Gears are toothed wheels that transmit motion from one shaft to another and determine the speed, torque, and direction of rotation of machine elements. Gears can be grouped into two major categories: those that operate in pairs to connect parallel shafts and those that connect nonparallel shafts. Parallel types include spur and helical. Nonparallel types include bevel, hypoid, and worm.

Types of gears

Spur gears. Cylindrical gears with teeth that are straight and parallel to the axis of rotation.

Helical gears. Cylindrical gears with teeth at an angle to the axis of rotation.

External gear. A gear with teeth on the outside of a cylinder or cone.

Internal gear. A gear with teeth on the inside of a hollow cylinder. Both spur and helical gears can be made as internal gears. The mating gear for an internal gear must be an external gear.

Bevel gears. Gears with teeth on the outside of a cone-shaped body. Teeth may be straight or spiral. Normally used at right angles (perpendicular to each other).

Worm gears. Gearsets in which one member of the pair has teeth wrapped around a cylinder like screw threads. Normally, this gear, called the worm, is at a right angle to the mating gear. Worm gears may be cylindrical, single-enveloping, or double-enveloping. Enveloping designs have curved worm or gear-tooth shapes to obtain more tooth contact area.

Face gears. Gears with teeth on the end of the cylinder.

Hypoid gears. Similar to bevel gears, but they operate on nonintersecting axes.

Pinion. Where two gears run together, the one with the smaller number of teeth is called the pinion.

Rack. A gear with teeth spaced along a straight line, and suitable for straightline motion.

Elements of gear teeth

Tooth surface. The side of a gear tooth.

Tooth profile. One side of a tooth in a cross section between the outside circle and the root circle.

Involute. A tooth profile generated from the involute of a circle. A common tooth shape for spur gears.

Base circle. The circle from which involute tooth profiles are derived.

Flank. The working or contacting side of a tooth. Usually has an involute profile in a transverse section.

Top land. The top surface of a gear tooth.

Bottom land. The surface at the bottom of the space between adjacent teeth.

Crown. A modification consisting of a slight outward bulge in the center of the tooth flank. The tooth becomes gradually thinner toward each end. A fully crowned tooth has a little material removed at the tip and root areas also. The purpose of crowning is to ensure that the center of the flank carries its full share of the load even if the gears are slightly misaligned or distorted.

Root circle. A tangent to the bottom of the tooth spaces in a cross section.

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