Gears are used to increase or decrease power or speed. Some common types of NXT gears are summarized below.

Rack and Pinion Gear

Worm Gear

A black gear that resembles a cylinder with a spiral wound around it.

The worm gear leads to an asymmetrical system; that is, you can use it to turn other gears , but it cannot be turned by other gears.

Worm gears enable you to use two axles operating at 90 degrees to each other. The worm gear must always be the driver gear.

Gear Ratio

Horizontal Arrangement

Diagonal Arrangement

Turntable

Because its teeth are on its side, the turntable cannot as easily engage other gears—except for the worm gear.

In the figure below, you can see how naturally a worm gear drives a turntable.

(Normally, there would be additional pieces to hold everything in place.)

The arrow around the worm gear's axle reminds us that the worm gear must always be the driver gear, and the arrow around the turntable shows us how the top part rotates, assuming the bottom part is secured to other pieces.

Suggestions

The worm gear can be used to quickly assemble a good geartrain, although worm gears will cause more power loss due to friction than other types of geartrains.

Each axle should be supported at two points by going through at least two girders or beams. These support girders should be separated from each other. If these two rules are followed, the axles will stay straight and not bind up inside the girders, creating a lot of friction.

When multiple girders support the same axle, these girders should be firmly attached to each other. If they are not perfectly aligned, the same binding problem described above may happen, and the gear train could lose a lot of power.

The axles can bend. Gears should not be dangling at the end of an unsupported axle. Gears should either be put between the girders supporting the axles or very close to the girders on the outside of the girders. Both cases are illustrated on the example gear train. If the gear is two or more LEGO units away from the outside of the girders, problems may arise.

Axles should not fit too tightly. After gears and spacers are put on an axle, the axle should be able to slide back and forth a little bit. It is very easy to lose a lot of power if spacers or gears are pressing up against the girders.