mainspring
/ˈmeɪnsprɪŋ/noun
1- the principal spring in a watch, clock, or other mechanism（钟表等的）主发条, 主弹簧。
1.1
- figurative something that plays a principal part in motivating or maintaining a movement, process, or activity〈喻〉主要动力; 主要动机：
they shared convictions that society was the mainspring of civilization.
他们有着共同的信念, 即社会是文明发展的主动力。