In the ongoing design and engineering crusade that is vehicle lightweighting, grams count. In some cases, tenths of a gram count. Regardless of how fuel price has moderated in the U.S. in recent years, the importance of global platforms and global markets means the drive to create more mass-efficient vehicle structures, subassemblies and components continues unabated.
Automakers are scrutinizing even the smallest parts in order to shed unnecessary weight on their final design.
The Multi-Material Lightweight Vehicle (MMLV), a concept collaboration by Ford and Magna under a project funded by the U.S. Department of Energy’s Vehicle Technologies Office, achieved nearly 25% overall vehicle weight savings, while increasing its fuel efficiency by 15% to 20%. These outstanding results were achieved by combining the aluminum-intensive structure with a variety of reinforced composites including carbon fiber and selective use of light metals such as magnesium and titanium. Other vehicle mass-optimization studies show similar opportunity.
Galvanic corrosion threat
Integrating MMLV principals into production vehicle programs at every level, including within major subsystems such as electrified propulsion systems, presents various challenges, noted Chris Needes and Hans-Jürgen Jäger, global market managers in the automotive chassis/powertrain and exterior areas, respectively, at Saint-Gobain.
“Multiple materials used within a small confine of space within systems such as a steering column, door hinge or motor stator, for example, can lead to an accelerated rate of corrosion and therefore risk damaging more expensive parts of the car over a shorter period of time – eliminating the benefits of weight saving in the short term,” Needes explained.
Jäger uses the example of a closure-panel hinge. “There is the possibility that steel and aluminum will be interacting in a tight space. This could be due to the hinge straps (aluminum) playing the role of a cathode, the bearing and pin (often steel or steel elements) the anode, and together with an electrolyte you’ve created a battery—a perfect environment for accelerating corrosion.”
The challenge, therefore, is to create a bind between two different metals without encouraging corrosion to occur and rust to take over. Needes noted that Saint-Gobain has invested heavily in R&D for multi-material applications in vehicle components and built considerable expertise in how they react with one another.
“In certain critical areas, such as: steering columns, door hinges and electric motor stator mounts, the use of innovative small, but important, parts can make a big difference to the overall mechanism performance.”
Grams saved add up
Needes explained that in order to remove weight from the vehicle for the goal of reducing its emission and fuel-consumption footprint, his team first had to identify what he calls potential “sacrificial elements” where dissimilar materials can be used in place of heavier metals.
“The collapsible steering column, for instance, poses an ideal candidate for light weighting,” he said, as it consists of interlocking shafts where steel can be replaced with aluminum. Specifically for this application, Saint-Gobain designed high-quality carbon steel-and-alloy-based tolerance rings. These radially-sprung component fasteners enable optimal joining between mating steering column shafts, thus enabling the use of dissimilar materials.
The tolerance rings’ design also dismisses a further manufacture and customer concern during the light weighting of vehicles–noise.
“By using specially developed tolerance rings we can ensure a perfect fit between steering shafts, which leads to reduced vibration, ensuring a noise-free environment inside the car,” Needes asserted.
Tolerance rings are often used to replace heat-to-press methods and adhesives, for example, in electric motor stator mounts as a way to save weight by allowing the stator casing to be made of lighter aluminum. According to the Saint-Gobain experts, tolerance rings streamline assembly by simply being inserted around the stator and press fitted into housings.
Needes noted that a contemporary passenger vehicle can contain up to 25 electric motors. A few grams’ saved on each individual motor, multiplied by 25, adds up to significant aggregate mass savings.
PTFE’s benefits in hinge design
The tolerance rings also compensate for differential expansion, greatly reducing the chance of assembly failure when dissimilar materials like steel and aluminum are used.
Where manufacturers can save on weight, they will—including in the doors, decklids, hoods and in particular, the hinges used for those closures. The number of closures on a typical passenger vehicles means the opportunity for weight saving can be substantial.
“In the car hinge application, automotive manufacturers and suppliers can now achieve up to a 40% weight saving of the total component compared to using purely steel parts, for example,” observed Jäger.
He explained that the bearings in these components sit between the hinge pin and housing to ensure smooth movement when the door is opened and closed. As opposed to the conventional bearing, a composite bearing’s Polytetrafluoroethylene (PTFE) liner (as used in Saint-Gobain’s Norglide bearings) also compensates for any misalignment between the straps and the pin within the car hinge, ensuring smooth movement, he claimed.
Composite bearings in this application extend the lifetime of the hinge, preventing paint from chipping and eliminating red rust. In addition, when the vehicle is in use, the PTFE liner within the composite bearing compensates for tolerance variations, such as thermal expansion, ensuring the perfect fit between the hinge pin and the housing. The PTFE properties in the layer greatly reduce the risk of the door squeaking when opened or closed throughout the car’s life cycle.
The multi-material vehicle is clearly a step forward in terms of optimizing material properties and joining techniques to achieve greater overall vehicle fuel efficiency and lower emissions. However, if manufacturers focus purely on the weight-reduction issue they run the risk of facing problems further down the line, such as the aforementioned corrosion and rust.
By collaborating with supplier partners and gaining an understanding of the bigger picture (including the most intricate details), OEMs can make the best use of their investment in multi-material vehicle lightweighting activities.
Chris Needes is Global Market Manager, Automotive Chassis, Norglide bearings and Rencol tolerance rings, Saint-Gobain and Hans-Jürgen Jäger is Global Market Manager, Automotive Exterior for the aforementioned product lines, Saint-Gobain.