The humble spring runs from Bronze Age tweezers to your tape measure
A tape measure snaps back into its case because of a thin steel band that is slightly curved across its width. Rolled up, it lies flat; pulled out, it springs back to its curve, pushing with an almost even force the whole way. That is just one of dozens of spring designs hiding in everyday objects.
The oldest springs had no coils at all. A bow, traditionally cut from flexible yew, stores energy as it is drawn and releases it into the arrow. Tweezers spread across many Bronze Age cultures, and Ctesibius of Alexandria found he could make springier bronze by adding extra tin and hammering the metal after casting. Coiled springs turned up in door locks early in the 15th century, and that same century produced the first spring-driven clocks, which shrank into large watches by the 1500s.
In 1676 Robert Hooke proposed that a spring's force grows in step with how far it is stretched. Real coil springs follow this rule well unless pushed past their elastic limit, though designs based on bending beams can respond in more complicated ways. Engineers describe stiffness with a spring rate, force per unit distance, and its inverse, compliance: a spring rated at 10 newtons per millimetre gives 0.1 millimetre for every newton. Put springs side by side and their stiffness adds up; chain them end to end and their compliance adds instead.
Most springs today are spring steel. Small ones are wound from wire that is already hardened, while big ones are shaped from softened steel and hardened afterwards. Phosphor bronze and titanium resist corrosion, and beryllium copper is chosen when a spring must carry electric current.
Shapes vary widely. Extension springs have touching coils and a hook at each end; compression springs have gaps between coils and need no fixings. Conical coils nest inside one another when squeezed, allowing longer travel. Leaf springs support vehicles, spiral mainsprings power music boxes and windup toys, and V-shaped springs once worked flintlock and wheellock firearms. In 1850 a Welsh smith, John Evans, began making carriage springs in New Haven, Connecticut, and his firm still operates.
Source: Spring (device)
