Updated 1 October 2011.
The use of uncoiling springs, rather than descending weights, as a driving force was an important factor in the development of clocks for general use, and appears to have started in the early fifteenth century. ‘Mainspring’ is primarily a horological term. Here, it is applied to any coiled spring used as a power source. Mainsprings are, and have been for many years, used as a power source (motor) in toys and models. Such toys and are usually referred to as clockwork toys. A motor powered by a mainspring is referred to as a clockwork motor. Mainsprings also have various engineering applications, and are sometimes called clock springs or motor springs. Usually, mainsprings are rectangular ribbons of spiral form, made from C-Mn spring steel heat treated to around 400 VPN. In horological terminology, the width of the ribbon is called the height, and the thickness the strength. The ends of a mainspring are usually annealed (softened) to permit machining for attachments.
The first known, reasonably well documented, metal fatigue failures were in clock mainsprings. For a description of a mainspring that failed in fatigue, click the following link. For the full reference see the Publications page. Mainspring 7
For a brief introduction to coiled springs as a power source click the following link. Coiled 4B . Pook L P. An introduction to coiled springs (mainsprings) as a power source. Int. J.Fatigue, 2011, 33(8), 1017-1024, is an extended version.