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Mechanisms: The Spring

Apr 27, 2023

Most people probably don't think about springs until one kinks up or snaps, but most of the world's springs are pretty crucial. The ones that aren't go by the name Slinky.

We all use and encounter dozens of different types of springs every day without realizing it. Look inside the world of springs and you’ll find hundreds of variations on the theme of bounce. The principle of the spring is simple enough that it can be extended to almost any shape and size that can be imagined and machined. Because it can take so many forms, the spring as a mechanism has thousands of applications. Look under your car, take apart a retractable pen, open up a stapler, an oven door, or a safety pin, and you’ll find a spring or two.

Technically speaking, a spring is an elastic object that stores mechanical energy and releases it when compressed, stretched, or twisted. The oldest form of spring known to man is likely the archery bow, followed by the flat springs of Bronze Age tweezers. Ancient Roman carriages used leaf spring suspensions for a smoother ride on those rocky roads.

Coiled springs, now the most common and identifiable, were first used in the 15th century in locks and clocks. In the 1800s, they began to pop up in beds and furniture. By the 1980s, they were buckling regularly inside of IBM keyboards.

Springs can be classified by their shape, their makeup, and their intended use. Most springs are either coiled or flat, but there are legions of custom springs in various shapes. The main uses of springs include returning something to its starting position, like a screen door; measuring force, like a scale; and storing energy for later use, like in a mechanical watch or a wind-up toy.

Springs are most often made from either metal or plastic; the choice depends on the intended application and environment. The majority are made from spring steel, which is a cold-drawn carbon steel alloy. Springs made from copper alloys like phosphorus bronze and beryllium copper provide high electrical and thermal conductivity. Plastic springs may seem impractical, but they’re quite useful in sensitive applications like food processing and x-ray equipment that call for non-metallic mechanisms made from inert materials.

No matter the composition, a good spring is one that can return to equilibrium again and again without any lasting dimensional damage. A well-designed spring should perform for a long time, which is probably why so many go unnoticed and unconsidered until they break.

Ask someone to draw a spring, and they will probably scribble out a compression spring. When squeezed or compressed, they get shorter. These are easily the most common and are found in everything from retractable pens to soap dispensers to pinball plungers to car suspensions.

The Slinky is a pre-compressed helical spring whose properties were toyed with to make it walk down stairs as slinkily as possible. Take the batteries out of something and you’ll probably find conical compression spring terminals that compress more flatly to save space.

Extension springs are the second most common. When stretched or pulled, they get longer. These are the springs that snap your screen door shut and give bounce to your trampoline. They are made much the same way as compression springs. The main difference is that extension springs have hooks or ears on the ends for connecting to the load or loads that will extend them. These ears are made by bending one or more of the end coils out, usually perpendicular the orientation of the remaining coils.

The other difference is the space between the coils. A compression spring is made to compress, so the coils are spaced wide enough to give room for compression. An extension spring is made to extend, so the coils touch in their unstretched position.

Torsion springs resemble compression springs, but are activated differently. Whereas compression strings are stressed in torsion, or by twisting the elastic material, torsion springs are stressed in bending. They are found in clothespins, pop-up electronics, and lots of other things with jaws.

The load is applied to the spring legs in the direction of the coil windings. Torsion springs are like little wound-up cantilevers that either supply torque, as in the clothespin, or withstand it as in the staple remover.

Another common torsion configuration is the torsion bar, used in some car suspensions and in the gullwing doors of the DeLorean DMC-12. Torsion springs can be flat, too, like the hairsprings in mechanical clocks, music boxes, and other wind-up toys.

The leaf spring is a type of non-torsional flat spring. Trucks and other large vehicles often have leaf spring suspensions that transfer the load evenly across the chassis.

The more complicated something is, the more springs it is likely to have. The same goes for the types of springs involved. It all depends on the need. A stapler has two: an extension spring that advances the magazine, and a leaf spring that returns the stapler to the ready position. Cars and cassette players each have many different springs, though the average car has it outnumbered by a few hundred.

Many things require specialized spring action, and for that there are many kinds of special springs, one-offs, and purpose-built wireforms, such as cotter pins or the handles of a binder clip. Springs can also be machined from a single piece of stock.

Wave springs are used in place of compression springs where tight load deflection is needed in a small space. Some electric motors pre-load their bearings with a type of wave spring that lowers noise.

Serpentine springs are the zig-zagging kind that chair and couch suspensions are made of. I suspect that the common potato masher design was based on such a spring.

Volute springs resemble a collapsible cup, and double volute springs look like croissants. Many types of tanks use volute spring suspensions in both vertical and horizontal configurations. That pair of hand pruners in your garage probably has a double volute; they don't seem to be used for much else.

The weirdest one might be the Belleville washer. These are conical disks that are stacked up in various configurations to provide different types of bounce and are most commonly found in land mines.

So what gives springs their resiliency and bounce? In the case of spring steel, the elasticity is a function of the way the molecules interact. At rest, the attracting and repelling forces within are balanced.

When a compression spring is squeezed, the repelling force builds, and releasing the spring pushes the molecules apart. In an extension spring, stretching the coils builds up the attracting force, and releasing the spring pulls the molecules together.

Whenever a force acts upon a spring of any kind, the amount of movement that results in the spring is known as its deflection. Most springs actuated within their elastic limit will obey Hooke's Law, which states that the force (F) needed to compress or extend a spring is equal to the product of the spring constant (k) times the distance traveled (x) (aka its deflection). In other words, the compression or extension of any spring is proportional to its force.

Not all of the coils of a spring will distort under load. Any spring with closed ends will have inactive coils that don't take any stress. Perhaps most importantly, spring designs must take into account the number of active coils needed to do the work.

For product manufacturers, there's no off-the-shelf solution for designs that call for springs. Their springs have to be made to order based on the desired dimensions and deflection.

Metal springs start with a coil of drawn steel cord that's either round, square, or rectangular in profile. The cord is fed into a forming machine that has been set up to make a specific type of spring. The machine might take an hour or more to set up, but once it's dialed in, it can make tens of thousands of springs per hour.

Compression springs are simply coiled with the desired end formations and cut. When making extension springs, the machine will bend the first coil or two out at a 90° angle to form a hook. Then it coils the rest of the spring, forms the other hook, and cuts it. Some hooks are formed by hand, depending on the material and customer spec.

The finished springs are then heated to relax the metal, which relieves the stresses from coiling and helps them retain their shape after flexing. Some springs are blasted with a high-speed stream of pellets for added strength in a process called shot-peening.

Most consumer-level springs are cold-wound. For wire diameters above 16 mm, they are hot-wound as you can see below. Hot-wound springs are found in automobiles, farm and construction equipment, and locomotive suspensions.

Let's say you have some design that could benefit from springiness. If you can't find the one you need in that spring assortment from Microcenter, you can make your own out of music wire if you have access to a lathe and a mandrel. [ThisOldTony] can help with the details. Have you made your own springs? Let us know in the comments.

Though we have just begun to scratch the surface of the subject of springs, we hope you’ve gained respect for these hard-working mechanisms.