A custom coil spring is open helical spring that offers resistance to a compressive force applied axially. Custom coil springs are usually coiled as a constant diameter cylinder. Most coil springs with material diameters under three-eighths inch are cold coiled. Almost all custom coil springs using over five-eighths in. bar stock are hot coiled. There is an overlapping area from three-eighths in. to five-eighths in. material size in which the custom coil springs may be either cold coiled or hot coiled.
The transportation industry is the largest user of hot coil springs. Agricultural implement manufacturers and construction equipment builders make up the next largest segment.
Custom hot coiled coil springs that are to be subjected to elevated temperatures or operate in a corrosive environment require the use of heat-treatable exotic alloys such as Inconel X 750, precipitation hardening stainless, and even en tungsten bearing tool steels. However, most hot-coiled coil springs are made from bars of carbon or alloy steel. These bars are produced to fine grain practice and are ordered as special bar quality to insure minimum surface imperfections. Surface imperfections can be prevented completely by cold finishing or centerless grinding the bars. Since a large portion of the cost is in the material, however, this can significantly increase the cost of hot coiled coil springs.
The finished hardness requirement for most hot coiled custom coil springs is in the range of Rockwell C 44-49. In cases involving special materials and improved surfaces, final hardness as high as Rockwell C 53 may be necessary to withstand and design stresses. It should be noted that as hardness increases above Rockwell C 48 ductility and toughness of the steel falls off rapidly. Custom coil springs made to a hardness of Rockwell C53, for example have been known to shatter under a constant load. A wire manufacturer should be consulted for the best and most readily available alloy and optimum hardness for each application.
Custom hot-coiled coil springs are usually preset. They are wound to a length greater than the finished length, heat treated, then compressed solid one or more times to bring them to the specified length. Presetting introduces beneficial residual stresses which allow the springs to support greater loads than would otherwise be possible. This does not increase service life. Actually, since it allows higher stresses, presetting can materially decrease the service life.
Service life is increased by shot peening, which is particularly effective when both the hardness and working stress are high. Hot-rolled bars, even when ordered as special bar quality, can have seams and significant decarburization. Making the material acceptable requires 90 percent minimum surface coverage and high peening intensities usually in the range of .006 - .010 C. Turned or centerless ground bars require less intensive peening typically in the range .012 - .020 A, but still with 90 percent coverage.
Rust leads to pitting and pitting to early failure, so some form of protective finish is recommended. The kind of coating required will depend on the environment which the springs are expected to encounter in storage and service. Commonly used coatings, and, in some in-stances, cadmium or zinc plating.
Custom coil springs which must be extremely reliable usually require magnetic particle inspection and may have to meet a specification that "no true indications of defects shall be found." There should be a clear understanding between coil spring manufacturer and purchaser on this point, for there can often be "indications" which will not significantly affect performance in a specific high-duty application.