK high-efficiency axles.
The Quantum bearings were designed by AAM’s in-house engineering team. The race profiles, surface finish and ball-bearing spacers are all “home-grown,” Guys noted. While AAM is not aligned with one specific bearing supplier for Quantum, the company may choose to partner with one, he said.
AAM had to learn about bearing manufacturing “in order to integrate certain features into our components,” Guys explained. “When you see Quantum you’ll see how we integrated features into the design. We’ve built over 100 beam axles to this architecture to indeed give ourselves confidence that it works from all perspectives.”
An AAM manufacturing-engineering team is currently at work to preparing Quantum axles for production, “looking at new techniques for machining-in certain features that I can’t yet show you,” Guys asserted. The engineers “are focused on how to fixture, how to cut, how to create the surface finishes on certain parts that allow us to achieve the performance results,” he said.
As OEMs confirm their light-truck driveline paths for the next decade, expect AAM Quantum axles to play a major role in reducing vehicle curb weight while boosting efficiency.