Springs are used for many purposes, and one spring may serve more than one purpose. Listed below are some of the more common of these functional purposes. As you read them, try to think of at least one familiar application of each.

1. To store energy for part of a functioning cycle.

2. To force a component to bear against, to maintain contact with, to engage, to disengage, or to remain clear of some other component.

4. To maintain electrical continuity.

5. To return a component to its original position after displacement.

6. To reduce shock or impact by gradually checking the motion of a moving weight.

7. To permit some freedom of movement between aligned components without disengaging them. These are sometimes called take-up springs.

As you read different books, you will find that authors do not agree on the classification of types of springs. The names are not as important as the types of work they do and the loads they can bear. The three basic types are (1) flat, (2) spiral, and (3) helical.

Flat springs include various forms of elliptic or leaf springs (fig. 11-7, A [1] and [2]), made up of flat or

    Figure 11-8. Bevel gear differential

slightly curved bars, plates, or leaves. They also include special flat springs (fig. 11-7, A [3]), made from a flat strip or bar formed into whatever shape or design best suited for a specific position and purpose.

Spiral springs are sometimes called clock, power (1 1-7, B), or coil springs. A well-known example is a watch or clock spring; after you wind (tighten) it, it gradually unwinds and releases power. Although other names for these springs arc based on good authority, we call them "spiral" in this text to avoid confusion.

Helical springs, also often called spiral (fig. 11-7, D), are probably the most common type of spring. They may be used in compression (fig. 11-7, D [1]), extension or tension (fig. 11-7, D [2]), or torsion (fig. 11-7, D [3]). A spring used in compression tends to shorten in action, while a tension spring lengthens in action. Torsion springs, which transmit a twist instead of a direct pull, operate by a coiling or an uncoiling action.

In addition to straight helical springs, cone, double-cone, keg, and volute springs are classified as helical. These types of springs are usually used in compression. A cone spring (11-7, D [4]), often called a valve spring because it is frequently used in valves, is formed by wire being wound on a tapered mandrel instead of a straight one. A double cone spring (not illustrated) consists of two cones joined at the small ends, and a keg spring (not illustrated) consists of two cone springs joined at their large ends.

Volute springs (fig. 11-7, D [5]) are conical springs made from a flat bar that is wound so that each coil partially overlaps the adjacent one. The width (and thickness) of the material gives it great strength or resistance.

You can press a conical spring flat so that it requires little space, and it is not likely to buckle sidewise.

Torsion bars (fig. 11-7, C) are straight bars that are acted on by torsion (twisting force). The bars may be circular or rectangular in cross section. They also may be tube shaped; other shapes are uncommon.

A special type of spring is a ring spring or disc spring (not illustrated). It is made of several metal rings or discs that overlap each other.