Constant force springs are a variety of extension springs. A strip of steel with a preset curvature is coiled tightly so that each turn of the strip rests on its inner neighbor. The spring is actuated in a pulling, linear motion with the deflection resistance originating from the material's stiffness and spring construction. Unlike other extension springs, a consistent degree of force is exerted despite the degree of deflection. Constant force extension springs should not be confused with power springs. 

Operation

The spring is mounted with the inner layers tightly coiled around itself and the spring terminal attached to the load force. The inner diameter may be attached to a rotating drum. The resistance is provided by the spring's attempt to return to its natural, wound condition. Material width and thickness, the natural radius curvature, and the diameter of the drum all contribute to extension resistance. Full load rate is achieved after the spring is extended to 1.25 times its diameter, so load capacity is reached quite easily.

Video credit: Vulcan Spring & Mfg. Co. via YouTube

Composition

The physical spring is most commonly made of 301 stainless steel, high-carbon steel, or Inconel®. Type 301 is an austenitic chromium-nickel stainless steel capable of attaining high tensile strength and ductility by cold working. It is not hardened by heat treatment, and while nonmagnetic in the annealed state, it gains magnetism with cold deformation. High-carbon steel owes its considerable hardness from the carbon (more than .3%) dissolved in the iron. Inconel is a lightweight nickel-chromium alloy, with extreme durability, performance, and heat- resistance. Inconel is a registered trademark of Special Metals Corp.

A drum attached to the inner diameter of the spring is frequently plastic, providing structural integrity, coil attachment, and less friction and inertia than a metal drum.

Configurations

Some constant force springs may implement a drum or spool, which can move as part of the spring deflection, while other constant force springs do not use a drum at all.

Coil Deflection

Mounting

Constant force springs offer unique types of mounting styles due to their gear/pulley-like rotation and interaction. Common mounting methods are detailed below.

Spool mounting is the most common mounting style. The spring's sliding friction is minimized by the use of the spool around an axis. The spool also provides a consistent shape with no deflection that will help maximize spring life, and also can include a brake to prevent over-extension.
Tandem mounting has two adjacent springs with attached coil strips. The springs spin in the same direction, at the same time, and offer a summarized return force.
Laminar mounting incorporates two interwound constant force springs, connected by welding across the strap width. This is used when optimal force is needed with minimal footprint.
Back-to-back mounting has two adjacent, unattached springs that spin conversely at the same rate. The sum of the torques and opposing rotations offer stable extension at long deflections.
Cavity mounting is an inexpensive mounting method, but does not feature a stop or brake. This high-friction design may limit spring lifespan.
Pulley mounting incorporates a pulley into the deflection to provide a mechanical advantage of up to 50%. However, notable disadvantages include a lower spring lifespan, and the spring cannot be fully retracted beyond the pulley in some instances. If the spring is subject to backbend in a block and tackle configuration, the spring will deform significantly.

End Details

Ends can be custom fabricated to an applications requirement, but common, predetermined ends like the ones below are readily available. End configurations vary more in constant force springs than in other types of springs because of the enlarged, flattened coil.

All sizes are metric. Image credit: Vulcan Spring

Specifications

Spring Dimensions

Width is the measurement across the flattest side of the coil.

Outside diameter is the farthest measurement across the overlapped, thin side of the coils. This varies greatly as the spring extends and retracts, so measurement is taken when the spring is at its lowest degree of deflection.

Inner diameter is usually only relevant for spool mounting, and could be found by measuring the outside diameter of the wind cavity and the remaining unraveled coils when the spring is at maximum deflection.

Length and maximum deflection are synonymous, and is the length of the coil at its fullest extension; 1 ½ coil wrap should remain unwound for safety.

Image credit: Shun Ming Industrial Ltd.

Performance

Spring life is rated by the number of cycles a spring can complete before warping, breaking, or the constant return force becomes inconsistent. Spring life varies depending on application and the performance characteristics listed below. Springs can be rated between 2,500 to over one million cycles.

Fatigue: A moderate or low number of required cycles will permit the use of a heavily pre-stressed spring with high return force available from a small footprint. Exposure to high temperatures, corrosive materials, and other hostile environments will increase fatigue.

Backbend: Unless otherwise designed to do so, backbending or reverse winding will permanently disfigure a spring.

Tensile load: The spring should not be withheld by its material construction, especially to alter a load; if a stop or brake is required, it should be applied at the spool or drum mechanism.

Acceleration: If the application requires a specific extension/retraction speed, spring and mounting testing may be necessary.

Torque: The return force of the device must be solely responsibility of the spring. No additional torque or friction should be applied to the drum or coiled body.

Stability: At long deflections the coil's natural curve may buckle, especially upon return. This can be remedied by the use of guides. Width and thickness also contribute to stability. The width of the coil should range between 50 and 250 times its thickness. Quality drum design and bearings will also add stability.

Initial load: At a deflection 1.25 times its diameter, the coil reaches its consistent, maximum return force. Initial load refers to the required weight to meet this constant force. The graph below illustrates this force exertion, though it is identified as initial extension.

Image credit: Springipedia

Safety

As the constant force spring deflects, potential energy is stored in the extended, unwound section. If the spring had reached its fatigue point, or if the load is suddenly removed, the spring will retract with uncontrolled speed and direction. This poses extreme danger--to operators and nearby equipment--especially from the coil's edges. Safety features can be implemented to reduce possible damage. If the spring is overextended a similar reaction can occur where the spring loses its mount and recoils in reverse, possibly damaging the load.

Applications

One of the most common but unrecognized use of constant force springs would be in retractable projection screens. They are also used in retail beverage racks to circulate products towards the cooler door. Constant force springs frequently appear in doors, cable and hose retractors, toys, hair dryers, and gym equipment. These springs serve many industries, like space and aviation, military, tool manufacturing, and automotive/ transportation.

Resources

Vulcan Spring Manufacturing Co. - Conforce® Constant Force Springs

Spring-i-pedia - Constant Force Springs

Power Spring - Constant Force Spring

Walker Corporation - Constant Force Extension Springs

Amtek - Hunter Spring Products - Neg'ator® Constant Spring Force

Images credits:

Gardner Spring, Inc. | Vulcan Spring