Helical gears have teeth set at an angle rather than parallel to the axis of rotation to enable them to transmit motion at greater speeds and torque. They can have either a parallel or crossed orientation: parallel helical gears transmit power between two parallel shafts, whereas the shafts of crossed helical gears are neither parallel nor intersecting (as seen with bevel gears). In mating two helical gears, the helix angle of the teeth must be the same.

Gears are available in a variety of materials, including stainless steel, cast iron, brass, and plastic, depending on application. There are several other factors to consider when selecting the appropriate gear for an application. The gear’s outside diameter is the longest linear distance across a gear. The bore diameter is the distance across the hole located in the center of a gear, where the mounting shaft will be inserted. The hub diameter is the distance from one tip of the shoulder (the hub) to the other. The thickness is the distance measured from the tip of a gear tooth to the gear’s bore or base (for a gear rack). The number of the teeth refers to the number of teeth around a gear. The pitch, also called the diametral pitch, is the number of teeth divided by the pitch diameter, and it can also serve as a guide for tooth size.

Helical gears are commonly used in automobiles, blowers, and elevators, among other machines.