|example of lego gears|
Gears are classified by the number of little ridges or "teeth" on them. Common Lego gears include the 8t, 16t, 24t, and the largest: the 40t.
Constructing a gear train is all about ratios. If you mesh an 8t gear with a 40t one, you'll get an 8 : 40 or 1 : 5 gear ratio. (This also happens to be the highest ratio attainable with only two Lego gears).
Compounding gears will help you increase power or speed though. This is best explained with the diagram below:
Once you've mastered this, you can work on implementing numerous gear trains in the same system, a topic I've covered here.