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Design Considerations for Constant Force Springs

illustration: Constant Force Springs

Since the introduction of constant force springs to industry, design engineers have been using these components to solve problems requiring a constant force or torque. Every constant force spring is produced to provide specific force which is exerted through the entire extension of the spring. The force is constant as long as the radius, due to diameter buildup causes the spring to load slightly as it is extended.


Several materials are used to make constant force springs. These include stainless steel, high carbon steel, beryllium copper, plastic and others as they are required. Type 301 Stainless Steel has proven to be superior for consistent quality, availability, stress retention and lowest product cost.

Estimate of Life

This is important, since the life of all constant force springs is very predictable. A life cycle is an extension and retraction of either the whole spring or any portion of it. A low estimate of the life will lead to early failure. A high estimate (the most common error) makes the spring larger and more expensive than required.

Establish the Load

The load of the spring should be equal to the force required in the application. The normal load tolerance for a constant force spring is +/- 10%

Determine the Space

For any load value there are several material thickness and width combinations that can be used. The natural coil diameter is dependant on the thickness, life, and load required on the spring. The end of a constants force spring does not extend tangent to the coiled body of the spring. To allow the spring to operate properly, a minimum distance of 0.8 x I.D. is required.

Calculate the Total Length

Allow for at least 1 1/2 turns to remain on the storage spool when the spring is fully extended.

Consider the Environment

Corrosive atmosphere or extreme temperatures can effect the spring life and material selection.