A torsion spring is a spiral spring, also known as a coil spring. They are designed to apply torque radially. They are opposite to compression springs used to keep the mechanism apart. The torsion spring fixes two mechanisms together, and its tightness is proportional to the energy stored inside. Tension must be eliminated in order for the spring to release stored energy.

Use torsion springs when rotational torque is required. There are two designs for torsion springs - single torsion springs and double torsion springs, with single torsion springs being the most common type. When assembling the torsion spring onto the shaft, it must be noted that when the spring rotates in the normal direction, the inner diameter will decrease, which can cause the spring to get stuck with the shaft and generate unnecessary stress on the spring; The inner diameter of the spring and the size of the shaft it acts on must be considered. Usually, when a tight bending radius is required for twisting spring legs, more ductile spring materials are used, such as piano steel wire ASTM A228, oil tempered steel wire ASTM A229, and 302 stainless steel ASTM A313. The leg configuration and large bending radius in any curved area help prevent the spring material used from breaking.

Twisted springs have multiple advantages, making them a popular choice in various applications.
Durability: Torque springs are designed to withstand heavy loads and high usage rates, and typically have a longer lifespan than other types of springs. Its sturdy structure allows it to withstand more cycles, thereby reducing the frequency of replacement and maintenance.
Design: The design of the torsion spring enables it to evenly distribute weight, making it an ideal choice for applications that require stable and controlled motion. This balanced design also helps to improve its long-term reliability and efficiency.
Smooth operation: The torsion spring provides smooth and controllable motion, which is particularly useful in applications that require gradually uniform force application. This smooth operation can reduce the pressure on connecting components, thereby extending the service life of the entire system.

Types of torsion springs
Single torsion spring: a single coil spring suitable for applications that require medium to high rotational force.
Double torsion spring: The double coil spring is wound in opposite directions, which can withstand higher loads and provide higher stability. They are an ideal choice for heavy-duty applications.
Bend Types
Radial bending: The coil bends along a radius, which is very suitable for applications that require vertical force, such as automotive components.
Axial bending: The coil bends along the axis, suitable for applications where the force is parallel to the spring axis, such as electronic devices.
Spiral bending: Continuous spiral bending provides smooth and consistent force, making it ideal for precision instruments and specialized machinery.
Tangential bending: The coil bends tangentially along the central axis, providing unique force characteristics.

Design a torsion spring
Designing a torsion spring is a systematic process that requires careful consideration of application requirements, material properties, and mechanical principles. The following step-by-step guide provides a structured method for creating effective torsion spring designs.
Step by Step Guide
1. Define application requirements
Required torque (M): Determine the torque required to perform the expected function.
Angular deflection (θ): Calculate the angle at which the spring must twist.
Environmental conditions: Evaluate factors such as temperature, corrosion, and exposure to chemicals.
2. Determine space limitations
Inner diameter (ID): must be suitable for any shaft or rod on which the spring is fitted. The inner diameter of the torsion spring should always be at least 15% larger than the shaft or rod on which the torsion spring is fitted. Why? Because when the torsion spring leg moves, the inner diameter will shrink, and you don't want the torsion spring to get stuck on the shaft. If the torsion spring gets stuck on the rod or shaft, the torsion spring will lose all torque and cannot function.
Outer diameter (OD): It should not interfere with surrounding components or shells.
Body length: Ensure that the length of the spring is suitable for the available space.
Leg length and orientation: Consider how the legs are connected to the application.
3. Choose materials
Performance requirements: Select materials that meet strength, flexibility, and environmental requirements.
Cost considerations: Balance performance with material and manufacturing costs.
4. Calculate key dimensions
Mean diameter (MD): MD=OD − d
Wire diameter (d): Estimate the value based on torque and space constraints.
Spring index: Spring index=MD ÷ d
The target value is between 5 and 12.
5. Determine the number of effective coils (N)
Angular deflection calculation:
Estimate N using the required angular deflection and material properties.
Balance:
Ensure that the number of coils allows for the required deflection and does not exceed the stress limit.
6. Design leg configuration
Function: The legs must effectively transmit torque to the application.
Simplicity: Keep the leg design simple to reduce manufacturing complexity.
Angle and Bend: Specify precise angles and lengths to suit the application.
7. Calculate the spring stiffness (k)
Using the spring rate formula: Rt=Ed ^ 4/10.8 DN S=10.2 M/d ^ 3
Adjustment:
Modify d, D, or N to obtain the desired k.
8. Prototype and testing
Build sample: Create a prototype based on the calculated dimensions.
Testing:
Install actual applications or test settings.
Measure torque, deflection, and observe performance.
Iteration:
Adjust design parameters based on test results.