­Pre-Winding Power Springs

Power Springs are used to create a compact power source in applications that require counterbalancing, sometimes through a cable/cord retraction mechanism. The small size and rotational output of Power Springs make them fantastic options for manufacturers and engineers. Vulcan’s Conpower Springs are pre-stressed, producing a flatter torque profile than conventional Power Springs. When a project calls for the use of a Power Spring or a Pre-Stressed Power Spring, the design discussion should address initial winding on the arbor. This initial winding is often referred to as pre-winding the spring. For most applications, we recommend Power Springs crafted from Type 301 stainless steel. However, they can be made from other materials, such as high-carbon steel and Inconel, depending on a given project’s requirements.

Industries that may benefit from Conpower Springs and conventional Power Springs include:

  • Aerospace and defense
  • Automotive and transportation
  • Industrial applications
  • Medical devices and healthcare
  • Point-of-purchase and retail displays
  • Windows and window shades
  • Oil and gas

During product assembly, the Power Spring is wound on an arbor to a predetermined number of turns. The main purpose of pre-winding the Power Spring is to maintain torque on the arbor when the retraction is complete. A secondary purpose of pre-winding is to increase the initial torque of the Power Spring.

A great example that shows how pre-winding can increase the initial torque of a Power Spring can be observed in a canister vacuum cleaner equipped with a retractable power cord. Sometimes, the last few inches of the cord retract slowly or not at all. If there were more turns available in the Power Spring, additional pre-wind would benefit the mechanism, increasing the Power Spring’s initial torque on the cord.

As shown in a typical Pre-Stressed Power Spring torque-turns curve (see below), torque increases with each additional turn. Pre-winding the Power Spring moves the start and end points along this curve. Using the stock Conpower Spring SCP13G59VS as an example, we can see that the torque increases steadily as the Pre-Stressed Power Spring is wound. Depending on your specific application, there may be options as to how far the Power Spring is wound, assuming we are not limited by the total number of turns required. This allows you to fine-tune your application to precise specifications.

The graph below shows the start and endpoints that are determined by the number of pre-winds on a Pre-Stressed Power Spring. For both examples, the number of working turns is 5.

  • Example 1 has only one pre-wind and will follow the torque curve to turn number 6.
  • Example 2 has been pre-wound 4 turns and works to turn number 9.

It can be seen by estimating the initial torque, that we can substantially increase the initial torque through pre-winding the Pre-Stressed Power Spring.  The same is true of a standard Power Spring.

It should be noted that each Power Spring design has a maximum number of available turns. The Power Spring should not be the mechanism used to stop the extension or retraction. Too much force on the Power Spring could damage both it and the mechanism. There should always be an external stop leaving at least one turn unused on each end of the working range. Cycling the Power Spring too close to the end, or too “solid” can cause strain on the outer attachment area and cause premature fatigue.

Use the information found here as a guideline when considering the types of springs needed for your design. At Vulcan Spring, we work with you to develop and finalize unique spring designs that are customized to meet your needs. Once we’ve settled on a design, we’ll provide you with a competitive quote and exceptional spring manufacturing solutions.

Contact us with questions about pre-wound springs or to discuss your custom Power Spring needs. Our responsive and helpful team is standing by.

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