How Springs work

What is a spring?

A typical spring is a tightly wound coil or spiral of metal that stretches when you pull it (apply a force) and goes back to its original shape when you let it go again (remove the force). In other words, a spring is elastic. I don't mean it's made from rubber; I mean that it has elasticity: it gets longer when stress is applied but (providing you don't stretch it too much) returns exactly to its original length when that stress is removed. Depending on how a spring is made, it can work in the opposite way too: if you squeeze it, it compresses but returns to its original length when the pushing force is removed.

You can make a spring out of more or less anything—even paper or orange peel!—but the kinds of springs we use in machines work effectively only if they're stiff enough to resist a pulling force and durable enough to be stretched many times without breaking. Typically that means they have to be made from materials such as stainless steel or tough alloys such as bronze. Some alloys have a property called shape-memory, which means they're naturally springy. Eyeglass frames are often made from a nickel- titanium shape-memory alloy called nitinol, sold under brand names such as Flexon®.

How does a spring work?

Imagine you have a piece of straight steel wire about 10cm (4 in) long—something like a long paperclip you've unwrapped. If you pull it with your fingers, it's extremely difficult to stretch it. Coil it around a pencil and with a bit of effort you can make yourself a small but perfectly functioning spring. Now pull or push it with your fingers and you'll find you can stretch and squeeze it quite easily.

Why has this once-stubborn piece of metal suddenly become so cooperative? Why is a spring really easy to stretch and squeeze when the same piece of metal, in the form of a wire, was so reluctant to change shape in the first place?

When the material is in its original form, stretching it involves tugging atoms out of their position in the metal's crystal lattice—and that's relatively hard to do. When you make a spring (as you'll discover if you try bending a paperclip), you have to work a little bit to bend the metal into shape, but it's nowhere near as difficult. As you bend the wire, you use energy in the process and some of that energy is stored in the spring; it's prestressed, in other words. Once the spring is formed, it's easy to change its shape a little bit more: the more windings of metal a spring has, the easier it is to stretch or squeeze it. You've only to shift each atom in a spiral spring by a small amount and the entire spring can stretch or squeeze by a surprising amount.

Springs are great for storing or absorbing energy. When you use a pushing or pulling force to stretch a spring, you're using a force over a distance so, in physics terms, you're doing work and using energy. The tighter the spring, the harder it is to deform, the more work you have to do, and the more energy you need. The energy you use isn't lost: most of it is stored as potential energy in the spring. Release a stretched spring and you can use it to do work for you. When you wind a mechanical clock or watch, you're storing energy by tightening a spring. As the spring loosens, the energy is slowly released to power the gears inside and turn the hands around the clockface for a day or more. Catapults and crossbows work in a similar way except that they use twists of elastic for their springs instead of coils and spirals of metal.