Open Coil Springs and Closed Coil Springs Explained
Open Coil Springs and Closed Coil Springs Explained
Open coil springs are springs whose coils have space or a pitch between each active coil, whereas closed coil springs have coils that touch each other with no visible gap. This difference in coil spacing is fundamental to how the spring functions. An open coil design (with pitch) allows the spring to compress under load, storing energy in the gaps between coils. In contrast, a closed coil design (no pitch) means the spring’s coils start out touching, so the spring must extend or twist to create space and store energy.
It’s important not to confuse open vs. closed coils with open vs. closed ends on a compression spring. Spring end types (like open ends, closed ends, ground ends, etc.) refer to how the ends of a compression spring are finished, which affects how the spring sits and distributes force. End types are a separate design consideration – for example, an open coil compression spring can have either open or closed ends. (For more detail on spring end types and their importance, see our guide on Compression Spring End Types .)
A compression spring is the most common example of an open coil spring. As the name suggests, a compression spring is designed to compress (shorten) under load. In its unloaded state, you can see visible gaps between its coils (the spring’s pitch). When you apply a force to a compression spring (for instance, pushing on it), the spring’s coils move closer together, closing the gap. This action stores mechanical energy in the spring, and when the force is removed, the spring springs back to its original length, releasing the energy as it pushes back against the load.
The open coil design is crucial for this function: the initial space between coils is what allows the spring to compress and store energy. If there were no space (pitch), the coils would already be touching and the spring couldn’t compress further. In a well-designed compression spring, even under maximum load the coils should not all touch; if the spring is compressed so far that all coils come into contact (called solid height ), the spring can no longer compress and may be damaged. This is why understanding spring design principles like coil pitch and total coils is important – it ensures the spring can safely carry the desired load without bottoming out.
You’ll find compression springs in countless everyday and industrial applications. For example, the spring inside a ballpoint pen that pushes the tip out and retracts it is a small compression spring. Large suspension springs in vehicles (coil over shocks) are compression springs that support the vehicle’s weight and absorb bumps. Other examples include shock absorbers, valve springs in engines, mattresses, and pressure-relief valves – all of these use open coil compression springs to cushion or resist a compressive force. In each case, the spring’s open-coil design allows it to absorb energy when compressed and then release that energy by pushing back, which is a fundamental aspect of spring mechanics.
An extension spring , also known as a tension spring, is a typical example of a closed coil spring. Extension springs are designed to extend (lengthen) under load – they resist a pulling force rather than a pushing force. If you look at an extension spring in its resting state, its coils are wound tightly together with no gaps (they are close-wound). In fact, the coils often touch one another and have an initial tension . This initial tension (built into the spring during manufacturing by winding the coils tightly) means the spring requires a certain amount of force before it even begins to stretch. The tightly wound coils s tore energy as tension ; when the spring is pulled apart, the coils separate and that tension provides resistance and the tendency to pull back.
Because extension springs work in tension, they usually have hooks, loops, or end attachments on both ends. These hooks allow the spring to be attached to two opposing components. In use, when those components try to pull away from each other, the extension spring’s hooks transmit the force to the spring, causing it to stretch. As the spring extends, it stores energy by the material’s deformation (similar to how a rubber band stores energy when stretched). Once the pulling force is removed or reduced, the stored energy causes the spring to contract, pulling the components back together. In essence, an extension spring pulls back against forces that try to separate the two ends.
Extension springs are widely used anywhere a return force is needed to pull components back together. For instance, the springs that pull a trampoline mat taut or the ones in a screen door that pull the door closed after you open it are extension springs. Other examples include automotive hood and trunk springs (on some cars that use extension springs), washing machine springs that stabilize the drum, and toys or farm machinery that require a pulling action. Even something as simple as a glove box door in a car or the spring on a vise-grip plier can be an extension spring. In all these cases, the extension spring’s closed-coil design keeps it compact until a tension force pulls it open, at which point it provides a restoring force to return parts to their original position.
A torsion spring is another type of spring that typically falls under the closed coil spring category. However, unlike compression or extension springs that work with linear forces (pushing or pulling), a torsion spring works with rotational or twisting forces. It is essentially a helical coil spring (often close-wound with coils touching) that has two ends called legs. When those legs are rotated relative to each other (imagine one leg anchored, and the other leg twisted), the spring’s coils either tighten or unwind slightly, and the spring exerts a torque (a twisting force) in the opposite direction to resist the twist. In other words, a torsion spring stores energy in a twisting deformation – the spring wants to return to its original wind, thereby exerting force to push the legs back to their starting positions.
In most torsion springs, the coils are initially close together (no significant gaps), similar to extension springs, which is why we call them “closed coils.” The coils being touching gives the spring a high initial stiffness, which is needed because torsion springs often must hold something in place firmly until enough torque is applied to move them. Some torsion spring designs do incorporate a small pitch (spacing) between coils to reduce friction when the spring twists, especially for applications requiring many cycles or where coil friction could affect performance. But generally, torsion springs are manufactured to be close-wound and then are loaded in a way that either further tightens the coils or only slightly separates them during operation.
