Advanced Mechanical Engineering

Constant Force Springs Design and Applications

Constant force spring, as the name implies, is used to refer a spring that experience and exhibits near-constant force over its range of motion. The concept behind constant force springs stems from metal strips that have undergone a series of changes. Which in return create tension in the metal strips, enabling them to execute that tension in a linear direction. Unlike common wires, the use of metal strips or spring steel rolled together forms a fully relaxed material when rolled up. When this state of the spring is altered or changed, the restoring force starts to take counteraction. A large portion of which is credited to the roll near the base of the spring. Since the configuration of that portion remains mostly constant during the unrolling of the spring, the force exhibited is also nearly constant.

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Configuration of Constant Force spring (Design aspects)

Being in its unique form, constant force spring differs in terms of design and composition. The design of the spring is set to follow pre-stressed steel or metal strips. Which exist a force to counter any recoiling attempt to exist in their relaxed state. The force exerted by the spring is nearly constant as the curvature has a constant radius. The force exerted by the spring can be claimed as perfectly constant if the minute buildup experienced by the spring is ignored. Due to the constant force spring’s enduring nature, matched with long extension capabilities it is used for a variety of equipment. To power many motors and devices, the constant torque of the spring matched with its near absence of inner friction makes it the ideal accessory to add for many designers. On a common note, the use of the spring can be seen in most movie cameras, typewriters, window stashes, as well as in many cable retractors and more.

 constant force spring design

How to select the constant force spring 

Albeit, the type of spring used in these equipment or other devices vary depending on the type of use. The purpose of the device is to be kept in mind before selecting the type of constant force spring. Usually, the keen area of interest revolves around the width and thickness of the coil. Moreover, the type of material used can widely impact the spring. Ideally, grade 301 steel is mostly selected. However, a variety of options are available ranging from high carbon steel to other types of steel. To reach the constant force level, the constant force spring is heavily reliant on its design and its diameters. Depending on the internal and external diameters, along with some changes in design will determine the intensity of the load the spring would have to overcome to execute constant force. In addition to this, the finishing details of the spring also play a major role. Depending upon the type of use, the outside end of the coil is selected. As the attachment of the spring widely depends upon the end of the coil, the robustness should be taken into consideration when selecting a constant force spring. Keeping in mind that the degree of tolerance of the spring is also subjected to its composition.

Constant Force Spring Applications


Applications of Constant Force Spring

The places where constant loading is required, the constant force spring is very useful. These are used in –

  • Motor operated toys
  • Door closer
  • Gym equipment
  • Toys
  • hair dryers
  • electric motors
  • home appliances
  • They are also used in space vehicles
  • Any returning and retrieving application requires a constant force spring .
  • Nail guns for nailing application in the wooden house building.
  • Automotive engineering requires these springs.
  • Power springs are used for power generations and many other purposes.

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Basic Mechanical Engineering is a small endeavor for the mechanical engineering students and fresh graduates. All the articles are written by mechanical "rocking" engineers \m/

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