Designing Constant Force Springs: Key Considerations and Best Practices

Constant force springs are an essential component in various mechanical devices. These springs are designed to exert a nearly constant force over a wide range of motion, making them ideal for applications that require a constant force throughout the operating range.

Designing constant force springs may seem simple, but there are key considerations and best practices that engineers must keep in mind to ensure optimal performance and reliability. Here, we will discuss these considerations and best practices.

Materials Selection

The material selection for a constant force spring is crucial for its performance. The material should have high strength and fatigue resistance to ensure the spring can withstand the required number of cycles. Common materials used for constant force springs include stainless steel, phosphor bronze, and beryllium copper.

Stainless steel is a popular choice due to its corrosion resistance, high strength, and long fatigue life. Phosphor bronze and beryllium copper are also frequently used due to their excellent spring properties and fatigue resistance.

When selecting the material, engineers must also consider the environmental conditions. For example, if the constant force spring will operate in a corrosive environment, the material must have excellent corrosion resistance.

Spring Geometry

The spring geometry is also critical for the performance of constant force springs. The geometry determines the amount of force the spring can exert over a given length of extension. The geometry includes the width and thickness of the spring material, as well as the coil diameter and number of coils.

One key consideration when designing constant force springs is the spring rate. The spring rate is the amount of force required to extend the spring by a specific distance. Engineers must design the spring geometry to ensure the spring rate remains relatively constant throughout its operating range.

Coiling Method

The coiling method is another essential consideration in the design of constant force springs. The coiling method determines the amount of residual stress in the spring, which can affect its performance and fatigue life.

There are two primary coiling methods used for constant force springs: the power-winding method and the clock-winding method. The power-winding method is the most common and involves coiling the material using a power-driven mandrel. This method produces a more uniform and consistent spring, leading to better performance.

The clock-winding method, on the other hand, involves coiling the material using a hand-cranked mandrel. This method can result in variations in the spring’s geometry, leading to lower performance.

End Fitting Design

The end fittings of a constant force spring are also crucial for its performance. The end fittings must securely attach the spring to the device it will operate in while minimizing stress concentrations that can lead to premature failure.

There are various end fitting designs available, including hooks, loops, and eyelets. The choice of end fitting design depends on the specific application requirements.

Testing and Validation

Finally, testing and validation are essential for ensuring the optimal performance and reliability of constant force springs. Engineers must develop a testing plan to evaluate the performance of the spring under various conditions, including its fatigue life and load-deflection characteristics.

Conclusion

In conclusion, designing constant force springs requires careful consideration of materials selection, spring geometry, coiling method, end fitting design, and testing and validation. By following these key considerations and best practices, engineers can ensure the optimal performance and reliability of the spring in various mechanical devices.