law, which states that the force a spring exerts is proportional to its extension. Types Classification Springs can be classified depending on how the load force is applied to them: Tension/extension spring The spring is designed to operate with a tension load, so the spring stretches as the load is applied to it. Compression spring Designed to operate with a compression load, so the spring gets shorter as the load is applied to it. Torsion spring Unlike the above types in which the load is an axial force, the load applied to a torsion spring is a torque or twisting force, and the end of the spring rotates through an angle as the load is applied. Constant spring Supported load remains the same throughout deflection cycle Variable spring Resistance of the coil to load varies during compression Variable stiffness spring Resistance of the coil to load can be dynamically varied for example by the control system, some types of these springs also vary their length thereby providing actuation capability as well They can also be classified based on their shape: Flat spring Made of a flat spring steel. Machined spring Manufactured by machining bar stock with a lathe and/or milling operation rather than a coiling operation. Since it is machined, the spring may incorporate features in addition to the elastic element. Machined springs can be made in the typical load cases of compression/extension, torsion, etc. Serpentine spring A zig-zag of thick wire, often used in modern upholstery/furniture. Garter spring A coiled steel spring that is connected at each end to create a circular shape. Common types The most common types of spring are: Cantilever spring A flat spring fixed only at one end like a cantilever, while the free-hanging end takes the load. Coil spring Also known as a helical spring. A spring (made by winding a wire around a cylinder) is of two types: Tension or extension springs are designed to become longer under load. Their turns (loops) are normally touching in the unloaded position, and they have a hook, eye or some other means of attachment at each end. Compression springs are designed to become shorter when loaded. Their turns (loops) are not touching in the unloaded position, and they need no attachment points. Hollow tubing springs can be either extension springs or compression springs. Hollow tubing is filled with oil and the means of changing hydrostatic pressure