That's Not the Way to Use That Spring!

Posted by Bob Gustafson on Mon, Oct 12, 2015 @ 09:20 AM

In power spring, Constant Force Spring, conpower spring, constant torque spring

In our experiences working with numerous customers and their diverse concepts, our representatives often learn a spring sample is being utilized in a design in a way the spring was not intended.That's Not the Way to Use That Spring! This misused prototype may work for a short time, but it will not perform long term as well as a properly designed solution.  Following are a few ways samples are often misused and how the situation can be resolved.


Winding the inner coil of a Constant Force Spring onto an arbor:

Situation – A constant force spring is placed in a cavity, and the outer end is fastened to the inside diameter of the cavity.  An inner arbor is attached to the inner coil of the spring, usually by manipulating the inner end.  The arbor then rotates, winding up the inner coils of the spring and using the torque to return the arbor to its starting position. 

Solution – In this situation a constant force spring is incorrectly being used as a power spring.  A constant force spring is designed to be used in a linear motion, not rotational.  With some discussion with one of our engineers, a properly designed power spring or Conpower® pre-stressed power spring can be created to optimize the available torque and turns.  This solution will ease final assembly, provide more available turns, increase life cycles and increase torque options.


Back-Bending a Constant Force Spring: 

Situation – A constant force spring is wound in the opposite direction of the coil set onto a spool to create torque. 

Constant Torque Spring DesignSolution – There are subtle differences between a Conforce® constant force spring and a Contorque constant torque spring.  The main problem with back winding a Conforce® spring in this manner is the spring could “take a set”,  meaning the original coil diameter can expand once used.  The result is a larger outside diameter and a loss of torque.  A better design utilizes a Contorque® spring.  This type of spring is designed and manufactured to be wound in the opposite direction of the coil set onto a recommended spool diameter.  Number of turns, torque and life cycles can then be accurately calculated.


Extending a Power Spring from its case: 

Situation – A power or Conpower® spring is wound into a case.  The outer end of the spring is extended out of the case and expected to retract back into the case.  The common comparison for this type of situation is the tape measure.  What is really happening here is that a spring made for rotational torque is being used for linear force.  

Solution – There are a few ways to accomplish the linear action this situation requires.  One way would be to design a Conforce® constant force spring to meet the extension, force and life cycle requirements.  The constant force spring is made for linear extension.  In other situations a better option may be to utilize the torque of the Conpower® spring by attaching a cable or cord of some type to the case housing the spring.  When the cable is extended the spring will wind onto an arbor,  creating torque that will retract the cable when released.  A third option is to create a Contorque® spring assembly.  This requires an output spool that will wind a spring and the cable.  The advantage to this is that the torque is relatively constant as compared to the Conpower® spring.


Attaching the inner end of a Constant Force Spring to a Spool: 

Situation – A constant force spring is modified to attach the inner end to the spool.  Sometimes this is done in order to provide a stop for the extension of the spring.  It is not recommended to use the spring as a stop, and there is an entire blog devoted to that concept. Read Springs Used as a Stopping Mechanism for more details.  Another reason this is done is to utilize the rotation of the spool in order to drive a mechanism. 

Solution – A constant force spring does not need to be attached to a spool.  An external stop needs to be put into place and the spring designed to be long enough to have at least 1-1/2 wraps of material remaining on the spool once fully extended.  If torque from a spool is required, it is best to review options such as a Conpower® spring or a Contorque® spring design. 


Using a Constant Torque Spring for extension because it is long:

Situation – A stock Contorque® constant torque spring is used for a linear application in an effort to take advantage of the long length of the spring.

Solution – While the Contorque® spring will provide linear force, it is neither calculated nor controlled.  This type of spring is designed and manufactured based on torque requirements.  In addition, a general rule is the extension length of a constant force spring should not exceed 50X the inside diameter of the spring. If used in this manner, the Contorque® spring will exceed this rule and could twist along its extended length.  A better solution for long extension would be to use a cable assembly mounted to a Contorque® Spring.