The under-development constant-velocity joint from GKN brings the joint and housings inboard, enhancing rotational stiffness and cutting weight. (GKN)
GKN turns the CV joint inside-out
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GKN Driveline is developing an innovative driveshaft concept that literally turns inside out the traditional configuration of the constant-velocity joint (CVJ), bringing potential benefits to both high-performance rear-wheel-drive vehicles—and future electric vehicles (EVs).
In a conventional design, CVJ housings face outward from the sideshaft, while the sealing systems on each end face inward; GKN has been testing a layout in which the CVJ reverses the positions of these components within a sideshaft system, integrating the CVJ housings within the sideshaft itself.
Dr. Wolfgang Hildebrandt, Director Advanced Engineering at GKN Driveline, said at the company’s Lohmar, Germany, R&D center: “The beauty of this design is that it incorporates existing GKN manufacturing expertise, but places the components in a different layout to provide a new benefit to our customers. In this respect, we have a significant advantage in not needing to invent or substantially change processes to deliver the design to automakers in the future.”
The change is said to bring significant design freedom to increase the rotational stiffness of the system, while simultaneously reducing the weight of each sideshaft by between 200 and 800 g (0.44 to 1.8 lb). On a conventional platform, this would result in a weight saving of up to 1.6 kg (3.5 lb), a weight reduction every EV producer will welcome as a contribution to longer range.
The new technology also should bring improved NVH levels, GKN claimed. There is no inherent reduction in the number of driveline components needed to transmit torque to the wheels in an EV, so NVH takes on new significance. Noise reduction in the cabin always is an important goal for quiet-running battery-electric vehicles that lack IC engine noise to drown out other mechanical ambient aural intrusions.
Electric-vehicle characteristics have highlighted a range of necessary improvements for future CVJ design, GKN stated. First, a CVJ’s efficiency directly affects a vehicle’s range; the company said that for every 1% increase in that efficiency, an OEM could expect better than 1% improvement in driving range.Range is invariably a crucial consideration for any EV purchase.
The long-established CVJ (2018 marks the 90th anniversary of its invention) might not be regarded as a salient element in EV development, but GKN’s latest CVJ project comes as a clear reminder that electric vehicle advances are not just about batteries and motors, but almost every aspect of automotive engineering technology.
CVJs, which particularly enabled a design advance for front-wheel drive vehicles, now feature in nearly every vehicle on the road; GKN Driveline manufactures more than 50 million units per year for the global automotive market. Rear-wheel-drive or front-wheel-drive cars normally use four CVJs to connect the differential to the wheel hubs. In more-complex all-wheel-drive layouts, there may be as many as 13 CVJs (four in the front, four in the rear, two in a one-piece propshaft to the front, and three in a two-piece propshaft to the rear) that connect the driveline.
GKN Driveline’s current Countertrack CVJ uses counter-running ball tracks to reduce internal friction and heat and cut overall weight. The latest evolution of Countertrack is the VL3 sideshaft system for rear-wheel drive vehicles; it has a shallow and contoured diaphragm boot seal that replaces the conventional conical CVJ boot design. It was designed to “significantly” reduce the volume of grease required, in turn saving weight and cost and improving packaging.
In an interview session with Automotive Engineering, Dr. Hildebrandt provided added detail regarding the new GKN technology:
How long has the new “inside-out” constant-velocity joint been in development?
“The idea of an ‘inside out’ joint has been around for years, but up until this point it has never been a practical solution because it was making the operating angles larger—which was undesirable. However, because we have developed the unique diaphragm boot, which shortens the length of the joints, GKN has been able to realize a significant benefit from the reversed position of the components.”
Is it still being tested—and if successful, when is production likely to start?
“This CVJ design is still at concept stage, where we are showing it to our customers and gathering their feedback. If there is positive feedback, we anticipate being able to quickly build up the hardware to perform application studies. Much of the concept factors-in existing and proven GKN Driveline design features, so we naturally have high confidence in its reliability and performance.”
Can it be used on any type of vehicle?
“The concept has an optimal design for high-performance, rear-wheel drive sports cars, where high stiffness can help to achieve a very direct torque response and more dynamic driving behavior. There are also benefits for high-stiffness drivelines in electric vehicles, where it enables more direct control of the e-motor output.”
Can you detail how the concept was conceived; is it unique to GKN?
“The ‘inside out’ concept was primarily developed at our research and development center in Lohmar, Germany, where it involved significant cooperation between the expert teams leading the development of both CVJs and sealing systems.”
How many patents are involved?
“We have applied for one core patent on the overall concept, but also have existing strong patent protections related to the CVJs and to the sealing system.”
Is this configuration likely to become the CVJ norm for GKN products?
“The reversed joint is a relatively niche design with a specific class of applications in mind. Its design is well suited to high-performance rear-wheel drive vehicles as well as electric cars. GKN Driveline often adapts its products to the needs of its customers and has developed bespoke solutions for numerous OEMs, so any future application of this ‘inside out’ design would of course be carefully tailored to suitable cars.
“Our development of the reverse-joint system reinforces what we have always said about CVJs, which is that they are far from being a commodity product. If you think about CVJs in this way, then you will probably not be maximizing their performance in your vehicles. The technology is constantly being [advanced] and the latest systems can deliver significant benefits for OEMs.”
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