Autonomous vehicles, the growing adoption of alternative powertrain technologies, increased car connectivity and the “shared” future of mobility are some of the key trends shaping the global automotive market in 2019. Many of them are driven by Millennials who are more willing to use car sharing or similar services.
Therefore, car manufacturers need to anticipate how such preferences might influence Millennials’ decisions to buy vehicles.
These developments raise new challenges throughout the design process. Car sharing, for one, means the interior mechanisms need to be adjusted more often for different drivers, each with their own preferences and sizes. Mechanisms in seats or car mirrors must accommodate this demand to ensure a smooth adjustment that is built to cope with increased use.
Also, more adjustment mechanisms are being introduced to enable more flexible interiors as autonomous driving becomes more popular.
This means car interior designs will shift and expand to swivel seats that may offer passengers increased comfort. Additionally, alternative powertrains, such as battery electrics, make less noise than internal-combustion engines, reducing the powertrain’s ability to mask interior squeaks and rattles. Thus, interior mechanisms need to be designed to be even quieter.
Meeting these growing demands for longer-lasting adjustment with low noise has become a dilemma for interior engineers. Bearings, a vital component in interior systems, often experience a variation in friction due to manufacturing tolerances of mating components. Here, larger bearing clearance can ensure low friction but also increases risk of rattle noise.
To address the need for low adjustment force without clearance, engineers can incorporate spring steel, ensuring clearance-free properties with consistently low forces to reduce or eliminate rattle noise altogether. This material is used in tolerance rings, where the spring-like properties compensate for manufacturing tolerances so the radial forces remain constant.
However, a tolerance ring made of spring steel isn’t low-friction in nature and generally is employed as a radial fastener, fixing the mating components together.
There is a simple but ingenious solution: adding a PTFE sliding layer to one side of the spring steel will provide the low-friction surface. Moreover, the PTFE layer eliminates the need for lubrication and ensures the part will last for the life of the vehicle. This unique combination allows consistent force throughout the mating component tolerance range with a low coefficient of friction to ensure smooth and easy adjustment without noise.
Saint-Gobain has studied this material combination at great length to ensure the viability of the concept. Tests have demonstrated that the alternative, injection-molded plastic, contains higher forces that are less consistent, while the innovative spring steel and PTFE solution shows greatly reduced operating forces that are consistent across the tolerance range.
Additionally, the combination of spring steel and PTFE significantly reduces stick-slip friction, or breakaway force, allowing for a higher-quality feel than the alternative system.
Furthermore, tests showed a sliding system experiencing vibrations from several directions saw a decrease in rattle noise of about 8 dB(A) for the spring steel and PTFE solution at low frequencies and a smaller decrease at high frequencies.
An 8-dB(A) increase in sound level is perceived as about 70% louder, so it is safe to say the new PTFE-coated spring steel solution minimizes rattle, reducing the overall noise significantly, and translates to a quieter automotive interior.
Finally, Saint-Gobain’s engineers performed tests on sliding force consistency at two different sliding speeds. The results revealed both the spring steel PTFE and the injection-molded plastic solutions are fairly consistent between the two sliding speeds, with only a minor increase in friction force at higher speed. However, the injection-molded plastic solution shows higher sliding force in both dry and lubricated conditions, as well as high stick-slip friction.
For end users, a high force means high effort required to operate the adjustment mechanisms and high stick-slip means a low-quality, clunky feel.
With the combination of spring steel and PTFE, engineers can eliminate the challenge of high friction variation or rattle noise and, without the need for lubrication, simultaneously reduce overall system costs and assembly complexity. This enables vehicle manufacturers to deliver the high-quality adjustment feel expected by Millennial consumers from the interior of a more technologically advanced car, such as an autonomous or electric vehicle.
As growing demands on the car of the future become more complex – from big data in connected vehicles, security and safety concerns for autonomous vehicles and creating a comfortable interior for car or ride-sharing purposes – seemingly small innovations in interior systems can make a big difference by enabling design engineers to create new mobility solutions.
Alfred Lethbridge, Ph.D., (above, left) is a content marketing executive at Saint-Gobain Performance Plastics. He is based in Bristol, U.K.