Types of Springs: Coil, Power & Constant Force Springs
Despite their simple appearance, springs have been a fundamental part of mechanical design for centuries. Though often small and easy to overlook, they play a critical role in countless machines and devices: watches, clocks, scooters, automobiles, and industrial equipment.
However, not all springs are designed the same. Depending on their geometry, they differ in how they store and release energy, allowing them to meet different mechanical requirements. In this article, we will explore the common types of springs and how they function.
What is a Spring?
A spring is an elastic mechanical device that stores potential energy when subjected to external force, such as compression, extension, or torsion. When force is applied, the spring temporarily changes shape and stores energy. Once the force is removed, it returns to its original form while releasing the stored energy. This ability allows springs to absorb shock, provide restoring force, and enable controlled motion in a wide range of mechanical systems.
Types of Springs
Springs come in many forms, each designed to store and release mechanical energy in different ways. While traditional springs are typically made from coiled metal wire, some specialized springs use flat metal strips to achieve unique force characteristics. The following are some of the most common spring types and how they differ in structure and function.
Compression Springs
Compression springs are the most common type of spring, typically made from metal wire wound into a cylindrical coil with visible gaps between each turn. They store energy by resisting compressive force. When pushed inward, the coils move closer together and store elastic energy, then expand back to their original length when the force is removed. Compression springs are widely used in products such as click pens, mattresses, and vehicle suspension systems.
Extension Springs
Extension springs also use a coiled wire structure, but unlike compression springs, their coils are wound tightly together with little or no gap between turns. They store energy when stretched, building tension as the coils are pulled farther apart. When released, the stored energy pulls the attached components back toward each other. Common applications include spring scales, screen doors, and trampolines.
Torsion Springs
Torsion springs are coiled springs designed to store and release energy through twisting motion rather than compression or extension. They typically consist of tightly wound coils with straight arms extending from both ends. When torque is applied, the spring twists and store elastic energy. Once released, it generates rotational force to return components to their original position. Common applications include clothespins, door closers, and counterbalance mechanisms.
Power Springs
Unlike the previous spring types, power springs are not made from coiled wire but from long, thin strips of spring steel wound into a flat spiral shape, resembling a rolled metal ribbon. They store energy when wound tighter around a central shaft, causing the flat metal strip to bend and store elastic energy. As the spring gradually unwinds, it releases stored energy as rotational force, making power springs ideal for compact mechanisms such as mechanical watches, clocks, and retractable power cords.
Constant Force Springs
Constant force springs look quite similar to power springs, as both are made from flat spiral spring steel. However, unlike power springs that exert rotational force during unwinding, constant force springs are designed to provide nearly uniform pulling force in a linear direction. This allows them to maintain almost the same resistance throughout extension, making them ideal for retracting and counterbalancing applications such as Monitor height adjustment systems, medical equipment lifting systems, fan height adjustment systems, etc.
Among these spring types, power springs and constant force springs operate quite differently from traditional coil springs. Instead of relying primarily on wire deformation, they use the bending behavior of flat spring steel to achieve compact energy storage and specialized force output. Because of these unique characteristics, the following sections will take a closer look at these two spring types.
Power Springs in Detail
A power spring, commonly known as a mainspring or clock spring in everyday applications, is a type of flat spiral spring made from a metal strip, typically high-carbon steel or stainless steel, wound into a compact coil. It is designed to store rotational energy when wound and release that energy as torque during unwinding.
Mechanism
The key function of a power spring is to store and release rotational energy in a compact space. When an external force winds the spring tighter toward the center, the flat metal strip bends and stores elastic potential energy. Once the release mechanism is activated, the spring naturally tries to return to its less tightly wound state. As it gradually unwinds, the spring continuously releases stored energy as torque, which can drive gears or other mechanical components.
A pull-back toy car is a simple everyday example of how a power spring works. When the car is pulled backward, the power spring inside is wound tightly and stores mechanical energy. Once released, the spring gradually unwinds and transfers the stored energy to the car’s gears and wheels, driving the car forward. This simple mechanism demonstrates how power springs store and release rotational energy in a controlled manner.
Advantages
Power springs offer several key advantages:
High Energy Storage Density
Power springs can store a large amount of rotational energy within a compact cylindrical space. This makes them ideal for applications where installation space is limited but energy output is still required.
