This article takes an in-depth look at torsion springs.
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A torsion spring is a mechanical component designed to store and release rotational energy.
The ends of a torsion spring are attached to a mechanical component. When the spring is rotated around its axis at one end, the winding tightens, storing potential energy.
During the winding process, one end of the torsion spring is deflected around the central axis, while the other end remains fixed. As the winding tightens and resists increasing rotational force, the spring accumulates more potential energy.
When a torsion spring is released, it unwinds and rebounds elastically, releasing the stored energy. This causes an equal rotational force to be exerted on the opposite end of the spring, applying torque to the attached mechanical component. Torsion springs are used to statically hold mechanical components in place.
The operation of torsion springs relies on their resistance to rotation or twisting. The mechanical energy generated by this resistance is stored and creates a torque that opposes the twisting force, which is proportional to the angle of twist. Common types of torsion springs include helical, torsion bars, and spiral wound springs. These springs can be made from materials such as wire, sprung steel, or rubber.
Torsion springs experience greater bending stress than rotational stress as they are twisted to achieve a tighter winding. Unlike other types of springs, torsion springs solely involve rotational force, without any linear force. This distinguishes them from compression and tension springs, which operate based on linear forces.
TThe mechanical performance of torsion springs relies on the elasticity of their material, which allows them to return to their original winding after being twisted. Torsion springs can be rotated to apply force in either a clockwise or counterclockwise direction, and they must be twisted in the direction of their winding to achieve maximum force.
Torsion springs are used in a diverse array of applications across nearly every industry. They are available in various configurations to suit different needs.
Torsion spring configurations are designed to store and release energy or to hold a mechanism in place by deflecting around the axis of the body’s centerline. When deflected in the correct direction, they reduce the body's diameter and increase its length.
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 dimension determines the outer diameter of a shaft or mandrel that can fit smoothly into the spring. For optimal operation, it is recommended that the inner diameter includes a 10% clearance to allow the inserted component to move freely.
The outer diameter of a torsion spring is the width outside the coil’s helix, measured perpendicular to the centerline axis. This dimension defines the diameter of the hole through which the spring is inserted, accounting for all necessary clearances to ensure the spring operates freely.
The wire diameter refers to the thickness of the wire used to coil and construct the torsion spring.
The mean diameter is calculated by subtracting the wire diameter from the outer diameter, and it is used in stress and spring rate calculations.
The body length of a torsion spring is the length measured when the spring is in an unloaded state, determined by measuring the outer surfaces of the end coils. As torque is applied, the body length increases while the spring diameter decreases.
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. It affects the load or torque required to store energy in the spring. Shorter legs require more torque to bend the coils. Additionally, the legs of a torsion spring can have different lengths.
TThe total coil of a torsion spring refers to the number of active coils in the winding. Active coils are those that twist or deflect under load and release energy when the spring is released. The total coil count is slightly less than the total number of coils due to the inactive coils accounted for by the legs. For torsion springs with a 0° leg angle in the free position, the total coil value is a whole number.
The pitch of a torsion spring is the centerline distance between two adjacent active coils. In closely-wound springs, the pitch is roughly equal to the wire diameter. However, closely-wound springs can generate significant friction during deflection. It is generally recommended to specify the total coil count and body length of the torsion spring instead of the pitch.
Torsion springs are wound in a specific 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 can be easily identified by observing the top of the torsion 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 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 unloaded, ranging from 0° to 360°. Common leg angles for standard torsion springs available in stores are 90°, 180°, 270°, and 360°. Additionally, manufacturers can customize the leg angle to meet specific client requirements.
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 the way the legs are bent relative to the spring diameter. Sharp bends in the legs can limit the spring's capacity, as stress tends to concentrate at the bent areas. Common types of leg orientations include axial, tangential, radial, and radial-tangential. Among these, the tangential leg configuration experiences the least stress.
The legs of a torsion spring can be twisted, bent, hooked, or looped to facilitate installation and operation. Below are the common leg styles for torsion springs, though custom leg styles can be provided upon customer request.
The performance of torsion springs is determined by the following properties and parameters:
The spring index is the ratio of the mean diameter to the wire diameter of a torsion spring. It provides insights into the spring’s coil tightness, strength, and manufacturability. Reducing the spring index increases the spring’s strength by either increasing the wire diameter or decreasing the outer diameter of the spring. A spring with a thicker wire offers greater strength compared to one with a thinner wire. Lowering the spring index tightens the coils and increases force, but also increases compressive stress on the coils. Springs with low indexes are more challenging to manufacture due to increased tooling wear and additional processing required to extend service life. Springs with indexes below 4 or above 25 are generally unmanufacturable, with an ideal range typically between 6 and 12.
Angular deflection is the angular distance that a leg of the torsion spring travels from its free position to its loaded condition.
The maximum allowed deflection is the greatest angular deflection a torsion spring can achieve under load without risking buckling or overstressing. If the spring surpasses this deflection, the coils may not return to their original position after the load is removed, due to material yielding.
The maximum angular deflection is the extent to which a torsion spring can be twisted while loaded before it buckles from overstressing. Typically, torsion springs with a larger diameter and more coils have a higher capacity for deflection. For instance, a garage door spring can endure multiple rotations without yielding, thanks to its high coil count and low design stresses.
The maximum load is the highest torque that can be applied to the leg of a torsion spring before it buckles. The capacity of a torsion spring is limited by either the maximum deflection or the maximum load, whichever is reached first.
The spring rate is the measure of rotational force applied to a torsion spring per unit of angular displacement. The equation below calculates the spring rate for round wire helical torsion springs:
Spring Rate per degree (lbs-in/degree) =. PL/Θ = E x d^4 / 3888 x D x Na
In this equation, P represents the load, L is the moment arm, Θ denotes 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 material. The constant 3888 is a theoretical factor that adjusts for friction between adjacent coils and between the spring body and the attached component.
The following table provides the modulus of elasticity for different types of torsion spring wires, which is essential for calculating the spring rate:
| 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, as described by the following equations. This relationship helps determine the amount of torque needed for a specific angular displacement or the angular displacement required to produce a certain amount of force.
Angular displacement = Torque/Spring Rate
Torque = Spring Rate x Angular Displacement
The bending stress in helical torsion springs can be calculated using the following equation:
Bending stress (psi) = 32 PLK/πd³
Here, K represents the bending stress correction factor. As torque is applied to a torsion spring, both the inside and outside diameters increase due to higher bending stress on the inner surface compared to the outer surface of the coils. For round wire helical torsion springs, the bending stress correction factor for the inside diameter is calculated using the following equation formulated by Wahl:
KID = [4C² – C – 1] / [4C (C-1)]
Here, \( C \) represents the spring index. The bending stress at the inner and outer diameters can be approximated using the following equations:
KID = [4C – 1] / [4C – 4]
KOD = [4C + 1] / [4C + 4]
Torsion springs should be loaded in a direction that causes the spring diameter to decrease, as residual forming stresses are beneficial when applied in this direction.
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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