There are many mechanical components to choose from for your device or application when it comes to constant force spring design. If your device requires a spring, consider constant force springs. These are a type of extension spring. They do not obey Hooke’s law, so their force remains constant, even when deflected.

Because of their flat force curve, constant force springs are suitable for applications that require a smooth range of motion and a constant load. Learn more about this type of spring to see if it could work in your device or application.

Already know what you need? Contact John Evans’ Sons.

Constant Force Spring Benefits

There are many benefits of constant force springs that make them suitable for a number of applications:

  • Have small space requirements
  • Provide a smooth range of motion
  • Have no inertia to overcome
  • Mount to existing hardware
  • Produce a counterbalance effect

For these reasons, constant force springs are versatile enough to work for a number of applications in which a constant load is required to counterbalance, tension, retract or dispense. This is why they’re frequently used to enhance surgical devices, deploy and retract doors on aircrafts, and counterbalance windows.

John Evans’ Sons Medical Constant Force Springs on blue gradient background.

Design Considerations

If you decide that constant force springs are ideal for your application, here are some considerations to think about in the design phase of your project.

Load Conditions

Constant force springs produce a force as the material is extended. Constant force springs are rated at a maximum operating length. This means that the maximum travel must be lower than the difference between the spring’s unextended length and the maximum safe operating length.


Another important design consideration is material. 301 stainless steel is commonly used for standard-issue springs. Stainless steel is well suited to resist corrosion and damage, so it’s ideal for applications in the medical sector. Most constant force spring selection charts will be based on a spring made of 301 stainless steel.

Other materials, including Inconel and Elgiloy, are able to withstand higher temperatures and pressures. Depending on your application, you may need a higher grade of material. Think about the operating environment when making this decision.


You should clearly define the lifecycle of your constant force spring to avoid premature failure, but to also save money on the design. Manufacturers’ spring selection charts will consider material (often stainless steel) and the spring’s thickness and coiled diameter.

Mounting Methods

Mounting methods will affect the design of your constant force spring. A manufacturer like John Evans’ Sons can help you choose the best mounting method for your application. In most instances, a constant force spring is wrapped on a drum, and then the free end is attached to the loading force. Again, there are several mounting methods, so this may depend on your application.

These are just some of the design factors. You and your constant force spring manufacturer will discuss these and other considerations prior to creating a prototype and testing.

Depending on your application, a constant force spring may be ideal. Ask your spring manufacturer in the design phase about constant force springs.

John Evans’ Sons is proud to be the international leader in constant force spring manufacturing. Considering constant force springs? View our spring selection charts and technical information for our constant force products.