8 Steps to a Constant Force Spring Design

Posted by Catherine Tyger on Tue, Jun 11, 2013 @ 09:19 AM

In Conforce®, Vulcan Spring, Constant Force Spring, environmental factors

design-guide-new-resized-600.jpgVulcan Spring has put together 8 steps to help you make some initial design calculations on your constant force spring. In addition to this article, our design guide is available for download online and will provide you with charts and more detailed information.

  1. Select Material – Most of the flat steel springs that Vulcan Spring produces are made from Type 301 Stainless Steel. This material is the most readily available and predictable. A Vulcan Engineer can assist in choosing a standard size to expedite the process. Other materials we have used to manufacture constant force springs include high carbon steel, Inconel and Elgiloy in various designs as required.

  2. Estimate the life cycle – The life of a constant force spring is very predictable, making it one of the most important considerations. Our previous blog, Constant Force Spring Life Cycles, has an in-depth explanation on the subject. It is important to be realistic since estimating a high life cycle can lead to a larger, more expensive design. Underestimating the life cycle of a constant force spring can lead to a premature failure. The Design Guide is separated into life cycle charts, which include commonly chosen life cycle amounts. Common life cycles range from 4,000 to about 25,000 life cycles. If a high life cycle is needed, considerations have to be made for increased tolerances of the spring’s diameter and force due to the low stress level and slight variances in material.

  3. Establish the force – The force of a constant force spring should be equal to the requirement of the application. For information on creating more force with a spring please see, Creating More Force with a Constant Force Spring. Standard tolerance for the force of a constant force spring is ± 10%.

  4. Determine the space – When reviewing the charts in our Design Guide, it will become obvious that there are several thickness and width combinations that will result in the same force. The inside diameter (I.D.) of the spring is dependent on the thickness, life cycle and force required. The outside diameter (O.D.) is dependent on the above with overall length as an additional consideration. A constant force spring does not extend tangent to the body of the spring. To allow for proper operation, a minimum distance of .80” X I.D. is required and should be considered during initial design. The standard tolerance for the I.D. and O.D. is ± 10%.

  5. Calculate the total length – The total length of the spring should take into account the actual extension of the spring in the application, plus at least enough material to keep 1-1/2 turns on the spring when fully extended. If mounting the spring on an oversized spool, additional material may be required. The basic formula for this estimation would be:

                     (I.D. X 3.14159 X 1.5) + maximum extension length.

    When using a spool diameter greater than the I.D., substitute the spool diameter in place of the I.D. for this calculation. For more information on spools please see Mounting a Constant Force Spring on a Spool.

    If there is a leader on the front of the spring (commonly called a pickup) that length should also be included.

  6. Select the end detail – Vulcan Spring has many standard end details available. A number of these ends can be found on our Conforce® Constant Force Spring web page. End details are listed by thickness and width of the material size used in the part. Vulcan Spring is equipped with a full tool and die department which is capable of creating many custom designs required by our customers.

  7. Consider the environment – Corrosive atmospheres or extreme temperatures will affect a spring’s life and material selection. Our blog, Environmental Factors for Constant Force Springs will assist in this determination.

  8. Finalize the design – Once all of the above criteria have been considered and the spring can be located or extrapolated from the design charts, a spring part number can be introduced. Vulcan’s system for assigning part numbers to designs is simple. Be aware however, that a part number does not include the end detail, pickup length or any other special requirements. The part number identifies the following:

      a.  The letter from the selected life table
      b.  The thickness of the spring in mils
      c.  The letter code of the width of the spring (taken from the charts)
      d.  The overall length in inches

    As an example, the spring with part number J12G21 specifies Type 301 Stainless Steel, 8,000 life cycles (J), .012” thick (12), ½” wide (G) and 21” long. From the design charts we also know the nominal I.D. is 1.05” and the nominal force is 3.42 lbs. Please note that we do not rely on the part numbers for exact specifications. Rather, these are used to distinguish the part from the others that we produce. Individual specifications, even if they vary slightly from the standard charts, are captured within the description. This explanation is a general overview of the part numbers that may be generated for you. 

Lastly, Vulcan has a wide range of stock constant force springs. These springs are often used to test or confirm the use of spring within its application prior to moving into a custom design. Vulcan constant force springs are readily available and can be utilized in prototypes, or even for production. Please review our Express Order at http://www.vulcanexpress.com/.