Power Spring Design Requirements & Calculations

What Are Power Springs?

Power springs are formed by winding strip material on an arbor and into a case or retaining ring. Power springs store and release torque through a central arbor or the case where it’s retained.

What Are Pre-Stressed Power Springs?

Pre-stressed power springs (Conpower®) undergo additional processing to yield a slightly flatter torque curve and increase the available turns.

Conpower® and Power Spring Uses

Conpower® and power springs are used in many applications where torque is required. Some well-known examples are clocks, toys, seat belts, canister vacuum cleaners, dog leashes and badge reels. To meet varying design demands, many parameters must be defined to gain enough information to begin.

In this blog, we’ll discuss a few of these critical parameters so that a designer can become more familiar with the details and requirements of a power spring.

When a Vulcan Spring representative suggests a power spring or Conpower® spring, you may be asked the following questions:

Conpower® vs. Power Spring

Conpower Spring in Retaining Ring

What’s the Inside Diameter of the Case?

Usually, the first question asked—what we are referring to here is the size of the case that the spring will fit. Often, the spring cavity is inside a rotating wheel that will pay off cable. In other instances, the case is constrained, and the central arbor will spin. In either configuration, knowing the inside diameter will allow us to calculate the number of turns available once the size of the spring material has been determined.

The spring material should only occupy approximately 40 – 50% of the available space in the case. If the spring exceeds this allotted space, the number of possible turns will decrease. If less space is needed, the spring could fit into a smaller housing, saving space and money while providing the same number of turns and torques curve.

What’s the Available Case Width?

This value is essential to determining the maximum width of the spring steel. We need to know how high the case can be in the event that we need to maximize the spring material to achieve the required torque. The spring case often does not need to consume all of the available space, but we can suggest a spring to fit if the design is already determined.

Spring in case and attached to the central arbor

What’s the Arbor Size?

The arbor is located in the center of the case. The inside of the spring will attach to this arbor, and the arbor diameter will help determine the space inside the case, the number of available turns, and spring and case size.

If the arbor is too small, the spring won’t fit properly and will compromise the torque and turns. If it’s too large, the maximum torque may not be achieved.

How Many Turns Are Required?

The number of active turns a power spring must provide is crucial information when determining spring size. As discovered through experience, it’s also a number that’s difficult to obtain early in the design stage.

Power springs achieve a maximum of 20–25 turns, while Conpower® springs can achieve more. Let’s begin by correcting a common misconception: torque.

What Is the Maximum Torque?

Torque is not the pull force on a cable. Torque is the rotational force exerted when the spring is unwinding. Friction causes hysteresis in a power spring, meaning that a greater torque is required to wind the spring than it will provide when unwinding.

When discussing torque, it’s best to agree to a maximum requirement since the torque of a power spring will increase as the spring is wound. Since the torque rises rapidly in the first and last few winds, power springs are usually designed to keep the first 20% of available turns and the last 20% before the spring is wound solid inactive.

Spring in Retaining Ring

What Is the Life Cycle Requirement or Goal?

A cycle is defined as one full winding of the material and one unwinding of the material. Power springs rarely have life cycles exceeding 200,000 winds and will likely be under 100,000 if space and torque are optimized.

Lifecycles are hard to predict because this figure depends on the number of winds used during cycling, and not every wind utilizes the entire spring.

For example, if the spring is only wound halfway for one cycle and then fully wound on the next, the life cycles will increase in part due to the utilization of the material within the housing.

Choosing the Power Spring for Your Design

There are many other options and factors to discuss when working with power springs and Conpower® springs. This information is a good starting point for understanding the many areas of concern during the power spring design process. Please consult your Vulcan Spring Product Manager early in the design process to understand your unique needs and achieve the most efficient results.

If you already know what you need, shop our selection of available springs.

Conpower® is a registered trademark of Vulcan Spring & Mfg. Co.

Please contact Vulcan with any questions.

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