%balance wheel%

1. (Horology) (a) A wheel which regulates the beats or pulses of a watch or chronometer, answering to the pendulum of a clock; -- often called simply a balance. (b) A ratchet-shaped scape wheel, which in some watches is acted upon by the axis of the balance wheel proper (in those watches called a balance). [ 1913 Webster ]

2. (Mach.) A wheel which imparts regularity to the movements of any engine or machine; a fly wheel. [ 1913 Webster ]