CONSTANT FORCE SPRINGS

A constant force spring is a roll of prestressed strip which exerts a nearly constant restraining force to resist uncoiling. The force is constant because the change in the radius of curvature is constant. This is true if the change in coil diameter due to buildup is disregarded.

Long extension capabilities, constant torque and virtual absence of intercoil friction have led many designers to specify constant force springs in such applications as brush springs for motors, counterbalance springs for window sashes and carriage return springs for typewriters. Constant force motor springs are used to drive mechanisms for timers, movie cameras and cable retractors.

REEL RETRIEVERS

Adaptable, strong and reliable, the Reel Retriever is a versatile, efficient device which offers counter-balance on any plane. A compact plastic box contains a spring-loaded retracting cable assembly which can be securely fastened to any surface.

The Reel Retriever is ideal for drawer returns, point-of-purchase displays of products that are intended to be handled by customers. They can also be utilized for any counter-balance application under 2 pounds.

Utilizing a nylon-coated, braided, 7x7 stainless steel aircraft cable, Reel Retriever is available in many pull forces with cable lengths ranging from 36" to 60".

A variety of interchangeable round or rectangular plastic ends (with or without adhesive) are also available.

In response to customer requests to expand its Stock Compression Spring offering to include Constant Force Springs, Lee Spring has added four series to their 2008 Print and Online Catalog. Lee now offers Constant Force Springs in four Life Cycle ranges: 2500, 4000, 13000 and 25000. Each spring is a roll of high yield Type 301 Stainless Steel strip exerting a nearly constant restraining force to resist uncoiling. When the strip is extended, the inherent stress resists the loading force at a nearly constant rate. Constant Force Springs are usually mounted by first tightly wrapping the spring on a drum, then attaching the free end to the loading force such as in a counterbalance application. This relationship can also be reversed.

For further information on Lee's Constant Force Springs or to order a catalog, visit their website at www.leespring.com, email marketing@leespring.com or call 888 SPRINGS.

Conforce springs offer high force output with very small space requirements, provide long linear reach with minimal force buildup, and store power indefinitely when extended. They are ideal for a wide variety of applications where constant force is needed, including applications requiring smooth returning and retrieving, counterbalancing applications, and tensioning and loading applications.

Because there are a number of different spring design possibilities for a given application it is suggested that you consult with a Vulcan engineer early in the design phase.

After you work through the product specifier below, submit your information and the specifier will create a specification sheet for submission to Vulcan.

The following parameters affect the constant force spring design:
Material - Typically Type 301 stainless steel, but high-carbon steel and other materials are available.

Life - An estimate which is too low leads to early failure, an estimate which is too high makes the spring larger and more expensive than necessary.

Force - should equal the requirement of the application, normal tolerance is +/- 10%.

Available Space - for proper operation, constant force springs require .8 x I.D. (see drawing).

Length - allow at least 1 and 1/2 turns to remain when spring is fully extended.

Environment - corrosive atmospheres and/or extreme temperatures will affect spring life and material selection.

End Detail - select END DETAIL to view a selection of VULCAN's many standard end detail designs. Custom designs are available.

Mounting Methods - select MOUNTING METHODS to view a selection of single and multiple mounting recommendations.

Stock sizes in stainless steel

Constant-force springs are a special variety of extension spring. They consist of a spiral of strip material with built-in curvature so that each turn of the strip wraps tightly on its inner neighbor. When the strip is extended (deflected) the inherent stress resists the loading force, just as in a common extension spring, but at a nearly constant (zero) rate. The accompanying load/deflection curves illustrate this.

The constant-force spring is will suited to long extensions with no load build-up. In use, the spring is usually mounted with the ID tightly wrapped on a drum and the free end attached to the loading force, such as in a counterbalance application. This relationship can be reversed, however, with the free end mounted stationary and the spring itself providing the working force, as with carbon brushes in electrical apparatus.

Considerable flexibility is possible with constant-force springs because the load capacity can be multiplied by using two or more strips in tandem, or back-to-back, as illustrated.

Material

• Type 301 stainless steel.

Note: Be sure to allow at least 1 1/2 coils of material on the drum at full extension. The spring ID will wrap tightly on the drum so that in most applications no fastening method on the drum is required.

MW Industries custom manufactures constant force springs in strip widths of .187" to 2",in thicknesses of .003" to .031", and lengths of 12" to 60" in all standard end treatments. They are ideal for motor brush springs, counterbalances, timing mechanisms, cable retractors and retail product displays. A constant force spring is a roll of prestressed strip, which exerts a constant restraining force as it resists uncoiling. Its long extension capability, constant torque and near absence of intercoil friction make it ideal for use in brush springs for motors, counterbalancing applications (such as window sashes), return springs, and cable retractors. MW specializes in constant force springs for use in retail product displays. We offer design assistance, protoype sampling, quick turnaround quoting and delivery, and can supply your needs for any quantity. Please contact us for product samples.

Lee Spring offers Custom Constant Force Springs in a variety of sizes and materials. This is in addition to the 40 Stock Stainless Steel Constant Force designs introducted in the 2008 Lee Spring Catalog.