Under weak loads the hairsprings of the zig-zag spring are hardly distorted. The maximum distance between the hairsprings' support points produces an elastic transmission between the driving force and the driven element. When the load increases, the hairsprings of the zig-zag spring bend appreciably. This makes the distance between the hairsprings' support points smaller. Consequently, the coupling is more rigid at that moment. Under unexpected overload, the deformity of the spring reaches its maximum value. The coupling can keep bearing bigger overloads despite having lost the elastic properties.