A gear is an element in a machine that transmits rotations and forces allowing one shaft to drive another. Its history dates back to ancient Greece, when Archimedes referenced the many different types of gears in his writing.
Spur gears are cylindrical, with a straight tooth line that is parallel to the shaft. They are the most widely used gears because they require relatively simple production processes but can achieve a high rate of accuracy. They are characterised by their lack of load in the axial direction, otherwise known as thrust load. The smaller of the meshing pair is the pinion and the larger is the gear.
Helical gears are cylindrical and have winding tooth lines. Like spur gears, they are used with parallel shafts, but their teeth meshing is better, allowing them to operate more quietly and transmit higher loads, which is an advantage in high-speed applications. Helical gears create a thrust force in the axial direction, making thrust bearings necessary. Meshing pairs require opposite hand gears because helical gears have right hand and left hand twist.
Gear racks consists of teeth, all the same size and shape, cut along a straight rod or flat surface. It is a type of cylindrical gear with an infinite pitch cylinder radius. When meshed with a cylindrical gear pinion, it creates linear motion by converting rotational motion. Gear racks can be separated into helical tooth racks and straight tooth racks, but both types have straight tooth lines. They can be connected end to end by machining their ends.
These cone-shaped gears are used to transmit force between two intersecting shafts. Bevel gears have teeth cut along the cone that serves as its pitch surface. Their different types include straight bevel gears, spiral bevel gears, angular bevel gears, and zerol bevel gears.
Spiral Bevel Gears
Spiral bevel gears have curved tooth lines and a higher tooth contact ratio, which makes them superior to straight bevel gears in noise, vibration, strength, and efficiency. They are, however, harder to produce and they create thrust forces in the axial direction due to their curved teeth.
These gears consist of two same-hand helical gears with a 45-degree twist angle on non-intersecting and non-parallel shafts. Their load carrying capacity is limited because the tooth contact is a point, making them unsuitable for large power transmission. Attention to lubrication is important because tooth surface sliding is what transmits power.
A type of bevel gear with a speed ratio of one, miter gears are used to change power transmission direction without changing speed. There are two types: straight and spiral. Miter gears with any shaft angle other than 90 degrees are known as angular miter gears.
Worm gears consist of a worm, which is a screw shape cut on a shaft, and a worm wheel, which is the mating gear. In addition to a cylindrical shape, these gears are available in an hourglass shape that can increase the contact ratio. Friction has to be reduced because the gear surfaces slide against one another.
Internal gears have teeth cut on the inside of cones or cylinders and are combined with external gears. Their primary uses are in gear type shaft couplings and planetary gear drives. Internal and external gears in mesh have the same rotational directions.