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.
During product assembly the spring is wound on an arbor to a predetermined number of turns. The main purpose for pre-winding the spring is to maintain torque on the arbor when the retraction is complete. A secondary purpose is to increase the initial torque of the Power Spring.
A great example of this can be found in a canister vacuum cleaner equipped with a retractable power cord. Sometimes the last few inches of the cord will retract slowly or not at all. If there were enough turns available in the spring, the design would benefit by adding an additional pre-wind, increasing the initial spring torque and force on the cord.
As shown in a typical Pre-Stressed Power Spring curve, the torque will increase through the available turns. Adding pre-winds 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 spring is wound. In certain applications, there are options as to how far this spring is pre-wound, assuming we are not limited by the total number of turns required.
The graph below shows start and end points that are determined by the number of pre-winds. 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 spring.
It should be noted that each spring design has a maximum number of available turns. The spring should not be the mechanism used to stop the extension. There should be an external stop leaving at least one turn unused at all times. Cycling the spring too close to the end, or to “solid”, can cause strain on the outer attachment area and cause premature fatigue.
Contacting a Vulcan project engineer to discuss the options is always a good idea. A good place to start the dialog is by using the on-line request form.The information on our form will help our engineer assist in determining the number of turns available and the torque for a specific application.