The teeth of a spur gear have an involute profile and mesh one tooth at a time. The involute form means that spur gears only produce radial forces (no axial forces), but the method of tooth meshing causes high stress on the gear teeth and high noise production.
Spur gears are a type of cylindrical gear, with shafts that are parallel and coplanar, and teeth that are straight and oriented parallel to the shafts.
There are two primary types of spur gears: external and internal. External gears have teeth that are cut on the outside surface of the cylinder. Two external gears mesh together and rotate in opposite directions. Internal gears, in contrast, have teeth that are cut on the inside surface of the cylinder. An external gear sits inside the internal gear, and the gears rotate in the same direction. Because the shafts are positioned closer together, internal gear assemblies are more compact than external gear assemblies. Internal gears are primarily used for planetary gear drives.
Spur gears are generally seen as best for applications that require speed reduction and torque multiplication, such as ball mills and crushing equipment.
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