Transmission
By Muhammad Zaim, Syazwan, Haslam and Muhammad Alif
iNTRODUCTION
Gear is a toothed wheel or disc, which meshes with another toothed wheel or disc to transmit motion.
Spur Gear
This is the most common type of gear tooth shape, and would be fitted in the accessory gearbox of an engine.
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The gear can be formed either internally or externally. An internal gear would be used where a change in speed is required without changing the axis of the drive. External spur gears are used where a change in speed is required but the shafts lie parallel to each other.
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Spur gears may be noisy owing to the impact of teeth upon other as they rotate.
Helical Gear
This is a smoother and Iess noisy running gear than the spur gear, the teeth are cut on a curve or helix, which produces a sliding engagement ofthe teeth, and more than one tooth is engagement at any one time.
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A disadvantage of this gear for is that it produces a heavy axial loads. This disadvantage can be eliminated by using double helical gears, with the teeth being cut in an opposite helix.
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An advantage of this type of gear is that it can accept and transmit a higher loading than a spur gear of the same size.
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Helical gears might be fitted in the reduction gearbox of a turbo prop engine or in the gearbox of an ordinary car.
Bevel Gear
This type of gear is used when the drive is required to be transmitted through an angle; in this case the gear teeth can be straight cut or in a helical form, when the axis of the shapes intersect.
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An example of use would be for transmission of drive from the main rotating assembly on a gas turbine to the accessory gearbox, or the tail rotor gearbox on a helicopter.
Worm Gear
This gear form is used where there is a large resistance to turning, and a large reduction in speed is required. The worm teeth are similar to a multi start thread, and are cut at an angle or on the skew, in which case the gear may be called a skew gear.
Planetary Gear
Planetary gear systems are typically used to reduce the propeller shaft speed on more powerful aircraft engines. This allows the engine to tum at a higher, economic rpm and develop more power. In a planetary gear system, the propeller mounts on a spider like cage that holds the planetary gears. These planetary gears rotate around a fixed central sun gear. some planetary gear systems, the sun gear is the drive gear and the ring gear is fixed in the nose section ofthe engine. In this situation, the planetary gears act as sample idler gears in the system
Hypoid Gear
This type of bevel gear, which is used where a change in speed is required, but the axis of the shafts do not intersect. This gear form is not normally used on gas turbines.