mainspring

From main +‎ spring. For figurative sense, compare spring (“source, origin”).

mainspring (plural mainsprings)

1. (horology) The principal spring of a clockwork mechanism, which drives it by uncoiling.
   • 1921 June, Margery Williams, “The Velveteen Rabbit: Or How Toys Become Real”, in Harper’s Bazar, volume LVI, number 6 (2504 overall), New York, N.Y.: International Magazine Company, →ISSN, →OCLC:

     He was wise, for he had seen a long succession of mechanical toys arrive to boast and swagger, and by-and-by break their mainsprings and pass away.

2. (figuratively) The most important reason for, or element of, something.

   My daughter is the mainspring of my life.

   • 2014 November 2, Paul Mason, “What Shakespeare taught me about Marxism”, in The Guardian‎:

     But in the England of Shakespeare’s history plays, the mainspring of the system has broken down.