Coil spring is a helical spring that offers resistance to a compressive force applied axially. Most coil springs with material diameter less than 9mm are cold coiled. Almost all custom coil springs using over 14in bar stock are hot coiled. There is an overlapping area from 9mm to 14mm material size in which the custom coil springs may be either cold coiled or hot coiled.
Custom hot –coiled helical springs are usually preset. They are wounded to a length greater than the finished length; heat treated, then compressed solid three 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 be otherwise possible.
Service life is increased by shot peening, which is particularly effective when both the hardness and working stress are high.
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. The transportation industry is the largest user of hot coil springs.