Custom Spring Design from A to ZJuly 1, 2017
The custom-spring designer’s first consideration is the environment in which a spring will be required to perform—temperature, for example. For instance, if the spring will experience heat in excess of 650 F, it will require a high-temperature metal alloy such as Inconel. Or, if the spring will work in lower-temperature environment, perhaps a less costly material will suffice, such as oil-tempered spring wire.
|Some factors to consider when sizing a spring: direction of wind, particularly when engineering a torsion spring (shown here), and assembly requirements. Wind direction dictates how energy is directed through the spring—right-hand (clockwise) or left-hand (counterclockwise).
Keeping with the theme of environment and material selection, the designer must establish early in the design process if the spring will come into contact with any form of moisture or chemicals, such as chlorine or bleach, or if will operate outdoors and be exposed to salt water. In this environment, professional spring designers likely would opt for Type 316 stainless steel. Though there are other costlier materials on the market, 316 stainless often is used in these applications. And, for springs subjected only to a high-moisture source (such as tap water), Type 302 stainless steel may suffice.
The next requirement in material selection to address is cycle life—the number of times the spring will cycle during its lifetime. For instance, if designers are engineering a spring that works in an engine and the spring will cycle several million times over its life, they might opt to engineer the spring using valve-spring-quality chrome-silicon or chrome-vanadium. Or, if the application calls for minimum cycle life, say a safety switch that may only cycle 1000 time or so during its life, music wire may suffice.
The next question: Where will the spring be installed/will it be working over a pin or a mandrel, for example? And, will the spring work inside of a cylinder or bore? If so, designers must include a safety margin in the design that accounts for these limitations. And, they must consider that springs are not machined products—they are coiled, and, therefore, the tolerances that a springmaker requires are much more substantial than those of machinists. For example, a machinist may be asked to hold a 1-in. block to a tolerance of ±0.0001 in., while a springmaker would hold a 1-in.-dia. spring to a tolerance of ±0.030 in. Therefore, when designing a spring that fits into a 1-in.dia. bore, the designer would be requested to build-in a safety factor of ±0.030 in., or an OD of 0.970 in. ±0.030 in.
Deflection and the 20-80 Rule
|To calculate the total amount of deflection within a compression spring (shown here), the designer considers three parameters: wire size, relaxed free length and solid height (wire size times the total number of coils in the spring). Then, to calculate total spring travel, subtract solid height from the relaxed free length of the spring.
To optimize spring performance, follow the 20/80 rule: Install the spring at no less than 20 percent of its total deflection, and at no more than 80 percent of its total deflection. To calculate the total amount of deflection within a compression spring, the designer considers three parameters: wire size, relaxed free length and solid height (wire size times the total number of coils in the spring). Then, to calculate total spring travel, subtract solid height from the relaxed free length of the spring.
For example, designing a spring from 1⁄4-in. wire with four coils and a relaxed free length of 3 in.:
1⁄4 in. x 4 coils =1 in. solid height
Next, in determining the total travel contained within the spring, the designer then will take the calculated relaxed free length of the spring and subtract the solid height of the spring. Using our example above:
Total travel = 3 in. – 1 in. = 2 in.