mainspring

The principal spring in a mechanical device, especially a watch or clock, that drives the mechanism by uncoiling.

The chief motivating force.

The principal spring of any piece of mechanism, as, in a gun-lock, the spring which operates the hammer; specifically, the coiled spring of a watch or other timepiece.

The impelling cause of any action; the inciting motive.

The principal or most important spring in a piece of mechanism, especially the moving spring of a watch or clock or the spring in a gunlock which impels the hammer.