CompositesWorld
Issue link: https://cw.epubxp.com/i/546021
AUGUST 2015 62 CompositesWorld FOCUS ON DESIGN Carbon composite spring: Big performance, small package Disc spring stack is lighter, reduces friction and can be performance-tuned. » Helical coil springs are a mainstay in today's automotive suspensions, with a basic design essentially unchanged since their advent. MW Industries (Rosemont, IL, US) is the largest US spring manufacturer/supplier and the largest supplier of high- performance springs to motorsports through its Hyperco division (Pontotoc, MS, US). Te company stays abreast of new mate- rials and, therefore, began investigating carbon fber compos- ites for high-performance springs several years ago, recalls Mark Campbell, corporate head of new product development: "We actually invited some composites specialists to come in and partner with us on designs." Tat collaboration resulted in a helical all-carbon composite spring design that seemed reasonable, says Campbell, but did not perform as expected, because "carbon does not perform well when placed in torsion." Further experiments included carbon composite "bump stops," suspension components designed to prevent metal-on-metal contact under extreme compression for NASCAR racing teams. Although these were ultimately banned by the racing body, the bump stop's compressible components led Campbell and Hyperco to an "ah-ha" moment, the fruit of which are lightweight disc springs, sold under the Carbon Composite Bellows Spring (CCBS) trademark. The Belleville washer Hyperco's CCBS incorporates a type of "Belleville washer," an industrial widget with a long and storied history. Invented in the mid-1800s by industrialist Julian Belleville, the truncated-cone- shaped steel washers could resist high loads in a small space, providing high spring force over a very short distance as well as high-energy storage capacity. Commonly used in industrial valve assemblies, the washers ultimately proved invaluable in warfare, particularly as return springs in large-caliber artillery. In fact, Belleville washer designs used by the U.S. military were classifed until the 1970s. Campbell's team believed that, given the success of the bump stops and the history of Belleville washers, a composite disc spring was achievable. By stacking multiple carbon composite discs (see drawing, p. 63), it seemed possible to achieve spring performance equivalent to a helical spring, or to steel disc springs, but at a weight reduction of as much as 60%. Stacked discs would be es- pecially appropriate for race cars, which typically weigh less than passenger cars and, therefore, require less spring stroke length but need stif springs for high-speed control. Despite the history and possibilities, data were scarce. "Te mathematical equations of how Bellevilles work are hard to come by, with no design formulas," asserts Campbell. "We had to de- velop our own equations for the shape and size of the composite versions, for the spring characteristics we wanted. It has taken a huge amount of time and testing." Te team had to consider many variables in search of optimum values for the overall disc diameter, the ratio of disc outer diameter (OD) to inside diameter (ID), the disc's laminate thickness and the pitch angle (degree of slope) of the disc's upper surface. By Sara Black / Technical Editor An alternative to the metal coil spring CCBS disc springs — stacks of slightly conical compression molded carbon/epoxy discs manufactured by the Hyperco division (Pontotoc, MS, US) of MW Industries (see above) — form strong, reliable suspension components in race cars, like this one (right), driven by SCCA racer Chris Farrell. Source (all photos) | Hyperco