Gears are easy to understand, make, and use, if you know the vocabulary and can space the gears at the correct distance apart. One nice thing about gears is that if you know any two things about them – let’s say outer diameter and number of teeth — you can use some simple equations to find everything else you need to know, including the correct center distance between them

have a look at the simple equations on the excellent (and free) design guide published by Boston Gear [PDF].

via [HackADay] source [MAKE]