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Springs are ubiquitous in all kinds of machines—from consumer products to heavy industrial equipment, take apart anything that involves a mechanism, and chances are, you’ll find a spring inside. Springs are storage devices for mechanical energy—think of them as analogous to batteries! The earliest spring-driven clocks appeared in the 1400s. Fast-forward some 600 years, you still have to charge the Apple Watch everyday, and it’s not nearly as fast as winding a clock…but I digress.
My personal fascination with springs began with Slinkies and wind-up toys, and countless mechanical pencils were sacrificed to satisfy my curiosity. These days, I appreciate the grown-up applications of springs, especially when I drive over potholes on Highway 101!
The most common way to classify springs is by how load is applied to them. You’re probably familiar with the following:
Compression spring: designed to operate with a compressive load and found in shock absorbers, spring mattresses, mechanical pencils, and retractable pens.
Extension spring: designed to operate with a tensile load. An archetypical example is a Slinky, but these are also found in luggage scales and garage door mechanisms.
Torsion spring: designed to operate with torque (twisting force); powers every clothespin and mouse trap.
Each of these types can be further characterized. We can look at the relationship between the force applied to a spring and its resulting displacement. Again, there are three classes of springs: linear (or constant rate) springs, variable rate springs, and constant force springs.
Linear springs obey Hooke’s Law (F=k*x), which means that the force needed to extend or compress such a spring by distance x is proportional to the distance, as long as the force doesn’t exceed the elastic limit of the spring. Torsion springs obey an analogous version of Hooke’s Law (F=k*θ, where θ is an angle). In both cases, k is the spring rate, and it stays constant, no matter the spring’s deflection. This is why linear springs are also known as constant rate springs.
On the other hand, a variable rate spring doesn’t have the same spring rate throughout its axial length. You can have a progressive change in the spring rate, or a more abrupt change—see the diagram below.
A familiar variable rate spring is the cone-shaped compression spring, most commonly found in battery boxes. The fully compressed height can be as low as one wire diameter. Variable rate springs also have the additional benefit of being laterally stable and less prone to buckling.
Self-explanatory by its name, a constant force spring requires nearly the same force, no matter how long the extension. Constant force springs are also called clock springs. This type of spring is usually a coiled ribbon of spring steel used in counterbalancing applications, such as height adjustment for monitors, and—you guessed it—clocks.
Springs can also be classified by how they’re made. The first spring that comes to most people’s minds is probably a metal coil spring, also known as a helical spring. However, there are many other types. Even an elastic band can be considered a variable rate spring, since it stores mechanical energy.
Lightweight coil springs are made by forming metal wires on an CNC coiling machine. The multi-axis CNC control allows you to create variable pitches and end conditions only limited by your imagination. Springs that come off the coiling machines do not have springy properties. They need to be heated to a high temperature (typically 500 degrees Fahrenheit or more) to relieve stress, then quenched to create shape memory.
In contrast, when making heavy duty coil springs, the wire is heated up before coiling. Check out this sweet video.
Flat springs come in all sorts of sizes and shapes: Spring washers, PCB spring contacts, and retainer clips are all examples of flat springs. Essentially sheet metal parts, they can be made by stamping. However, there are coiled flat springs as well, such as clock springs and volute springs. They also need be heat treated for shape memory.
Machined springs and die springs are used for heavy duty applications with high strength and precision requirements. As the name suggests, machined springs are made on CNC lathes and mills.
Plastic or composite springs are commonly found in corrosive environments, such as food production, medical, and marine applications. Due to creep, they should only be used in intermittent cycles. Compared to metal springs, they are relative newcomers to the space, and supply is not as abundant.
Now that you’ve gotten a braindump of all these types of springs, you might be eager to try some out. Coil springs and certain types of flat springs are available in so many stock sizes and materials that you will most likely find one that suits your application in a catalogue.
Here are some great places to source off-the-shelf springs:
The Spring Store
If you find yourself needing a custom spring, come back soon to check out our design guide for springs.