Gear train

A gear train is formed by mounting gear on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next

gear,

 

Compound gear trains have two or more pairs of gears in mesh, so that they rotate together.
 

This compound gear train has gears on three shafts.
The gear on the input shaft meshes with a larger gear on a counter-shaft or cluster gear. The counter-shaft has a smaller gear formed on it, in mesh with the output shaft gear.
The motion of the input is transferred through the large gear, along the counter-shaft to the smaller gear, to the output.
The output turns in the same direction as the input, but at a reduced ratio, depending on the relative sizes of the gears.
Since two pairs of gears are involved, their ratios are “compounded”, or multiplied together.