Compression springs are used more than extension springs in critical applications due to their self-limiting properties. A compression spring cannot be pushed past its breaking point, whereas an extension spring can be easily overloaded to the point of failure. But in many situations, extension springs are fine because the installation itself limits their range of travel. Consider the many extension springs used in an automotive drum brake, a definitely critical application.

Compression springs can be manufactured in a variety of shapes besides the standard straight coil, including conical, barrel, and hourglass forms which are used in special applications. Compression springs, though usually made from round wire, can also be made from square or rectangular wires. They can also be manufactured in shapes other than round coils, such as rectangular. Extension springs also can be made in many shapes besides the basic coiled round wire.

An important consideration for both compression springs and extension springs are their ends. Coil springs often are used with seats and grinding the ends flat allows them to set fully into a seat. This is especially true of heavy-duty springs such as those used in engine valve trains. Compression springs used for lighter duty applications will often just be made with single extra loops at the ends which lay flat compared with the spring helixes. Extension springs are available with many varieties of hooks and loops for their ends which serve as attachments to posts, holes, etc. Often the spring if overloaded will break at the hook, not the coil.

Materials for coil springs range from music wire to any number of spring steel alloys. Some materials offer good corrosion resistance, relaxation resistance, electrical conductivity, etc. Generally, coil springs are stress relieved after forming to remove any residual stresses imparted during the manufacturing process. For a more complete discussion on the different materials used in the manufacturing of springs, see our related guide on the types of spring materials.

Torsion springs look similar to compression and extension springs but instead of applying force through a longitudinal axis apply a radial force opposite to the direction of the coil winding. The same material considerations of compression and extension springs apply. Ends are another consideration but for torsion springs the ends usually extend out from the body of the spring where they form lever arms. The orientation of these lever arms with respect to each other is a consideration, another is the handedness of the spring. Torsion springs wound as concentric spirals from flat stock are sometimes called spring motors because of their use in mechanical watches, windup toys, etc.

Drawbar, volute, and garter springs all rely on the mechanism of the coil spring to function. Flat springs are sometimes called leaf springs and are often custom-formed from flat spring steel for specific purposes. Many trucks and some automobiles use leaf springs for their suspensions.

Gas and air springs are somewhat different in their actuating methods than most of the mechanical springs discussed here. Instead of relying on the twist in a length of straight or coiled metal, gas and air springs use pressurized gas to produce a spring effect.

Spring manufacturing is a heavily made-to-order business, in that most manufacturers can make any spring you want based on a number of specifications including wire diameter, number of coils, coil diameter, etc. A number of manufacturers publish catalogs of their stock springs which cover a wide range of choices over a discrete interval of dimensions and sizes.