Spiral Bevel Gear
Spiral bevel gears are bevel gears with curved tooth lines. Because of higher tooth get in touch with ratio, they are superior to directly bevel gears in productivity, strength, vibration and sound. However, they are more difficult to produce. Also, for the reason that tooth are curved, they trigger thrust forces in the axial direction. Within the spiral bevel gears, the one with the zero twisting position is called zerol bevel gear.
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Bevel Gear
Bevel gears possess a cone shaped overall look and are used to transmit force between two shafts which intersect for one point (intersecting shafts). A bevel gear has a cone as its pitch surface and its own teeth are trim along the cone. Varieties of bevel gears involve direct bevel gears, helical bevel gears, spiral bevel gears, miter gears, angular bevel gears, crown gears, zerol bevel gears and hypoid gears.
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Gear Rack
Same sized and shaped teeth cut at equivalent distances along a flat surface or a straight rod is named a gear rack. A gear rack is a cylindrical gear with the radius of the pitch cylinder currently being infinite. By meshing with a cylindrical equipment pinion, it converts rotational motion into linear movement. Gear racks could be broadly divided into directly tooth racks and helical tooth racks, but both include straight tooth lines. By machining the ends of equipment racks, it is possible to connect gear racks end to end.
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Helical Gear
Helical gears are used with parallel shafts related to spur gears and so are cylindrical gears with winding tooth lines. They possess better teeth meshing than spur gears and have superior quietness and can transmit bigger loads, making them suitable for high rate applications. When working with helical gears, they create thrust force in the axial direction, necessitating the application of thrust bearings. Helical gears come with right palm and left palm twist requiring opposite palm gears 
for a meshing couple.
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