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Unit 3.4

Gear train combining a series of gears
External gears have the teeth cut into the outside rim of a metal or plastic disk.
Internal gears have the teeth located on the inner surface of a hollow cylinder or ring.
Helical gears Gears that are cylindrical in shape but the teeth are curved or helical.
Worm gears a special helical gear that has only one tooth that spirals about a central axis like a screw.
Bevel gears angled like a cone with teeth cut into the conical faces.
Rack and pinion gear train consists of a single spur or helical gear that meshes with teeth in a straight rod with teeth cut into it.
Planetary gear train uses a combination of spur gears and a ring gear in a concentric arrangement that maintains a central axis of rotation.
Diametral pitch represents the number of teeth on a gear measured in teeth per inch.
Pitch circle an imaginary circle that passes through the midpoint of each tooth.
Pressure angle the angle of the face of the tooth that sits above the pitch circle.
Addendum the portion of the tooth that is above the pitch circle.
Addendum circle an imaginary circle that traces through the tops of the teeth, equaling the maximum diameter of the gear.
Dededum the portion of the tooth that is below the pitch circle.
Dededum circle an imaginary circle that traces through the bottom of each tooth and tooth space.
Tooth clearance the space between the top of a tooth and the bottom of the tooth space on two meshing gears.
Bore a pre-sized hole drilled through the center of the gear to accommodate a shaft or axle.
Keyway a small square channel cut into the gear across the borehole or cut into the axle shaft.
Key a small metal prism that locks the gear and the shaft together by fitting into the keyway, essentially overlapping the joint.
Fulcrum 1. The pivot point of a lever. 2. The axis of rotation of a moment arm.
Lever arm the distance of the lever from the fulcrum to where a force is applied or a weight sits.
Gear ratio the ratio of the pitch diameters of each pair of meshing gears
Driver gear the gear turned by the motor
Output gear the gear that is turned or driven by the driver gear
Backlash excessive distance between the tooth on one gear and the tooth space or gap on the mating gear.
Gear gnashing occurs when gear teeth jump over tooth spaces and the tops of gears grind past one another.
Sprocket A wheel with metal teeth that mesh with a chain.
Compound gear train at least two of the gears have the same axis of rotation.
Created by: BHS Engineering