Gear

a cylindrical or conical part using a tooth or screw-based surface configuration to mechanically transmit power from one portion of a machine to another. Gear designs are based in part on the shaft alignment: parallel, angled, over-and-under, etc. Selected gear types are:

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Spur gear Has teeth on the outside of a cylindrical body that are straight and parallel to the axis of rotation.
Helical gear Has teeth that spiral around the outside of a cylindrical body at an angle.
Internal gear Has teeth on the inside of a hollow cylindrical shape.
Bevel or miter gear Has teeth on the outside of a conical body. They may be straight cut (as in the plain bevel gear), or spiral cut (as in the spiral bevel gear). Both transmit motion between intersecting shafts at various angles.
Hypoid gear Has teeth cut in spiral bevel pattern, but set on non-intersecting shafts crossing at a right angle (over-and-under).
Worm gear Has threads that wrap around a cylindrical body.
Herringbone gear Has two separate rows of adjoining teeth on the same gear, cut in the configuration of two connected helical gears with teeth angled in a V-shaped alignment.
Crown gear Has teeth set in the rim perpendicular to the rotation plane of the gear.
Straight gear (rack and pinion) Has a toothed bar into which a worm or spur type ‘pinion’ meshes, normally used to translate rotating motion into reciprocating motion.
Ratchet and pawl Has a toothed wheel or bar which catches a ‘pawl’ (a mechanical device that allows rotation only in one direction).
Sprocket Has a gearlike wheel which drives or is driven by a chain as opposed to direct mesh.

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