What's constant force spring

A constant force spring (also referred to as power springs or constant torque springs) is a roll of pre-stressed strip that exerts a nearly constant restraining force to resist uncoiling. The force is constant because the change in the radius of curvature is constant. This is true if the change in coil diameter due to buildup is disregarded. Long extension capabilities, constant torque, and a virtual absence of intercoil friction have led many designers to specify constant force springs in such applications as brush springs for motors, counterbalance springs for window sashes, and carriage return springs for typewriters. Constant force springs are used to drive mechanisms for timers, movie cameras, and cable retractors. It is recommended to leave at least two wraps of the spring coiled at all times during use. 

Applications of Constant Force Spring

The constant force spring is an excellent device for applications where a constant load is required. Some of the many applications of constant force springs are in counterbalances, door closers, cable retractors, hose retrievers, tool head returns, cabinet furniture components, gym equipment, hair dryers, toys, electric motors, appliances, space vehicles, and other long-motion functions.

Motor springs are used for cable retraction, power generation etc. Power springs are used for retracting applications such as seat belt webbing, tape measures and dog leads, seat recliner, window regulator mechanisms etc.

Manufacturing Constant Force Springs

Constant force springs can be manufactured using the same spring coiling machines used for helical springs. The coiler will have to be modified to accommodate the flat strip of metal that makes up the constant force spring. AIM offers modifications for constant force spring manufacturing as part of their turn-key coiling system design services.

Once the equipment is calibrated, constant force spring manufacturers have to decide on a number of parameters before beginning production:

Material, width and thickness of strips: these three parameters go a long way in determining the properties of constant force springs, much in the same way that wire material and diameter determine the properties of helical springsInitial load: the force that must be overcome in order to reach “constant” forceInternal and External Diameter: the diameter inside the coil, which often surrounds a drum, and the diameter outside the coilEnd type: The outside ends of constant force springs often attach to another part, which will act directly to coil and uncoil the spring.