Mainspring (noun)

1. A coiled metal spring in a mechanical device, such as a clock or watch, that provides the energy to power its movement.
2. Something that provides the driving force or motivation for a particular activity or situation.

Origin:

Early 18th century: from main (in the sense "main or chief") + spring.

Examples:

1. The mainspring in the grandfather clock had finally worn out and needed to be replaced.
2. The CEO's passion for the company was the mainspring of its success.
3. The scientist's love of discovery was the mainspring of his research.
4. The mainspring in the old pocket watch was still ticking after over a hundred years.
5. The mainspring of the revolution was the desire for freedom and equality.