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Power Springs: An Overview

  • A power spring, in its simplest form, is a flat strip of spring material that is cut to length, wound around a forming arbor, and then unwound into a housing.

  • Common Names for Power Springs
  • Power springs are known by many names, including coil springs, hair springs, spiral springs, clock springs, motor springs, mainsprings, recoil springs, wind springs, retractor springs, and flat springs.

  • Visible Key Factors Defining Spring Performance
  • The performance of a power spring is determined by:
  • - Material Dimensions: Thickness (t), Width (b), and Length (L)
  • - Available Space: Housing Diameter (D) and Arbor Diameter (a)
  • Despite these basic parameters, designing and predicting the performance of a power spring is a highly complex task well understood by few.

  • Simplifying Complexity with Mecca C&S Power Spring Design Software
  • Mecca C&S Power Spring Design Software bridges this gap by making the intricate design process accessible. Leveraging advanced formulas and finite element analysis, engineers can input basic parameters and fine-tune manufacturing details to predict potential spring performance. This not only enhances accuracy but also saves significant development time.

  • This summary offers a foundational understanding of power springs while highlighting some of the complexities involved in their design and performance.

Understanding Power Spring Types: Conventional vs. Prestressed

One of the most subtle yet critical distinctions in power springs lies in their free state or shape when not retained in a housing.
Conventional SpringsA conventional power spring spirals outward from the center in the same direction as it is wound.
Prestressed (Backwound) SpringsA prestressed or backwound power spring features a section of its shape that spirals in the reverse direction of the winding.
Key DifferencesWhen housed, both types of springs may appear identical, making it challenging to distinguish them visually. However, the prestressed spring generates significantly higher stresses, resulting in greater torque.
Choosing the Right SpringEach spring type has its own set of advantages and disadvantages, with the optimal choice depending on the specific application and performance requirements. By carefully considering the design and intended use, engineers can determine which type best suits their needs.
Conventional
Advantages - Lower stress, Best cycle life, Easiest to manufacture.
Disadvantages - Lower torque, Lower turns, Torque not as flat
Prestressed
Advantages - Higher torque, Flatter Torque
Disadvantages - Lower cycle life, More costly to manufacture, Size and Forming methods sometimes limited.
Conventional SpringsConventional springs are typically manufactured by winding the spring material from a flat condition, making them easier and more cost-effective to produce.
Prestressed SpringsPrestressed springs require additional steps in the manufacturing process. The material is first coiled—either before, during, or after cutting—then stress-relieved through heat treatment, and finally backwound into the housing. These extra steps allow prestressed springs to achieve higher torque, but they add complexity to the manufacturing process.
Advanced Modeling with Our Mecca C&S Power Spring Design SoftwareOur Power Spring Design Software accurately models both conventional and prestressed springs by calculating the free state for each design. This information is then used to predict spring performance, enabling engineers to optimize designs for specific applications with confidence and precision.
Comparing Torque Outputs: Conventional vs. Prestressed Power SpringsThe torque graph and spring simulations above illustrate the performance differences between similarly sized power springs manufactured using different methods.
Higher Torque with Prestressed FabricationPrestressed fabrication methods generate higher stresses within the spring, resulting in a significantly higher torque output compared to conventional methods.
Impact of PrecoilingPrecoiling, further enhances torque output. This method is identifiable by a reduction in "dead turns" — the turns visible when the spring is unwound within its housing.
Notable Turn VariationsBoth prestressing and precoiling methods lead to slight variations in the number of active turns, contributing to differences in overall spring performance.

Torque & Turns: Key Factors in Power Spring Design

In power spring design, torque and turns are the most critical outputs.
Torque CharacteristicsTorque typically starts at zero and increases rapidly during the initial turns. As the spring continues to wind, the rate of torque increase slows, resulting in a flattened torque curve. This characteristic (as shown in the torque graph below) is advantageous for many applications, providing consistent performance across a wide range of motion.
Understanding the Torque GraphThe torque graph displays two lines:Red Line: Represents the winding (input) torque.Blue Line: Represents the unwinding (output) torque.The unwinding torque is always lower than the winding torque due to friction or hysteresis.
Minimizing FrictionWhile some friction is unavoidable, it can be reduced with thoughtful design, proper lubrication, and specialized manufacturing techniques. These adjustments improve efficiency and enhance the performance of the power spring in its intended application.

Geometry and Design Turns in Power Springs

The geometry of a power spring is a critical factor in determining the number of design turns it can achieve.
Space UtilizationThe available space between the housing and the arbor defines the spring’s design potential. The percentage of this space occupied by the spring material (referred to as the "% full") directly impacts the number of turns the spring can produce.
% Full and Design TurnsThe % full graph provides a simple relationship between space utilization and design turns:
As the % full increases from 0%, the number of design turns rises rapidly.However, as the % full approaches 50%, the rate of increase in turns reduces significantly, resulting in diminishing returns despite adding material length.This parabolic relationship highlights the importance of optimizing spring geometry to balance material usage with performance requirements, ensuring maximum efficiency and effectiveness in design. As material is added to increase the % full from 45% to 50%, the resulting increase in turns is often minimal, leading to inefficient use of material. Beyond 50%, the number of turns decreases, further exacerbating material waste. However, certain applications requiring extended cycle life may benefit from higher fill rates despite the reduced efficiency.
Key Ratios for Efficient DesignEfficient power spring design also depends on:- Arbor Diameter to Material Thickness (a/t): The ratio influences the spring's ability to perform within the available space.- Length to Material Thickness (L/t): This ratio impacts the spring’s ability to store energy and maintain durability. Streamlining Design with Mecca C&S Power Spring Design SoftwareThe complexity of calculating spring turns and torque is simplified with Mecca C&S Power Spring Design Software. The software not only performs precise calculations but also enables fine-tuning of parameters to achieve optimal efficiency.
By adjusting material thickness or eliminating unnecessary material, you can create springs that save resources and reduce costs, providing long-term value for your products.

