This article takes an in-depth look at torsion springs.
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A torsion spring is a mechanical device crafted for storing and releasing rotational energy efficiently.
The ends of a torsion spring are connected to a mechanical component. As one end spins around the spring's axis, the coil tightens, accumulating potential energy.
In the winding process, one end of the torsion spring moves around the central axis while the other remains stationary. As the coil tightens and withstands the increase in rotational force, the spring builds up more potential energy.
Upon release, a torsion spring unwinds and elastically rebounds, discharging the stored energy. This results in an equivalent rotational force being exerted on the opposite end, applying torque to the connected mechanical part. Torsion springs are commonly used to provide static support for mechanical components.
The functioning of torsion springs is based on their resistance to rotational or twisting forces. The mechanical energy produced by resisting this motion is stored and generates torque that counters the twisting effort, in proportion to the twist's angle. Typical torsion spring varieties include helical, torsion bars, and spiral wound springs, which can be manufactured from materials like wire, sprung steel, or rubber.
Torsion springs primarily endure bending stress rather than rotational stress due to their tighter winding under twisting. Unlike other spring types, such as compression and tension springs that work with linear forces, torsion springs exclusively handle rotational forces.
The mechanical capabilities of torsion springs rely on the elasticity of the materials they are made from, allowing them to revert to their original shape after twisting. These springs can be rotated to exert force either clockwise or counterclockwise and must be twisted along their winding direction to achieve the greatest force impact.
Widely utilized across a multitude of industries, torsion springs find applications in various settings and come in numerous configurations to meet diverse requirements.
Torsion springs are helical mechanical components engineered to store and release rotational energy, or to maintain a mechanism in a specific position by twisting along the axis of their body’s centerline. Unlike compression springs or extension springs, torsion springs generate torque rather than linear force, making them uniquely suited for applications that require angular force or controlled rotational movement. Common uses include hinges, garage door mechanisms, clothespins, automotive assemblies, and industrial machinery.
During the process of deflection — when the spring is twisted in the intended direction — a torsion spring’s body diameter decreases while its overall length increases, allowing it to efficiently store mechanical energy and exert rotational force in critical operations. This distinctive mechanical principle is foundational for both custom-torsion spring design and commercial off-the-shelf spring selection.
The winding direction of a torsion spring must align with the specific requirements of its application. During assembly, the load-bearing leg should be positioned on the correct side, either left or right, to ensure proper alignment. Torsion springs are supported by a mandrel that corresponds to the hinge line of the application.
The inner diameter of a torsion spring is the width within the coil's helix, measured perpendicular to the centerline axis. This critical dimension determines the maximum outer diameter of a shaft or mandrel that can fit smoothly into the spring. For optimal operation in rotating assemblies or high-cycle applications, it is recommended that the inner diameter includes a 10% clearance to allow the inserted component to move freely and prevent binding or excessive wear.
Engineers and designers must consider material expansion or contraction, especially for high-torque or high-temperature environments, when specifying the required inner diameter for precision performance.
The outer diameter of a torsion spring is the complete outside width of the coil’s helix, measured perpendicular to the centerline axis. This dimension directly defines the diameter of the hole or housing into which the spring is inserted, accounting for all necessary tolerances to ensure the spring operates freely while also preventing excessive lateral movement that could lead to alignment issues or spring fatigue.
The wire diameter is the thickness of the wire used to coil and construct the torsion spring. This measurement significantly influences the spring's maximum torque output, durability, and stress resistance under repeated loading cycles.
The mean diameter is calculated by subtracting the wire diameter from the outer diameter (Mean Diameter = Outer Diameter – Wire Diameter). Accurate calculation of mean diameter is essential for determining both the stress distribution within the coils and for spring rate calculations that impact reliability in dynamic applications.
The body length of a torsion spring is the length measured when the spring is in an unloaded (free) state, determined by measuring the outer surfaces of the end coils. As torque or angular force is applied, the spring's body length increases while its active coil diameter decreases, impacting the torque available for rotational mechanisms such as lift gates or lever arms.
The leg length of a torsion spring is the distance from the end of the spring’s leg to the centerline axis of the coil. Leg length directly affects the applied load or torque required to deflect and store energy in the spring. Springs with shorter legs require greater torque to initiate coiling. Additionally, the legs of a torsion spring can have different lengths, allowing for custom configurations in tight or complex assemblies, such as those found in precision instruments or automation systems.
The total coil of a torsion spring refers to the full number of active coils in the winding. Active coils are those that twist or deflect under load, storing and subsequently releasing energy as the spring unwinds. The total coil count is slightly less than the total number of physical coils due to the inactive portions represented by the spring's legs. For torsion springs with a 0° leg angle in the free position, the total coil value is a whole number, an important specification for both standard and custom torsion spring design.
