Time to read: 5 min
Springs are common 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, analogous to the electrical storage capacity of batteries. The earliest spring-driven clocks appeared in the 1400s. Fast-forward some 600 years, you still have to charge your Apple Watch every day, and it’s not nearly as fast as winding a clock… so yay progress, I guess?
My personal fascination with springs began with Slinkies and wind-up toys, and countless mechanical pencils were sacrificed and deconstructed to satisfy my curiosity. These days, I appreciate the more sophisticated applications of springs — especially when I drive over potholes, thanks to my car’s modern coil spring suspension that still delivers a smooth ride!
Spring Types and Their Applications
The most common way to classify springs is by how the load is applied to them. These are the most common classifications of springs:
- Compression springs: designed to operate with a compressive load. Found in shock absorbers, spring mattresses, mechanical pencils, and retractable pens.
- Extension springs: designed to operate with a tensile load. An archetypical example is a Slinky, but extension springs are also found in luggage scales and garage door mechanisms.
- Torsion springs: designed to operate with torque (twisting force). These springs power every clothespin and mouse trap.
Within each of these different spring types are further characterizations and classifications. Next, let’s 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.
Pro-Tip: When choosing springs for an assembly, make sure to follow Design for Assembly Rules. Use our free 10 DFA Rules to Live By Checklist.
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.
Hook’s Law for Linear Springs:
F=- k * x
Where: x = distance or (L2 – L1)
F= force needed to extend or compress the spring ‘x’ distance
k=spring constant or spring rate
The negative sign is present in Hook’s Law because the restoring force is in the opposite direction of the applied force; pulling a spring down will cause a downward extension but a resultant upward force.
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.
Hook’s Law for Torsion Springs:
F=- k * Θ
Where Θ=angle of deflection or twist
Variable Rate Springs
On the other hand, a variable rate spring doesn’t have the same spring rate throughout its axial length, in other terms – k is not constant. 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. Here are some additional resources for conical spring calculations for stiffness and allowable working stress.
Constant Force Springs
Constant force springs are uniquely formed with pre-tensioned metal strips, not wire as standard springs are formed from. 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.
Also, the term constant force spring is a little misleading. The reality is that the spring’s full load must be overcome by extending the spring to 125% of its original diameter — then a nearly constant force can be utilized to continue expanding the spring.
Springs can also be classified by how they’re made, because there are many methods for making springs. The most commonly-known is probably a metal coil spring, also known as a helical spring, but there are many other types of springs. 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 a 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 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 the shape memory.
In contrast, when making heavy duty coil springs, the wire is heated up before coiling, which you can see in this video.
Flat springs come in all sizes and shapes. Spring washers, PCB spring contacts, and retainer clips are all examples of flat springs. These springs are essentially sheet metal parts made by stamping. However, there are coiled flat springs as well, such as clock springs and volute springs — and all flat springs need be heat treated for shape memory.
Belleville washers and conical springs are both common terms used in exchange for disk springs, which are disk-shped with a concave surface. They are typically made by stamping, plasma cutting, or blanking a flat sheet of metal, then the concave shape is typically machined. Disk springs can look like deformed metal washers, but they serve much more complex purposes.
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 susceptibility, they should only be used in intermittent cycles. Compared to metal springs, plastic springs are relative newcomers to the space, and supply is not as abundant.
Now that you know more about the types of springs, it’s time to try some out! Coil springs and certain types of flat springs are widely available in a range of stock sizes and materials that will likely suit your application. When sourcing, be sure to specify your spring material. Here are some of the most common spring materials:
- Beryllium copper alloy
- One-directional glass fiber composite materials
- Steel alloys
Here are some suppliers of off-the-shelf springs:
And here are some other spring-related resources to learn more:
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