8 Steps to a Constant Torque Spring Design

A constant torque spring is a specially stressed constant force spring traveling between two spools.  The spring is stored on a storage spool and reverse wound onto an output spool.  When released, torque is obtained from the output spool as the spring returns to its natural curvature on the storage spool.

The spring does not need to be attached to the storage spool.  The spring can be housed in a cavity, eliminating the need for a storage spool.  However, friction is then introduced and can change the torque requirements. 

When considering the use of a constant torque spring it is highly recommended that you speak with a Vulcan Engineer and work through various design parameters.  However, it is possible to make some initial design calculations by following the 8 steps described below.   It is also recommended that you download our Design Guide.The torque produced by a constant torque spring will increase slightly over many turns.  Constant torque, or even negative torque, is also possible.  Negative gradients as much as 50% can be achieved in certain applications.

    1. Select the material – Most of the springs we produce are made from Type 301 Stainless Steel.   This material is usually the most readily available and predictable.  A Vulcan Engineer can assist in choosing a standard size to expedite the process.  Vulcan has also used high carbon steel, Inconel and Elgiloy in various designs as required.
    2. Estimate the life – This is one of the most important considerations since the life of a constant torque spring is very predictable.  It is important to be realistic since estimating a high life cycle requirement can lead to a larger, more expensive design.  Of course, under estimating the life cycles can lead to a premature failure.  The Design Guide is separated into life cycle charts.  Commonly chosen life cycle amounts are from 4,000 life cycles to about 25,000 life cycles.  Designs can be made to higher life cycles except consideration for increased tolerances of diameter and force may need to be considered.  This is mainly due to the low stress level of such a spring and the slight variances in material.
    3. Establish the torque – The torque of the spring should be equal to the requirement of the application.  Standard tolerance for the force of constant force spring is ± 10%.
    4. Determine the space – When reviewing the charts in the Design Guide it will become obvious that there are several thickness and width combinations that can be used to create the same torque.  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.  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 in account the actual number of turns of the spring on the output spool plus at least enough material to keep 1-1/2 turns on the spring when it is fully rotated.  The formula for determining length is as follows:L=π N (Do +Nt) +10 DoThe above formula takes into account: diameter buildup, number of revolutions required (N), thickness of material (t) and output spool diameter (Do).
    6. Select the end detail – This end detail will connect the spring to the output spool.  Vulcan has many standard end details available.  A number of these ends can be found on our web page.  End details are listed by thickness and width of the material size used in the part.  Please let us know if the end requirement is not found on the page.  Vulcan Spring has a full tool and die department capable of creating many designs required by our customers.
    7. Calculate the Minimum Spool Spacing – The distance between spool centers (dimension “S”) must be greater than the radius of the spring when fully wound on the storage spool, PLUS the radius of the spring when fully wound on the output spool.
    8. Finalize the design – Once all of the above criteria have been reviewed, and the spring can be located or extrapolated from the design charts, a spring part number can be introduced.  Vulcan’s system is quite easy when assigning part numbers to designs.  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:
      1. The letter from the selected life table
      2. The thickness of the spring in mils
      3. The letter code of the width of the spring (taken from the charts)
      4. The overall length in inches

As an example, the spring with part number V5D46 specifies Type 301 Stainless Steel, 4,000 life cycles (V), .005” thick (5), 1/4” wide (D) and 46” long.  From the design charts we also know the torque will be .52 inch-pounds when mounted on a .58” diameter storage spool and a 1.01” output spool.  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.

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