The pitch of a torsion spring is the centerline distance between two adjacent active coils. In most closely-wound torsion springs, often used in high-cycle fatigue-resistant applications, the pitch is nearly equal to the wire diameter. However, close winding increases friction during deflection, so designers typically specify the total coil count and body length rather than pitch to optimize reliability and performance in custom-torsion spring manufacturing.
Torsion springs are manufactured with a specific winding direction, which can be either right-hand or left-hand. Right-hand winding rotates the coils clockwise, while left-hand winding rotates them counterclockwise. The winding direction is chosen based on the application’s direction of applied force, and is easily identified by observing the orientation at the top of the spring.
Torsion springs are designed so that the load and winding direction are aligned. If the load and winding direction need to be opposite, both the load and angular deflection must be reduced. Understanding this relationship is crucial for ensuring the proper mechanical function and long service life of the spring, especially in critical applications like automotive latches, industrial doors, or electromechanical devices.
Understanding the direction of winding is crucial for the proper function of a torsion spring, as it determines the direction of deflection. The placement of the torsion spring in an application relies on the winding direction, which affects how the front and back legs will be positioned and move.
In a right-hand wound torsion spring, the back leg will torque clockwise while the front leg torques counterclockwise. For left-hand wound torsion springs, this is reversed: the back leg will travel counterclockwise, and the front leg will move clockwise.
The leg angle of a torsion spring is the angle between its legs when the spring is in an unloaded state, ranging from 0° to 360°. Typical leg angles for standard torsion springs available in stores are 90°, 180°, 270°, and 360°, but many spring manufacturers can supply custom leg angles to meet specific engineering or installation requirements for specialized machinery or industrial applications.
The leg angle affects the total coil count of a torsion spring. As previously noted, the total coil is slightly less than the total number of coils in the winding. The following equation describes the relationship between the leg angle and the total coil.
Leg Angle at Free Position = Number of Inactive Coils (fractional value) x 360°
The leg orientation of a torsion spring refers to how the legs are bent relative to the spring diameter and the axis of installation. Sharp bends in the legs can limit the spring's torque capacity, as stress concentrations are highest at bent areas. Common leg orientation types include axial, tangential, radial, and radial-tangential arrangements. Among these, the tangential leg configuration typically experiences the least stress, enhancing the lifespan of high-cycle torsion springs used in demanding environments.
Torsion spring legs can be twisted, bent, hooked, or looped to facilitate installation, maintain proper seating, and deliver ideal force application. Below are the most common leg styles for torsion springs found in industrial and commercial spring catalogs, though custom leg styles and special forming processes can be provided upon customer request for unique motion control challenges:
The performance of torsion springs is determined by the following engineering properties and parameters, which are essential for proper selection and application:
The spring index is the ratio of the mean diameter to the wire diameter of a torsion spring. This critical value provides insight into the spring’s coil tightness, strength, compression resistance, and overall manufacturability. A lower spring index (tighter coils or thicker wire) increases spring strength and force capacity, but raises manufacturing complexity and internal stress.
A spring with a thicker wire or smaller mean diameter offers greater load capacity and fatigue resistance, which is crucial for heavy-duty applications like overhead doors and machinery clutch assemblies. However, springs with indices below 4 or above 25 are generally unmanufacturable, with an ideal range for most torsion spring manufacturing between 6 and 12.
Angular deflection is the angular distance (in degrees or radians) that the leg of a torsion spring travels from its free (unloaded) position to its loaded (maximum torque) condition. This parameter determines the range of rotational motion the spring can safely accommodate, making it a key factor for applications like spring hinges, lever arms, and actuators.
The maximum allowed deflection (also called maximum angular travel) is the greatest angular deflection a torsion spring can safely achieve under load without risking coil buckling, permanent deformation, or loss of mechanical performance. Exceeding the specified maximum deflection may lead to material yielding, reduced cycle life, and spring failure. Torsion springs engineered for high rotation applications, such as garage door springs and heavy-duty counterbalances, typically feature a higher coil count and lower design stresses for extended durability.
The maximum load or maximum torque is the highest rotational force that can be safely applied to a torsion spring leg before yielding, buckling, or permanent deformation occurs. The operational capacity of a torsion spring is limited by either the maximum deflection angle or the maximum torque — whichever limit is reached first. Maximum load specifications should always be matched to the application's actual requirements to prevent premature spring failure or unsafe conditions.
The spring rate (or torque rate) is a measure of the rotational force (torque) applied per unit of angular displacement (degree or radian) to a torsion spring. Spring rate is crucial for motion control, ensuring that the spring delivers predictable, repeatable torque in precision assemblies. The following equation calculates the spring rate for round wire helical torsion springs:
Spring Rate per degree (lbs-in/degree) = PL/Θ = E × d^4 / 3888 × D × Na
Where P is the applied force, L is the moment arm, Θ is the angular displacement, d is the wire diameter, D is the mean diameter, Na is the number of active coils, and E is the modulus of elasticity of the spring material. The constant 3888 corrects for friction and geometry factors in real-world applications.
