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Standard Gears
Nearly all models will require at least one gear, as rarely will all of the axles be running at the same speed and be in the same position. Gears can be used to reduce speed, and there are four basic Lego gears. I will use the names 'small', 'medium', 'normal' and 'large' to describe them.

	The small gear has eight teeth and a radius of half a Lego unit.
	The medium gear has 16 teeth and a radius of one Lego unit.
	The standard gear (don't ask me why I call it a standard sized gear!) has 24 teeth and a radius of one and a half Lego units.
	The large gear has 40 teeth and a radius of two and a half Lego units.

In order to reduce or increase the gear speed, you must combine one or more of the gears shown. In the example on the left, by turning the near handle you turn the eight toothed cog, which turns the 24 toothed cog which turns the other handle (the blue dot is a marker). The ratio for the decrease in speed in 24:8, which you can cancel down to 3:1. This means that the large handle turns around once for every three turns of the smaller handle. Another important thing to note is that the gears will turn any gears touching them in the opposite direction, an important consideration if you are building a model.

Here is another example. The handle is again connected to a small cog, but this time it turns a 40 toothed cog. The ratio of speed is this time 40:8 which cancels down like a fraction of 5:1.

But standard gears aren't all perfect. In the two examples above, only three of the four main cogs have been used. This is because the medium sized one will only combine with itself on a beam. You can see this by looking at the radii of each cog. You cannot mesh a small gear and medium gear as no Lego beam has holes which are 1.5 spaces apart. There is a way to get around this though, and that is by meshing them on a beam, like in the picture opposite. You will need to position a half bushing between the two crossblocks to making the spacing correct. As a general rule, the sum of the radii of two meshing gears must equal the distance between their centres.

Another way to join two gears in which one is a medium sized gear, is to attach them vertically. You will need to make sure that there is a 1/3 plate between the two beams in order to space it properly. The gap is a little wider than it needs to be, but is still enough to fit.