CompositesWorld
Issue link: https://cw.epubxp.com/i/546021
CompositesWorld.com 63 L-type disc Discs interlock when stacked via outer fange/ inner lip designs Pitch angle (degree of slope: 7-10°) of disc's surface is a key performance parameter F-type disc DISCS IN CROSS SECTION 60-100 mm OD Ratio of OD to ID is a key performance parameter (optimum range: 1.7 to 2.2) ASSEMBLED DISC SPRING Disc stack replaces steel helical coil spring/shock assembly N-type disc (no lip/fange) closes out each end of stack Illustration / Karl Reque Tunable Disc Springs Hyperco Carbon Composite Bellows Spring (CCBS) › Small carbon composite discs, molded at pitched angles, are stacked alternately, in series. Te team also needed a means of mechanically interlocking adjacent discs, when stacked. Further, a disc spring made with carbon composites would take advantage of carbon fber's high tensile properties in bending but had not been tried for high- frequency cyclic loading typical of an automotive suspension (between 0.3 to 33 Hz). What was obvious, says Campbell, is that the design would eliminate a major disadvantage of the coil spring, which "when axially loaded, always produces undesirable torsional side loads, and when installed in a coil-over spring/shock assembly, these side loads create signifcant friction on the shock shaft bearing and seal." Tis phenomenon, referred to as stiction (from static and friction), adversely afects the shock action, and puts stress on the car's suspension that can lead to handling problems. Te composite discs, by comparison, would generate no side loads, since they simply fatten out under axial load. Friction is a problem, however, in the key disc-spring variable of spring rate, the amount of load required to defect a spring one inch. Te disc design needed to deliver consistent spring rates, from part to part. Here, Hyperco had to investigate the spring phenomenon known as hysteresis. Te term refers to the fact that when load is applied to the spring, causing it to compress a certain distance (e.g., 10 mm), that load (in kilograms) is greater than the load measured during extension (springback) or unloading of the spring, for the same measured defection, says team member and carbon composite program manager Greg Hazard. As a result, the suspension unit does not move as freely as it should. Spring manu- facturers work hard to reduce hysteresis — that is, to ensure that the defection value is the same during compression and exten- sion, and, thus, contribute to a more predictable, controllable suspension system that reacts faster and more precisely to imper- fections in the road surface. "In a disc spring," says Hazard, "hysteresis is caused by the friction at the mating surface of two adjacent discs." Tat friction adds load to the spring during compression, but reduces load during extension. Hyperco believed it could use the molding process to vary the shape of the disc contact region to manipulate the friction to damp suspension load energy for some application requirements. Hazard says, "Hysteresis exists, and we might as well be able to use it as a tuning tool to the driver's advantage." › The disc design fattens under axial load, eliminating undesirable spring side loads. › Stacks can be "tuned" to meet specifc compres- sion and extension conditions.