Spring (device)

A spring is a device consisting of an elastic but largely rigid material (typically metal) bent or molded into a form (especially a coil) that can return into shape after being compressed, extended or twisted. Springs can store energy when compressed, when extended, and/or when twisted. In everyday use, the term most often refers to coil springs, but there are many different spring designs. Modern springs are typically manufactured from spring steel. An example of a non-metallic spring is the bow, made traditionally of flexible yew wood, which when drawn stores energy to propel an arrow. When a conventional spring, without stiffness variability features, is compressed or stretched from its resting position, it exerts an opposing force 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 of the force versus deflection curve. 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 proportional to the angle. A torsion spring's rate is in units of torque divided by angle, such as N·m/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 manufacture. Some non-ferrous metals are also used, including phosphor bronze and titanium for parts requiring corrosion resistance, and low-resistance beryllium copper for springs carrying electric current.

History Simple non-coiled springs have been 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 springs out of an alloy of bronze with an increased proportion of tin, hardened by hammering after it was cast. Coiled springs 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. On March 8, 1850, John Evans, Founder of John Evans' Sons, Incorporated, opened his business in New Haven, Connecticut, manufacturing flat springs for carriages and other vehicles, as well as the machinery to manufacture the springs. Evans was a Welsh blacksmith and springmaker who emigrated to the United States in 1847, John Evans' Sons became "America's oldest springmaker" which continues to operate today.

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. Often used in torsion bar vehicle suspension systems. There are many other ways to potentially classify and subclassify springs, such as their shape - coil springs are common, but so are leaf springs, for example. Garter springs are arc springs with a specific arc in mind; both are typically made by bending a coil spring into a position.

Common types The most common types of spring are:

Balance spring Also known as a hairspring. A delicate spiral spring used in watches, galvanometers, and places where electricity must be carried to partially rotating devices such as steering wheels without hindering the rotation. 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. Arc spring A pre-curved or arc-shaped helical (coiled) compression spring, which is able to transmit a torque around an axis. Garter spring An arc spring where the arc is a full circle. They can be compression or extension springs. Volute spring A coil spring in the form of a cone so that under compression the coils are not forced against each other, thus permitting longer travel. Hollow tubing spring 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. Leaf spring A flat spring used in vehicle suspensions, electrical switches, and bows. V-spring Also known as Chevron spring. Used in antique firearm mechanisms such as the wheellock, flintlock and percussion cap locks. Also used as a door-lock spring, such as in antique door latch mechanisms.

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