Torsion Springs

What are Torsion Springs?

A torsion spring is a helical spring that works by twisting around its coil axis rather than compressing or extending along it. When a force is applied to the legs, the spring body rotates, storing mechanical energy as the wire is stressed in bending. When the force is released, the spring unwinds back to its free position. This rotational mechanism differs fundamentally from compression and extension springs, where the wire is stressed in torsion along its length—in a torsion spring, the wire bends, and the axis itself rotates.

Torsion springs appear wherever angular force or return torque is required. Clothespins, mousetraps, garage door counterbalance systems, clipboard clips, vehicle suspension torsion bars, door hinges with self-return, and electrical switch mechanisms all run on torsion springs. The spring converts linear force applied to the leg into angular displacement of the coil body, and the stored energy returns the mechanism to its starting position when the force is removed.

How Torsion Springs Work

A torsion spring stores energy in bending, not twisting. As the legs are deflected angularly, each cross-section of the wire experiences bending stress proportional to the applied moment. The spring rotates around its central axis, and the coils either wind tighter (closing) or unwind (opening), depending on the deflection direction relative to the wind direction. A right-hand-wound spring tightened in the clockwise direction sees the coils close and the body diameter shrink; deflected counter-clockwise, the coils open and the body diameter expands.

The torque delivered rises linearly with angular deflection: M = kθ, where k is the spring constant in N·mm/° and θ is the angular deflection from the free position. The spring constant depends on the wire diameter, coil diameter, number of coils, and the material’s modulus of elasticity in bending. Unlike compression springs where coil count divides the rate, more coils in a torsion spring mean a softer spring—each coil adds more wire length to bend, reducing the stiffness per degree of deflection.

Three critical angles define a torsion spring’s working envelope: the free angle (legs at rest), the maximum safe deflection angle (at which the wire reaches its design stress), and the solid angle (coils fully closed). The usable working range is the difference between the free angle and the maximum safe deflection. For a single torsion spring, this range is typically 90° to 270°. Double torsion springs double this range by using two coil bodies in series.

Torsion Spring Classification System

Torsion springs are classified first by leg configuration—how the force enters and leaves the spring—then by coil body style, and finally by the performance regime. Leg geometry is the most important specification layer because the leg is where external force is applied and where stress concentrates at the transition from the coil body.

Leg types determine how the spring mounts and interfaces with the actuating mechanism. A straight leg is simply the wire end left straight, tangent to the coil body—the simplest form, used where the leg bears against a flat surface or slot. A bent leg adds a defined angle—typically 90°—to the wire end, creating a positive engagement point for a pin, hole, or tab. A hooked leg forms a small U-bend at the end, capturing a mating pin or wire and preventing the leg from slipping off under deflection. A loop end closes the wire into a full circle, distributing load across the loop and connecting to a shaft or pin that passes through.

Body configurations control the torque output and angular range. A single torsion spring has one continuous coil body between two legs. A double torsion spring places two coil bodies between three legs—the center leg is common, and the outer legs move in opposite directions. This doubles the torque for the same angular deflection while keeping the spring compact. A cylindrical torsion spring has constant coil diameter throughout. A conical torsion spring tapers in diameter—when deflected, the larger-diameter coils absorb more bending, creating a progressive torque characteristic that rises non-linearly with deflection.

Performance types push the operating bounds. Heavy-duty torsion springs use oversized wire diameters and alloy steels for applications like industrial door counterbalances, agricultural equipment, and railway carriage linkages. Precision torsion springs are wound to tighter angular tolerances—typically ±2° free angle—for instruments, aerospace actuators, and medical devices. Micro torsion springs with wire diameters from 0.08 mm serve watch mechanisms, micro-relays, and surgical instruments. Constant-torque torsion springs use a prestressed coil that delivers near-constant torque over a wide angular range, eliminating the linear rise of standard torsion springs.

Types of Torsion Springs

The twelve types below span the four classification groups. The leg type is the primary specification layer—every torsion spring must have defined leg geometry—while the body configuration and performance category refine the torque, angular range, and operating life.

