A spring is an elastic object that stores mechanical energy. In everyday use the term often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel, although some non-metallic objects like the bow are also springs. When a conventional spring, without stiffness variability features, is compressed or stretched from its resting position, it exerts an opposing

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

approximately proportional to its change in length (this approximation breaks down for larger deflections). The ''rate'' or ''spring constant'' of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the

gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...

of the force versus deflection

curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line, but that does not have to be straight. Intuitively, a curve may be thought of as the trace left by a moving point. This is the definition that ...

. An extension or compression spring's rate is expressed in units of force divided by distance, for example or N/m or lbf/in. A torsion spring is a spring that works by twisting; when it is twisted about its axis by an angle, it produces a

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

proportional to the angle. A torsion spring's rate is in units of torque divided by angle, such as

N·m The newton-metre (also newton metre or newton meter; symbol N⋅m or N m) is the unit of torque (also called ) in the International System of Units (SI). One newton-metre is equal to the torque resulting from a force of one newton applied ...

/ rad or ft·lbf/degree. The inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness (or rate) of springs in parallel is additive, as is the compliance of springs in series. Springs are made from a variety of elastic materials, the most common being spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication. Some non-ferrous metals are also used including phosphor bronze and

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current (because of its low electrical resistance).

History

Simple non-coiled springs were used throughout human history, e.g. the bow (and arrow). In the Bronze Age more sophisticated spring devices were used, as shown by the spread of tweezers in many cultures. Ctesibius of Alexandria developed a method for making bronze with spring-like characteristics by producing an alloy of bronze with an increased proportion of tin, and then hardening it by hammering after it was cast.

Coiled springs A selection of conical coil springs The most common type of spring is the coil spring, which is made out of a long piece of metal that is wound around itself. Coil springs were in use in Roman times, evidence of this can be found in bronze Fib ...

appeared early in the 15th century, in door locks. The first spring powered-clocks appeared in that century and evolved into the first large watches by the 16th century. In 1676 British physicist Robert Hooke postulated Hooke's law, which states that the force a spring exerts is proportional to its extension.

Types

A spiral torsion spring, or hairspring, in an alarm clock.">alarm_clock.html" ;"title="hairspring, in an alarm clock">hairspring, in an thumb">Battery contacts often have a variable spring A volute spring. Under compression the coils slide over each other, so affording longer travel. Vertical volute springs of Stuart tank 180x180px, Selection of various arc springs and arc spring systems (systems consisting of inner and outer arc springs). Tension springs in a folded line reverberation device. A torsion bar twisted under load Leaf spring on a truck

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

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 A cantilever is a rigid structural element that extends horizontally and is supported at only one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cant ...

, 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 inside the tubing such as a membrane or miniature piston etc. to harden or relax the spring, much like it happens with water pressure inside a garden hose. Alternatively tubing's cross-section is chosen of a shape that it changes its area when tubing is subjected to torsional deformation: change of the cross-section area translates into change of tubing's inside volume and the flow of oil in/out of the spring that can be controlled by valve thereby controlling stiffness. There are many other designs of springs of hollow tubing which can change stiffness with any desired frequency, change stiffness by a multiple or move like a linear actuator in addition to its spring qualities. ; Arc spring: A pre-curved or arc-shaped helical compression spring, which is able to transmit a torque around an axis. ;

Volute spring A volute spring, also known as a conical spring, is a compression spring in the form of a cone (somewhat like the classical volute decorative architectural ornament). Under compression, the coils slide past each other, thus enabling the spring to ...

: A compression coil spring in the form of a cone so that under compression the coils are not forced against each other, thus permitting longer travel. ; Balance spring: Also known as a hairspring. A delicate spiral spring used in

watch A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached ...

es, galvanometers, and places where electricity must be carried to partially rotating devices such as steering wheels without hindering the rotation. ; Leaf spring: A flat spring used in vehicle

suspensions In chemistry, a suspension is a heterogeneous mixture of a fluid that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometer, and will eventua ...

, electrical

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...

es, and bows. ; V-spring: Used in antique firearm mechanisms such as the wheellock, flintlock and percussion cap locks. Also door-lock spring, as used in antique door latch mechanisms.

Other types

Other types include: ; Belleville washer: A disc shaped spring commonly used to apply tension to a bolt (and also in the initiation mechanism of pressure-activated

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) ; Constant-force spring: A tightly rolled ribbon that exerts a nearly constant force as it is unrolled ; Gas spring: A volume of compressed gas. ; Ideal Spring: A notional spring used in physics: it has no weight, mass, or damping losses. The force exerted by the spring is proportional to the distance the spring is stretched or compressed from its relaxed positio