Torsion springs are very common in mechanisms that require a rotational force or a self-resetting hinge. A classic example is a clothespin – the spring that keeps the clip closed is a small torsion spring. Similarly, the spring on a mousetrap or a spring-loaded clip (like on a clipboard) are torsion springs. Door hinges on self-closing doors, like those in commercial buildings or screen doors, often have torsion springs built in so that the door will automatically swing shut. You’ll also find torsion springs in automotive suspension systems (anti-sway bars or older car suspensions sometimes use torsion bars, which work on the same principle), and in gadgets like retractable pen mechanisms or wind-up toys and clocks. Torsion springs are valued for providing a strong angular force in a compact form and for their durability in cyclic applications.
The design of the coil – specifically whether coils are open (pitched) or closed (touching) – directly determines how a spring behaves under load. This comes down to basic spring mechanics and spring design principles:
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Open Coil (with Pitch): If a spring is manufactured with space between coils (an open coil spring), it means the spring can be compressed. The amount of space (pitch) sets how much the spring can compress and also influences the spring’s rate (stiffness). A larger pitch (more gap) generally allows more travel but might result in a lower spring rate for a given wire size, whereas a tighter pitch (smaller gap) makes the spring stiffer. Open coils are necessary for compression springs because they need room to deflect. That open space is what lets the spring store energy when squeezed and then release it as it expands back. Without enough pitch, a compression spring would “bottom out” too soon. Manufacturers carefully calculate the required number of coils, wire diameter, and pitch to achieve the desired strength and deflection. In essence, the coil pitch in a compression spring controls how it will compress under force and how much load it can take before the coils start touching.
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Closed Coil (No Pitch): Springs made with coils touching (close-wound) are inherently stiff initially because there’s no free space between coils. This design is used when a spring’s natural position is already at or near solid (no gaps), and the spring’s function is to resist stretching or provide torque. For extension springs, the closed coil design provides an initial tension that must be overcome before the spring extends. That initial tension is a key design parameter – it ensures the spring stays snug and doesn’t freely dangle when not under load, and it provides a baseline resistance to any pulling force. For torsion springs, a close-wound coil maximizes the number of coils (and thus the torque the spring can exert) in a given space, and it ensures a firm preload, so the spring can immediately start exerting force when one leg is rotated. One thing to watch out for with closed coil designs is coil friction: when the spring starts to move (either extending or twisting), the coils will rub against each other. This can cause internal friction that slightly affects the spring’s response (for example, extension springs may not extend completely smoothly at first due to the coils unlatching from each other). In high-cycle or precise applications, designers sometimes include a small gap (or use a slight pitch or a spacer) even in torsion springs to minimize this friction and ensure more consistent performance. Overall, the lack of pitch in a closed coil spring is what allows it to pull or hold things tightly until force is applied, at which point the spring either extends (creating gap between coils) or twists.
In summary, coil spacing is directly related to spring function: a spring with open coil spacing is meant to be compressed (pushed), and a spring with closed coil spacing is meant to be extended or twisted (pulled or rotated). Understanding this aspect of spring coil design is a fundamental part of a spring design guide for engineers. It helps in choosing the right type of spring for an application and in customizing spring properties. Whether you’re designing a small gadget or a large piece of industrial machinery, knowing how coil pitch affects behavior will guide you in specifying the correct spring dimensions and type.
In summary, open coil and closed coil springs are fundamental concepts that distinguish compression springs from extension and torsion springs. Knowing the differences in their design and function will help you make the right choice in any project. Here are the key takeaways:
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Open vs. Closed Coil: Open coil springs have spaced coils (pitch) allowing compression, while closed coil springs have touching coils and work in tension or torsion. This coil design directly relates to whether a spring pushes, pulls, or twists.
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Compression Springs (Open Coil): Designed to shorten under load and push back against forces. They are common in applications like suspensions, buttons, and shock absorbers, where they absorb energy and provide resistance.
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Extension Springs (Closed Coil): Designed to stretch under load and pull components back together. They have hooks for attachments and are used in things like trampolines, garage doors, and many gadgets, providing a restoring tension force.
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Torsion Springs (Closed Coil): Designed to work by twisting, exerting a rotational force. Commonly found in hinges, clips, and counterbalance mechanisms (e.g. clothespins, door closers, mousetraps, etc.), they offer torque to rotate objects back to position.
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Spring Design Considerations: Coil spacing (pitch), material, wire diameter, number of coils, and end types all affect a spring’s behavior. For compression springs, end types (open vs. closed ends, ground or not) are important for how the spring sits and distributes load – see Compression Spring End Types for details. Always consider your application’s requirements (force, distance, space, and type of motion) when choosing a spring, and remember that custom spring design is available if standard springs don’t fit your needs.
Choosing the correct spring type is crucial for any mechanical design, but you don’t have to figure it out alone. If you need help selecting or designing a spring, or if you have unique requirements, feel free to contact Acxess Spring. Our team of experienced spring engineers is ready to assist with spring design and manufacturing, ensuring you get the ideal coil spring for your project. Whether it’s a one-off prototype or a large industrial order, we’re here to help bring your project to life with the right spring solution. Contact us today for expert guidance and turn your ideas into reality!