Helically wound springs that deflect torque rotationally, torsion springs express mechanical energy through the property of elasticity: the spring action happens when it is twisted rather than compressed or pulled. Despite the name, torsion springs are subject to bending stress--not torsion--as the torque is carried through the length of the wound material. The spring coil's diameter reduces and the body length increases as the legs (work points) of the spring are deflected.

Operation

The legs of the torsion springs are attached to separate components, with the flexibility of the wound material providing opposite angular torque around the spring's axis. This force can hold parts in place or store and release the mechanical energy on cue. A higher potential energy results in a tighter, leaner spring.

Image credit: General Wire Spring Company

Production

Torsion springs are typically steel because of the stiffness of the material. Hard drawn steel, music wire, spring steel and stainless steel are most common, but plastic can be considered for light-duty needs. Torsion springs are typically close-wound, but sometimes space is provided between coils (i.e. 'pitch') in order to reduce friction in high-energy springs. Manufacturers will often provide design units for torsion springs in both imperial and metric standards.

Video credit: Master Spring & Wire Form Co. via YouTube

Configurations

Configurations include wind direction, leg arrangement and number of segments.

Wind Direction

Torsion springs are wound in either a clockwise or counter-clockwise direction. Loads should be applied in the direction of the wind.

Legs

Leg arrangement is the most critical variable amongst torsion springs.

Double Torsion Spring

Double torsion springs are two torsion springs, one wound clockwise and one wound counter-clockwise, each with an independent leg but intermediated by an unwound section. Total torque available is their sum.

Image credit: Advanex USA

Specifications

Important specifications for torsion springs include dimensions, performance, materials and finishes.

Dimensions

• Inner diameter is typically specified when the spring is required to slip over a mandrel with sufficient clearance to operate freely. It is generally recommend the spring is afforded 10% clearance.
• Outer diameter is typically specified when the spring is required to fit into a circular hole with sufficient exterior clearance to operate freely, or if there are outer housing clearance issues.
• Wire diameter is the diameter of the actual spring wire.
• Body length is the length of the spring coil.

Image credit: Century Spring

Performance

• Spring rate is the constant return torque provided per unit of angular displacement. Simply, it is the consistent attempt of the spring to return to its normal shape. Planet Spring offers an automated calculator to determine spring rate
• Maximum deflection is the total degree rotation a torsion spring can assume.
• Maximum load is the total torque that can be exerted on a spring leg.

Materials

Steel is the most common material used for torsion springs, though custom fabrication options exist.

• Hard drawn steel is inexpensive, but has a low working load.
• Music wire is a common and relatively inexpensive high-carbon steel alloy used for spring manufacture. It is cold drawn and offers uniform tensile strength.
• Spring steel is a standard industrial grade of steel specifically used for spring making. It exhibits good elastic and return properties.
• Stainless steel exhibits good corrosion resistance for specialty applications
• Specialty metals and alloys can be customized per application. This may include beryllium copper, beryllium nickel, niobium, tantalum, and titanium.
• Plastics are rigid and corrosive resistant, but are best in light-duty usage.

Finishes

Coating methods can offer additional properties to untreated springs

• Black oxide coating is an inexpensive way to prevent corrosion with a durable finish.
• Gold iridite is a chemical coating offering durability and conductivity.
• Passivation adds a thin layer of oxide or nitride onto the metal, enhancing corrosion resistance.
• Zinc offers a galvanized layer of protection to the coils.
• Other finishes may be offered by individual manufacturers.

Applications

Torsion springs are familiar in use and are commonly employed in clothespins, clipboards, mousetraps, vehicle doors, and garage doors. Ancient Greeks and Romans constructed siege catapults using similar torsional designs, using heavy rope as the coil.

Resources

Lee Spring - Learn About Torsion Springs

Springipedia - Torsion Springs

Engineers Edge - Spring Torsion Calculator

Optimum Spring - Torsion Spring

Image credit:

Gardner Spring, Inc.