The table below provides the modulus of elasticity for popular torsion spring wire materials, which is essential when calculating the spring rate for high-performance torsion springs:
| Modulus of Elasticity of Spring Wires | |
|---|---|
| Spring Wire | Modulus of Elasticity (psi x 106) |
| Music Wire | 30 |
| Stainless Steel Grades 302, 304, and 316 | 28 |
| Stainless Steel Grade 17-7 PH | 29.5 |
| Chrome Vandadium | 30 |
| Chrome Silicon | 30 |
| Phosphor Bronze | 15 |
The spring constant is linked to torque and angular displacement, enabling engineers to determine the torque required for a specific angle, or the displacement resulting from a given force. Use the following relationships:
Angular displacement = Torque / Spring Rate
Torque = Spring Rate × Angular Displacement
The bending stress in helical torsion springs is a vital factor in ensuring reliable performance and long-term fatigue resistance. Stress can be calculated using:
Bending stress (psi) = 32 PLK / πd³
In this equation, K is the bending stress correction factor. As rotational force is applied, the inside and outside diameters of the spring increase due to higher bending stresses, especially on the inner coils. For round wire helical torsion springs, Wahl's correction formula for the inside diameter is:
KID = [4C2 – C – 1] / [4C (C-1)]
Here, C is the spring index. For quick approximations, use:
KID = [4C – 1] / [4C – 4]
KOD = [4C + 1] / [4C + 4]
Best design practice is to load torsion springs so that forces cause the coil diameter to decrease, as this takes advantage of beneficial residual forming stresses and improves spring longevity — a critical concern for mission-critical and safety applications.
Material selection and surface finishing (such as shot peening, coating, or corrosion-resistant alloys) further impact stress resistance, load capacity, and cycle life. Always consult a torsion spring manufacturer or specialist to optimize spring design for your specific use case.
When selecting or specifying a torsion spring for a product or mechanical system, consider factors such as required torque, available installation space, rotational travel, cycle life expectations, operating environment (temperature and corrosion), and safety considerations. Whether sourcing standard torsion springs or commissioning custom torsion springs, key criteria include material properties, spring geometry, wire finish, fatigue rating, and mounting requirements. Industry-sectors such as automotive, aerospace, medical devices, agricultural machinery, and industrial automation all have unique performance specifications and quality certifications for torsion spring suppliers.
For optimal results:
By understanding the mechanics and dimensions described above, engineers, designers, and procurement professionals can confidently evaluate and purchase torsion springs that ensure peak mechanical performance and long-term reliability for their products or manufacturing processes.
Torsion spring manufacturers provide a broad range of torsion springs designed to suit various applications. The diverse types of torsion springs make them highly valuable across multiple industries, as they can deflect and return countless times without requiring replacement.
TTorsion springs are utilized in a wide range of applications, from clipboard clips to the demanding environments of construction and automobile manufacturing. Their simple structure and versatility make them an invaluable tool across various industries.
Single torsion helical springs are the most common type of torsion springs. They are made from wire coiled into a helix, with the ends extended to form legs. These legs are where the load is applied to twist the spring around its axis.
Double torsion helical springs feature two coils—one right-hand and one left-hand—wound from a single length of wire and separated by central legs connected in an 180-degree bend to reduce friction. The coils operate in parallel, with the total torque of the spring being the sum of the individual torques exerted by each coil. These springs are commonly used to rotate, lift, neutralize, and center rotating loads.
Torsion bars are flexible and elastic straight bars designed to twist within their elastic limit. When torque is applied at their ends, they experience shear stress around their axes. Typically made from rubber or steel, torsion bars are commonly used in heavy-duty applications.
Torsion fibers are a type of torsion bar used in light-duty applications and sensitive devices. They often require tension to provide a return torque and are typically made from materials such as glass, silk, or quartz fibers.
Spiral wound torsion springs are crafted from wire coiled into a flat spiral, with the load applied to the free end while the central end remains fixed. The coils surround each other rather than stacking up, allowing these springs to achieve large angular displacements and multiple revolutions. This design enables spiral wound torsion springs to maintain relatively constant torque over a wide range of angular displacements, making them ideal for applications that require a steady energy output.
Torsion springs are made of steel due to its stiffness with hard drawn steel, stainless steel, music wire, and spring steels being the most common materials. When light duty springs are required, certain varieties of high strength plastics are used.. The main characteristic of torsion springs is their extremely close winding, which is necessary to create their torque.
