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 through the case that it is retained in. Pre-stressed power springs (Conpower®) undergo additional processing to yield a slightly flatter torque curve and increase the available turns.
Power and Conpower® 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. In order to meet the varying demands of these designs, many parameters must be defined in order to gain enough information to begin.
In this blog, we will discuss a few of these key parameters so that a designer can become more familiar with the requirements. In the second part of this blog, we will get into further details and a few design calculations that will also help.
For now, let’s assume that it will be up to the Vulcan Spring representative to suggest either a power spring or a Conpower® spring as the design parameters are similar.
Inside Diameter of Case
This is usually one of the first questions we will ask. What we are referring to here is the case size that the spring will fit into. Many times the spring cavity is inside of a rotating wheel that will pay off cable. Other times, 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 consumes more space than that the number of turns available will actually decrease. If less space is filled than the spring could fit into a smaller housing, saving space and money while providing the same number of turns and torques curve.
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. Many times the spring case does not need to consume all of the available space, but we can suggest a spring to fit if the design is already determined.
The arbor is located in the center of the case. The inside of the spring will attach to this arbor and much discussion will need to take place regarding this connection. The arbor diameter affects the available space inside the case and therefore the number of available turns. If the arbor is too small the spring will take a set that will compromise the torque and turns. If it is too large the maximum torque may not be achieved.
The number of active turns a power spring needs to provide is crucial information. As discovered through experience, it is also a number that is difficult to obtain early in the design stage. Power springs achieve approximately 20 – 25 turns maximum while Conpower springs can achieve more than that. Torque Required Let’s begin by correcting a common misconception; 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. This means that a greater torque is required to wind the spring than it will provide when unwinding. When discussing torque it is best to agree to a maximum requirement since the torque of a power spring will increase as the spring is wound. Since the torque increases at a rapid rate 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.
Life Cycle requirement
A cycle is defined as one full winding of the material and one unwinding of the material. Power springs will rarely have life cycles exceeding 200,000 cycles and will more than likely be under 100,000 if space and torque is optimized. The cycle life is hard to predict because it is dependent on the amount of winds used during cycling. For example, if the spring is only wound halfway for one cycle and then fully wound on the next the life cycles will increase. This is due in part to the utilization of the material within the housing.
There are many other options that we can discuss when working with power springs and Conpower® springs. This information is a basic guide to understand the many areas that are of concern during a design. Please be sure to consult your Vulcan Spring Product Manager as early in your design as possible. Already know what you want? Buy springs now.
Conpower® is a registered trademark of Vulcan Spring & Mfg. Co.