Gears are mechanical devices that transmit power and motion between axes. There are a number of different gear types. Examples include spur gears, change gears, cluster gears, and helical gears. Spur gears are straight-toothed and transmit power and motion between parallel axes. Change gears resemble spur gears and are hubless with a keyway that mates with a hub for rapid changing. Cluster gears are spur gears with an integral hub whose end can accept a second, hubless gear. Helical gears a helical-toothed and transmit power and motion between parallel axes (opposite hand) or right-angle axes (same-hand). In herringbone gears, each tooth comprises two opposing helices. They transmit power and motion between parallel axes and may or may not have center groove.

Selecting gears requires an analysis of specifications. The number of teeth on a gear, along with desired pitch diameter, determines the gear pitch or module. English gears are measured in design units such as inches (in.) and fractions of inches. They use diametral pitch (DP) or circular pitch (CP) as tooth pitch designations.  Metric gears use design units such as millimeters (mm) and centimeters (cm). The pitch is designated as the module, or metric distance between teeth. 

Gears are mechanical devices that transmit power and motion between axes. There are a number of different gear types. Examples include spur gears, change gears, cluster gears, and helical gears. Spur gears are straight-toothed and transmit power and motion between parallel axes. Change gears resemble spur gears and are hubless with a keyway that mates with a hub for rapid changing. Cluster gears are spur gears with an integral hub whose end can accept a second, hubless gear. Helical gears a helical-toothed and transmit power and motion between parallel axes (opposite hand) or right-angle axes (same-hand). In herringbone gears, each tooth comprises two opposing helices. They transmit power and motion between parallel axes and may or may not have center groove.

Selecting gears requires an analysis of specifications. The number of teeth on a gear, along with desired pitch diameter, determines the gear pitch or module. English gears are measured in design units such as inches (in.) and fractions of inches. They use diametral pitch (DP) or circular pitch (CP) as tooth pitch designations.  Metric gears use design units such as millimeters (mm) and centimeters (cm). The pitch is designated as the module, or metric distance between teeth. 

In straight tooth gears, tooth axis is parallel to the axis of rotation. Spiral or helical teeth curve along the face. Miter or bevel gears have a conical face and operate on intersecting axes, which are often at right angles. Miter or bevel gears sets are matched sets with defined ratios. A hypoid gear is similar to a bevel gear, but driven by a worm-like drive on non-intersecting axes. A differential end gear mates with a differential assembly. An internal gear typically has straight teeth on the inner diameter (ID), a circular outer diameter (OD), and mates with spur gear on the inside. A worm gear, also known as a worm wheel, is used for high- ratio speed reduction between non-intersecting right-angle axes. A gear stock or pinion wire is a length of rod with gear teeth already furnished for custom width machining. In a gear blank, the hub and OD are already formed. Gear blanks may be hobbed or ground to any tooth configuration.

Spiral bevel and miter gears have a right or left hand specification, which describes the direction of tooth curve. If they have spiral teeth, bevel and miter gear sets must have opposite hand specifications to mesh. In other words, a left-hand pinion will drive a right-hand gear, and vice versa. Matching items for sets are listed according to the hand specification of the pinion. These gears may have a center bore, a circular center bore for mounting onto a shaft or an integral shaft, in which the gear is on a non-separable cylindrical center shaft.

Gears differ in terms of related components, tooth type, and materials of construction. Anti-backlash gears have a mechanical assist such as a spring to take up any play between meshing gear teeth, thus avoiding backlash when gear direction changes. Ground teeth provide smoother, higher-precision tooth form. Materials for gears may include aluminum, brass, bronze, cast iron, carbon or alloy steel, hardened steel, stainless steel, and a wide variety of plastic materials. A rack is a straight component with gear teeth; typically straight-toothed that mates with pinion (spur gear). Pinions are drive gears that drive racks or larger gears.  They can be physically similar to spur gears, and occasionally helical gears.  The "pinion" designation is primarily used to indicate that the device is the driving gear in a rack and pinion configuration.