Materials

Music Wire (ASTM A228)

• High-carbon steel, cold-drawn to high tensile strength; standard for general torsion springs

• Maximum operating temperature 120 °C; good fatigue resistance for deflection within the design stress limit

• The leg-to-coil transition is the fatigue weak point; stress-relieve after forming to reduce residual bending stress

Stainless Steel 302/304 (ASTM A313)

• Corrosion-resistant, non-magnetic after cold working; operating range −200 °C to 290 °C

• Lower modulus of elasticity than music wire—spring constant reduced by approximately 10% at equal dimensions

• Required for food processing, marine, medical, and outdoor torsion spring applications

Stainless Steel 316

• Molybdenum addition for pitting resistance in chloride environments; coastal and chemical processing

• Slightly lower strength than 302; match grade to the rest of the assembly to avoid galvanic corrosion

Chrome Silicon (ASTM A401, AISI 9254)

• High fatigue life under cyclic bending; quenched and tempered to HRC 47–51

• Maximum operating temperature 250 °C; specified for high-cycle torsion springs in automotive and industrial machinery

Inconel X-750 / Inconel 718

• Nickel-chromium superalloys for continuous bending stress at temperatures to 450 °C

• Maintain torque output where carbon and stainless steels would relax and lose spring force

• Specified for turbine actuators, exhaust valve return mechanisms, and downhole petroleum tools

How to Choose the Right Torsion Spring

Step 1: Torque and deflection—what moment at what angle?

Define the torque required at the installed (preloaded) angle and at the maximum deflection angle. The difference divided by the angular travel gives the spring constant k in N·mm/°. Ensure the maximum deflection stays within the spring’s safe working range—typically 60–75% of the theoretical solid deflection—to avoid over-stressing the wire in bending.

Step 2: Leg configuration and mounting

The legs deliver force from the actuator to the coil body and back. Straight legs work for flat-surface or slot mounting. For pinned or hole-mounted applications, bent legs or hook ends give positive engagement that prevents the leg from walking off the seat under repeated cycling. A loop end distributes load around a shaft and is the strongest leg configuration for high-torque applications.

Step 3: Space and angular range

A single torsion spring covers 90°–270° of working deflection. If the mechanism requires more than 270°, a double torsion spring places two coil bodies in series, each absorbing half the total deflection. If the spring must deliver near-constant torque rather than a linear ramp, a constant-torque spring wound with prestress eliminates the rising-torque characteristic of standard torsion springs.

Frequently Asked Questions

What is the difference between a torsion spring and a tension spring? A torsion spring twists around its axis and exerts rotational torque; a tension spring stretches along its axis and exerts linear pulling force. Torsion springs stress the wire in bending; tension springs stress it in torsion. The two spring types serve fundamentally different loading modes.

Why does a torsion spring need to be wound in the correct direction? The spring body diameter changes under deflection: winding tighter (closing coils) shrinks the diameter; unwinding expands it. If installed on a shaft, the spring must deflect in the closing direction to avoid binding against the shaft surface. Install direction is specified as right-hand or left-hand wound.

How is the torque of a torsion spring calculated? Torque M = Ed⁴θ/(3,670ND), where E is the modulus of elasticity, d is wire diameter, θ is angular deflection in degrees, N is active coils, and D is mean coil diameter. Doubling the wire diameter increases torque by a factor of 16.

Can a torsion spring be used as a compression spring? A torsion spring’s legs deflect angularly around the coil axis, not axially along it. If compressed axially, the coils would close with negligible travel and the spring would not function. Torsion springs are designed exclusively for angular loading.

What is a double torsion spring and when is it used? A double torsion spring has two coil bodies wound in opposite directions sharing a central leg. It doubles the torque output for a given deflection while fitting within roughly the same axial space as a single torsion spring. Standard in garage door counterbalance systems and dual-action switches.

How do I measure the free angle of a torsion spring? The free angle is the angle between the legs when the spring is unloaded. It is measured from leg centerline to leg centerline around the coil axis. Manufacturing tolerance is typically ±5° for standard springs and ±2° for precision-grade springs.