Constant force springs maintain a constant recoil force due to their unique design, unlike ordinary springs which follow Hooke’s law. They are used in self-retracting devices such as tape measures and seat belts to ensure consistent force in a small space.
In most types of springs, the force exerted when that spring is compressed and then relaxed is proportional to the distance it has been stretched. A constant force spring, unlike an ordinary spring, does not obey this rule. As a result of the physical design of the constant force spring, the power with which recoil remains constant. Constant force springs are, therefore, useful in many applications where a constant 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 return spring in the return direction is equal to the distance the spring has moved from equilibrium – in other words, how much it has been stretched – multiplied by a mathematical constant, the rate of the spring. The spring retracts in the opposite direction to that in which it was stretched. Because many elastic materials behave this way, Hooke’s law has broad applications in physics.
Hooke’s law does not apply to constant force springs because their recoil force remains constant throughout their range of motion. This is achieved by their unique physical design, which consists of a thin coiled ribbon of steel. Constant force springs are in a relaxed state when fully coiled and charge with force when partially or fully uncoiled. When released, the spring rewinds pulling the extended tape back into a tight roll. Since the recoil force comes mostly from the part closest to the wound roller, a constant force is maintained.
A common application of the constant force spring can be seen in tape measures used in home and construction applications. A typical tape measure mechanism consists of a thin metal tape wound around an internal drum inside a plastic casing. When the tape measure is pulled out and locked, a force is imparted to the spring. Once the lock is released, the tape automatically retracts into the plastic casing. This retraction is achieved by the recoil action of the roller, which works like a constant force spring.
Many other self-retracting devices also use the constant force spring. Examples include automobile seat belts, window weights, seat recliners, and staple guns. In all these applications, the generating force of the device must be consistent, behaving the same way each time acting in a small space. Constant force springs satisfy both of these conditions, as they exert a constant force and can also be wound tightly, fitting a large length of web into a modestly sized area.
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