1) Gearing 2) Gears 3) Gear train 1) Mechanical device 
• (n.) A combination of wheels, and their connection, in a machine or mechanism.
(from the article `clock`) The wheelwork, or train, of a clock is the series of moving wheels (gears) that transmit motion from a weight or spring, via the escapement, to the ...
Wheel'work` noun (Machinery) A combination of wheels, and their connection, in a machine or mechanism. 
[n] - mechanical device including an arrangement of wheel in a machine (especially a train of gears)
noun mechanical device including an arrangement of wheel in a machine (especially a train of gears)