What is a constant force spring?

A constant force spring, also known as clock spring, is typically a roll of pre-stressed strip that can exert a mostly constant force to withstand uncoiling. This is achieved because the changes in the curvature’s radius are also constant. However, the force is just truly constant when this variation is disregarded.
Constance force springs are generally designed as a rolled ribbon of steel spring which is quite similar to a rolled-up unit when relaxed. Due to their virtual absence of inter-coil friction, constant torque, and long-extension capabilities, many designers have used them in many industrial applications which require a counterbalance against heavy-moving components such as motors or van doors.

Operation of constant force springs

A constant force spring can be tightly mounted with the terminal attached to its load force and interior layers which are coiled around itself. The interior diameter might be tied to a moving drum. During operation, the resistance is generated by the spring's effort to return to the natural, wound state. The thickness and width of the material, the drum’s diameter, and natural radius curvature all determine its extension resistance. Once the spring has been extended to 1.25 times the diameter, it will achieve a full- load rate and easily reach the loading capacity.

Technical features of constant force springs

When choosing constant force springs, keep in mind the following technical features:

Constant force springs are mostly constructed from 301 stainless steel due to its durability and low cost. It is able to attain ductility and highly tensile strength in cold working conditions. In addition, Inconel and high-carbon steel are other common materials.
The drum attracted to the interior diameter is often made of plastic, which provides coil attachment, structural integrity, as well as reduce friction compared to a metal unit.

Estimated life cycles
The life of constant force springs isn’t time-dependent. Instead, it can be determined by the relation between the material’s thickness and its diameter. A higher estimate makes the unit bigger and more expensive, but these units can also last longer.

Required force
The force of the springs should be equal to the application’s requirements. Basically, the usual tolerant should be approximately 10%.

Available space
There are a few combinations of material width and thickness which could be applied for any force values. The natural diameter of the coil will depend on the force, life, and thickness required.

Mounting methods
There is a variety of mounting options for constant force springs. These include single, dual, front-to-front dual, back-to-back dual, and back-to-back dual-dual

Common applications
As said earlier, this type of spring is applied in many industries thanks to its constant torque and long-extension capabilities. Some of the applications include door closers, hose retrievers, cable retractors, counterbalances, space vehicles, appliances, electric motors, toys, hair dryers, gym equipment, furniture & cabinet components, and other devices which require constant loads. Also, you can find these units in retail beverage racks to deliver products to the cooler door.


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    Central Birmingham Springs Limited
    Unit 7D. Waterfall Lane Trading Estate
    Waterfall Lane. Cradley Heath
    T: 0121 248 1112
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    Company No. 04256582