By Mark Shortt
It’s not easy to make a custom spring a reality—that is, to complete the transformation from an idea or concept to a tangible, finished part. Recognizing this, Ritch Froelich set out to help designers understand some of the key considerations required to engineer a custom spring.
Froelich is general manager and chief design engineer at Ace Wire Spring & Form Company, Inc., an ISO certified and ITAR registered manufacturer of custom compression springs, extension springs, torsion springs, and wire forms for diverse applications. He wrote a guide, “Proper Requirements of Spring Design,” which Ace Wire Spring has made available to spring designers. The guide covers important factors that designers should consider, for example, when selecting the right material or establishing tolerances for specific applications.
Material selection, Froelich said, begins by considering the environment in which the custom spring will perform. Temperature is a key part of that equation. If the part needs to function in temperatures exceeding 650 degrees Fahrenheit, a high-temperature material, such as Inconel, should be specified. “Whereas, if the design requires the spring to work in an ambient temperature environment, perhaps a less costly material, such as oil tempered (spring wire), can be introduced,” Froelich wrote.
Another key consideration is whether or not the spring will come into contact with any form of moisture or chemicals, such as chlorine or bleach, according to Froelich. If the part will be used in an outdoor environment that includes exposure to salt water, most professional spring designers would start with T-316 stainless steel, he noted. “Though there are other, more costly materials on the market, T-316 stainless is highly used in these applications,” he wrote. “If the spring were simply subjected to a high-moisture source, such as tap water, conceivably, a design using T-302 can be implemented.”
Other key factors to be considered are cycle life—the amount of cycles that the spring will encounter over its service life—and exactly where the spring will be installed in the application.
“If the designer is engineering a spring that works in an engine, and the spring were to be cycled several million times over its lifetime, a material such as chrome silicon valve or chrome vanadium valve should be the material of choice,” Froelich wrote. “If the application calls for minimum cycle life, say, a safety switch that may only be implemented around 1,000 cycles over its lifetime, perhaps the most efficient material to implement would be music wire.”
The designer also needs to know, for example, if the spring will function in a location over a pin or a mandrel, and if it will be working inside a cylinder or bore. This information is relevant because the designer needs to leave a “safety margin” within any design that requires these limitations. It’s important to keep in mind that springs are coiled when they are manufactured, as opposed to being machined, Froelich noted. “Therefore, the tolerances that a spring maker requires are much more substantial than those of machinists,” he wrote.
Ace Wire Spring & Form’s engineers (www.acewirespring.com) offer design engineering assistance—including recommendations on part design, tolerances, and wire selection—to support the design of various compression springs, extension springs, torsion springs, coil springs, garter springs, assembly/specialty springs, and wire forms. The company is headquartered in McKees Rocks, Pennsylvania.