Spiral Torsion Springs

Spiral torsion springs, also known as clock springs or hair springs, are made from flat strips of metal wound into concentric spirals designed to store and release energy through rotational force (torque) over an angle, typically less than 360 degrees. They are ideal for applications where space is limited axially (along the center axis) but more space is available radially (around the center).
Design and Function
The function of a spiral torsion spring is based on the application of torque, rather than the compression or extension forces of other spring types.
Torque Generation: When one end of the spring is fixed (usually the outer coil to a housing) and the other is rotated (inner coil to an arbor or shaft), the spring resists the twisting force and stores energy.
Energy Release: When the applied force is removed, the spring unwinds to its original position, releasing the stored energy as a rotational force to drive a mechanism.
Linear Torque: For rotations up to approximately 360 degrees, the spring typically provides a linear torque curve, allowing for precise control of force.
Coil Contact: The coils are designed with spacing so they generally do not touch during operation to minimize friction. If the rotation exceeds its design limit, the coils begin to "close-out", which increases friction and raises the torque rapidly.
Materials
The material choice impacts the spring's strength, durability, and resistance to environmental factors.
High Carbon Steel (e.g., Music Wire): A common, economical option with high tensile strength for high-stress applications.
Stainless Steel (e.g., Type 302, 316, 17-7PH): Offers excellent corrosion resistance and performs well across a wide range of temperatures, ideal for medical or marine applications.
Specialty Alloys: Materials like Inconel are used in extreme conditions for enhanced heat or chemical resistance.
Common Applications
Spiral torsion springs are widely used where compact size and precise rotational force are necessary.
Timekeeping Devices: Utilized in the mechanisms of mechanical clocks and watches to drive the movement of the hands.
Motor Brush Assemblies: Maintain consistent pressure on carbon brushes in electric motors, ensuring optimal electrical contact.
Counterbalance Systems: Used in heavy mechanisms like armored hatch covers, jet stairs, and cargo doors to assist with opening and closing, reducing operator effort.
Retractable Mechanisms: Found in car seatbelts, retractable power cords for home appliances, and toys that wind and rewind.
Industrial Controls: Employed in switches, relays, valves, and actuators that require a controlled, short-angle rotational movement.