Spring steel is a group of industrial-grade materials known for its high resilience, pliability, and strength. It can be compressed, bent, extended, and twisted to its elastic limit, and then return to its original shape without being deformed. These springs also have high fatigue strength and durability and are inexpensive. Spring steels contain high carbon concentrations. The types of spring steels are:
What stainless steel grades are commonly used in the production of torsion springs, and how do they compare in terms of mechanical properties and corrosion resistance?
Which alloy spring steels are commonly used in torsion springs, and how do elements like vanadium, manganese, silicon, chromium, nickel, and molybdenum enhance their suitability for high impact and shock applications?
Copper-based alloys offer excellent electrical properties, corrosion resistance, and performance in subzero temperatures. They are known for their high strength and ductility but tend to be more expensive than spring steel and stainless steel. Common copper-based alloys used in torsion springs include:
Nickel-based alloys provide excellent corrosion resistance and perform well in both elevated and subzero temperatures, making them suitable for harsh environments. However, they have high electrical resistance, which makes them unsuitable for electrical applications. Common nickel-based alloys used in torsion springs include:
Torsion springs can be made from round, square, and rectangular wires. Round wires are the most common and readily available. For square and rectangular wires, sharp corners are avoided to prevent stress concentration; the corners are rounded to mitigate this issue.
The process of manufacturing torsion springs from steel wires involves the following steps:
Spring Winding
The production of torsion springs starts with coiling a piece of wire to form the spring body. It is performed by a CNC spring coiler or a spring coiler through the help of a mandrel. Winding can be performed with the wire at room temperature (cold winding) or an extremely elevated temperature (hot winding).
Hot winding is preferred for thicker wires and bar stocks. In this method, the wire is heated at a very high temperature to increase its flexibility and then wound over the mandrel while it is red hot. Subsequently, the wire is removed from the spring coiler and plunged immediately to an oil bath to cool and harden it at a rapid rate. The spring produced at this stage is too brittle and needs to be tempered.
The ends of the torsion spring are bent after winding.
Heat Treating
The spring winding step has generated stress within the material. Heat treatment is necessary to relieve the material from stress, restore its resiliency, and completely harden. The coiled spring is heated at a predetermined temperature and duration and then slowly cooled.
Shot peening
Shot peening is a cold working process involving striking the spring with steel, ceramic, or glass shots to compress the layers beneath the surface. This process strengthens the torsion spring to resist fatigue, corrosion fatigue, cracking, galling, and erosion from cavitation. Shot peening should not be performed on small wire diameters since it can open up the spring and cause the free angle to grow.
Finishing
A thin protective layer is added to the spring to prevent corrosion, increase aesthetic value, and impart special properties (e.g., enhanced electrical conductivity). The common surface finishes for torsion springs include zinc, gold, chromium, nickel, black oxide, and rubber. Finishing can be accomplished by a plating, powder coating, dip coating, or passivation process.
Torsion springs are utilized in a variety of products, including:
A clothespin is one of the simplest applications of torsion springs. The torsion spring mechanism allows the prongs of the clothespin to open and securely grip the cloth when finger pressure is released. This same principle is also used in the clips of clipboards.
Spring-loaded hinges incorporate a torsion spring through the knuckles, with the spring’s legs attached to rectangular plates. This setup provides a self-closing mechanism for doors in residential, commercial, automotive, agricultural, and garage applications. When the applied force is released, the torsion spring ensures the door remains closed. Additionally, the spring can be configured to hold the door open if desired.
Clock springs, or main spring, are a type of spiral wound torsion spring. This spring is known to provide constant force output, and it can make large angular deflections of many revolutions while having a little variation in torque. Clock springs are available in square, rectangle, and D-shaped inside diameters.
Mechanical watches frequently use clock springs to function. When the knob is rotated, the clock spring stores energy, which is then released to drive the clock’s wheels as the spring unwinds. This mechanism is also employed in clocks, timers, metronomes, wind-up toys, and music boxes.
A clock spring is commonly located within the steering mechanism of vehicles, positioned between the steering wheel and the steering column. It ensures that all electrical connections—such as those for the airbag, horn, radio, and other steering-related systems—remain intact. The clock spring allows the steering wheel to be turned multiple times in various directions without damaging the electrical wiring. As the steering wheel rotates, the spiral winding of the clock spring coils and uncoils around a disc, preventing the wiring from becoming tangled or damaged. Without the clock spring, these wires could get twisted and potentially break during steering wheel movement.
Clock springs in vehicles are also referred to by several other names, including spiral cables, coil spring units, coil assemblies, cable reel assemblies, contact reels, and airbag clock springs (for vehicles with airbags). Regardless of the name used, the function of clock springs remains consistent across all vehicles: they ensure uninterrupted electrical connections while allowing the steering wheel to rotate freely.
Torsion bar suspensions use torsion bars in automobiles to support trailing arms when lateral or vertical forces act on the wheels. As these forces are applied, the torsion bar twists around its axis, reducing deflection in the trailing arms and helping to maintain vehicle stability.
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