Sustained Torque Output
As a power spring unwinds, its torque output gradually decreases until the spring returns to its natural state. In practical applications, power springs are often installed with pre-turns, meaning the spring is partially wound at the starting point to provide sufficient initial torque and maintain usable torque throughout operation.
No External Power Required
Because power springs operate purely through mechanical energy storage, they do not require batteries or external electrical power. This makes them highly suitable for applications where electrical power is unavailable, undesirable, or difficult to maintain.
Applications
Power springs are widely used in everyday products and industrial equipment due to their compact design and ability to deliver stable rewind force or driving torque. Common applications include:
Automatic Rewind Devices
Power springs are commonly used in automatic rewind devices such as Seat belt retractors, vacuum cleaner cord reels, extension cord retractors, and pet leashes. In these applications, the spring provides rewind force that smoothly pulls the cable or strap back into place after use.
Mechanical Timing Devices
Power springs are widely used in mechanical timing devices such as traditional wind-up watches, mechanical alarm clocks, and kitchen timers. They store mechanical energy when wound and release it gradually to drive the timing mechanism at a controlled rate.
Toys and Entertainment Products
In toys and entertainment products, power springs provide the driving force needed to create motion. Common examples include classic wind-up toys and Music boxes, where the stored energy is converted into movement or sound.
Industrial Equipment
Power springs are also used in industrial equipment such as air hose reels, tool balancers, and non-powered lifting devices. In these applications, they help store and release energy in a controlled manner to improve operational convenience and efficiency.
Constant Force Springs in Detail
Unlike traditional springs, constant force springs provide nearly uniform pulling force throughout extension. This means the spring maintains almost the same pulling resistance over most of its travel range, instead of becoming significantly harder to pull as it extends further.
Mechanism
The nearly constant pulling force of a constant force spring comes from how it stores and releases energy during extension. The spring consists of a tightly coiled flat metal strip that naturally tends to remain curved.
When the strip is pulled outward, most of the spring remains either tightly coiled or fully straightened. Only the small transition zone between the coiled and straight sections actively resists the pulling force, as this is the area where the metal is continuously changing from a curved shape to a straight shape.
Because this working region remains nearly the same size throughout extension, the curvature and bending radius also stay nearly constant. As a result, the resisting force remains constant instead of increasing significantly like a traditional spring.
When the external pulling force is reduced or removed, the spring gradually recoils back into its coiled shape and releases the stored energy. This retraction allows the spring to provide smooth rewind force to counterbalancing support in practical applications.
Advantages
Constant force springs offer several key advantages:
Nearly Constant Pulling Force
Constant force springs provide smooth and consistent force output over most of their extension range. This makes them ideal for applications that require predictable and stable movement.
Suitable for Long Travel Applications
Because the pulling force remains nearly constant even over long extension distances, constant force springs perform especially well in applications involving long strokes or repeated retracting motion.
Compact Coil Structure
Despite providing force over long travel distances, constant force springs can be tightly wound into compact circular coils, making them space-efficient for product design.
Applications
Because constant force springs provide a smooth, nearly constant pulling force over long travel distances, they are widely used in mechanisms that require stable force output, automatic retraction, or weight balancing.
Height-Adjustable Fans and Adjustable Desks
Constant force springs provide a pulling force that offsets the weight of the fan or desktop, allowing users to adjust the height easily while keeping it at the desired position.
Cordless Blinds and Window Balancers
Constant force springs allow blinds or vertically sliding windows to stay at nearly any height without dropping due to gravity or snapping back too quickly.
Vending Machines and Shelf Pushers
These springs provide steady pushing force behind products. As items at the front are removed, the remaining products are smoothly pushed forward, keeping them accessible and properly positioned.
Conclusion
Although power springs and constant force springs share a similar flat spiral structure, they are designed for different mechanical requirements. Power springs are ideal for applications that require rotational torque and compact energy storage, while constant force springs are better suited for applications requiring smooth, nearly constant pulling force over long travel distances.
Choosing the right spring depends on your application’s force requirements, space constraints, and motion design. With extensive manufacturing experience in both power springs and constant force springs, Teng Yao provides customized spring solutions tailored to different industrial needs. Contact us today to discuss your application requirements and find the right spring solution for your product or equipment.