spur gear
The simplest type of gear wheel, with teeth projecting radially, parallel to its axis.
spur gear: the workhorse toothed wheel, straight teeth radiating outward
A spur gear is a cylindrical wheel with teeth cut straight across its circumference, each tooth parallel to the gear's axis of rotation. When two spur gears mesh, their teeth engage in a sliding contact that transmits torque between parallel shafts. This is the most common gear type in machinery because it is simple to manufacture, reliable in operation, and efficient at moderate to high speeds.
The teeth on a spur gear are defined by a pitch diameter, which is the theoretical circle on which gear calculations are based, and a module or diametral pitch, which governs the tooth size and spacing. A module of 2 mm, for example, means each millimetre of pitch diameter accommodates one tooth. Two spur gears mesh only if they have the same module; a 40-tooth gear with module 3 will not engage a 50-tooth gear with module 2.5. The pressure angle, typically 20 degrees in modern gears, determines the angle at which teeth meet and affects load distribution and noise.
Where spur gears work and fail
Spur gears excel in compact assemblies where shafts must be close together and performance must be predictable. However, they produce considerable noise because all teeth on the meshing pair engage and disengage almost simultaneously, creating impact forces. At very high speeds or high loads, helical gears, which introduce a diagonal tooth angle, are preferred because their gradual engagement is quieter and distributes force over more tooth surfaces. Spur gears can experience tooth bending fatigue if undersized, surface pitting from inadequate lubrication, and wear if meshed with poor alignment.
The term "spur" refers to the teeth projecting outward like spurs on a rider's heel. Spur gears are distinguished from helical gears (angled teeth), bevel gears (conical wheels for non-parallel shafts), and worm gears (screw-like drives). In power transmission systems, spur gears remain the default choice for cost-sensitive applications where noise is tolerable and speeds remain below roughly 3,000 rpm for small industrial gears.