Power Spring Cycle Life: Design and Optimization

A power spring typically operates within a defined range of turns. For example, in a hose reel application, the spring may be preloaded to 6 turns and extended to 16 turns during operation. Retracting the hose returns the spring to its initial 6 turns, completing one cycle.
Factors Affecting Cycle LifeCycle life is influenced by numerous factors, including:- Stress Amplitude & Alternating Stress: The range and variation of stresses during operation.- Coil Friction: Resistance caused by the spring’s interactions within its housing.- Spring Forming Methods: Techniques used to manufacture the spring.- Material Type & Edge Condition: The quality and properties of the material used.- Lubrication: Reducing friction to extend life.- Special Design Configurations: Tailored features for specific applications.Due to these complexities, calculating cycle life is challenging. Typically, stress calculations are paired with S-N logarithmic endurance curves to estimate the cycle life for a given material. These calculations are often compared to tested prototypes or similar springs to refine estimates.
Enhancing Efficiency with Mecca C&S Power Spring Design SoftwareOur design software simplifies this process by estimating cycle life based on calculated stresses and automatically adjusting as design changes are made. While no cycle life calculation is perfect, the software provides valuable initial guidance, allowing you to refine designs with tested prototypes.
By using our software, you can:- Save time and money during the development process.- Quickly adjust designs to improve cycle performance.- Reduce the need for extensive manual calculations and testing.This streamlined approach helps you achieve more accurate results while significantly shortening development time.

Power Spring Manufacturing Methods

Power spring manufacturing processes are relatively straightforward, though the equipment used is often highly specialized.
Conventional Manufacturing Process1. Cutting: The spring material is cut to the required length.2. Winding: The material is wrapped around a forming arbor and unwound into a container. Prestressed Spring Process1. Coiling/Spooling: The material is first coiled or spooled to create the desired preloaded shape.2. Stress Relief: Parts are then stress relieved using heat treatment. 3. Reverse Winding: The material is then wrapped in the opposite direction around the forming arbor before being placed into the housing. These processes, while simple in principle, require precise equipment and techniques to ensure accurate spring performance and reliability.
Precision in Power Spring Design and ManufacturingWhile the basic process of manufacturing power springs is straightforward, many details are highly intricate, including:- End Forms and Shapes: Custom configurations for specific applications.- Coiling Equipment: Specialized machinery for forming the spring.- Cutting Dies: Precision tools for shaping material.- Heating Equipment: Used for annealing and stress relief.- Winding Equipment: Ensures accurate placement in the housing.- Special Forming Methods: Techniques tailored to enhance performance. Advanced Design Control with Mecca C&S Power Spring Design SoftwareMecca C&S Power Spring Design Software empowers designers to control and refine their designs by adjusting critical manufacturing details. Key parameters such as:- Arbor Size- Anneal Length- Free Coil Diameter- Precoiling Detailsare both estimated and customizable, allowing designers to fine-tune their designs with precision. This level of control not only optimizes the design output but also prepares the design for seamless production, bridging the gap between design and manufacturing.

Advanced Techniques and Features in Power Spring Manufacturing


PrecoilingPrecoiling is a method used to enhance torque and turns by preserving the original material shape. It is often identified by a reduction in "dead turns" when the spring is unwound. During initial wrapping, stresses exceeding the material yield stress result in permanent deformation into a spiral shape. Precoiling modifies this process by reducing forming stress to create a larger spiral using techniques such as:- Adding a shim during winding.- Utilizing multiple winding arbors.- Combining both methods.Advantages:- Increased torque and total turns.Disadvantages: - Extra production steps.- Potential stress concentrations that may reduce cycle life.Special forming processes can mitigate stress concentrations to improve reliability.
AnnealingAnnealing addresses high surface stresses caused when material is formed to a small radius, reducing the risk of fractures or cracks.- The material is heated to a glowing red state and allowed to cool.- This transformation reduces hardness and allows for tighter forming without cracking.- Annealing a longer length at the inner end of the material creates a tighter wrap around the arbor.- Increases the required arbor diameter, producing a precoiling effect. Winding Arbor vs. Final Arbor- Final Arbor (Customer Arbor): Used to wind the spring in its end application.- Winding Arbor: Typically smaller than the final arbor to properly form the material’s inside diameter. Special Situations:Using a winding arbor larger than the final arbor can cause:- Loss of turns.- Reduced torque output.- Shortened cycle life.Proper winding arbor design is critical to prevent stress concentrations and maintain material integrity.
End ConfigurationsConnecting the spring material to the arbor and housing involves various methods:- Sharp Angles: Formed to create hooks or bends.- Piercing or Notching: Creates secure connection points.- Spot Welding: Ensures durability.The goal is to achieve a secure and cost-effective connection.
Retaining MethodsPower springs must be contained within a structure to function properly. Common retaining methods include:- Metal or plastic housings.- Bands or rings.- "C"-shaped keepers.- Riveted outer wraps (integral bands). Special FeaturesAdding a leaf spring, or bridle, to the outer end of the spring enhances performance by:- Increasing torque.- Reducing hysteresis.- Improving cycle life.Similarly, a well-designed outer end connection can achieve these benefits, ensuring optimal spring functionality. By combining these techniques and features, power spring designs can be optimized for specific applications, ensuring efficiency, durability, and cost-effectiveness.