What Is a Constant-Force Spring?

In most types of springs, the force exerted when that spring is compressed and then relaxed is proportional to the distance that it has been stretched. A constant-force spring, unlike an ordinary spring, does not obey this rule. As a result of the constant-force spring’s physical design, the power with which it recoils remains constant. Constant-force springs are, therefore, useful in many applications where a consistent recoil force is important.

The behavior of ordinary springs can be described by a mathematical equation known as Hooke’s law. This law states that the force of a recoiling spring in the direction of recoil is equal to the distance of the spring’s displacement from equilibrium — in other words, how far it has been stretched — multiplied by a mathematical constant, the rate of spring. The spring recoils in the direction opposite of that in which it has been stretched. Since many elastic materials behave in this way, Hooke’s law has wide applications in the field of physics.

Hooke’s law does not apply to constant-force springs because their force of recoil remains constant across their range of motion. This is achieved by their unique physical design, which consists of a thin, coiled ribbon of steel. Constant-force springs lie in a state of relaxation when they are fully rolled up, and become charged with force when they are partially or completely unrolled. When released, the spring recoils by pulling the extended ribbon back into a tight roll. Since the recoil force comes mostly from part nearest to the coiled roll, a constant force is maintained.

A common application of the constant-force spring can be seen in the tape measures used in household applications and construction. A typical tape measure mechanism consists of a thin metal ribbon coiled around an inner drum inside a plastic casing. When the measuring tape is pulled out and locked, force is imparted to the spring. Once the lock is released, the ribbon automatically retracts back into the plastic casing. This retraction is achieved due to the recoil action of the roll, which functions as a constant-force spring.

Many other devices that feature an automatic retraction also make use of the constant-force spring. Some examples include automobile seatbelts, window counter-balances, seat recliners, and staple guns. In all of these applications, the device generating force must be consistent, behaving the same way each time while acting within a small space. Constant-force springs meet both these conditions, as they exert a constant force and can also be coiled tightly, fitting a great length of ribbon into a modestly-sized area.