What Is Aircraft Propeller Reduction Gearing?

Propellers are aerodynamic devices that change rotary motion from an airplane engine or other related power source into propulsive force, allowing the aircraft to traverse the sky with ease. They can be fastened directly on a crankshaft of a piston engine or powered through a reduction gear box (RGB) attached to a piston or jet engine. The RGB converts the high rotation speed of the engine into the appropriate speed for propeller operation.

The increased brake horsepower generated by high horsepower engines results partly from increased crankshaft rpm. As such, it is necessary to provide reduction gear to limit the propeller rotation speed to a set value that maintains efficiency. In cases where the speed of the blade tips surpasses the speed of sound, propeller efficiency is dramatically reduced. That being said, reduction gearing for engines enables the engine to operate at a higher rpm with ease.

It is important to note that reduction gearing allows the engine to develop more power while slowing down the propeller’s rpm, preventing the propeller’s overall efficiency from decreasing. Since reduction gearing must have the capacity to handle high stresses, the gears are usually made of steel forgings. More than that, there are three common types of reduction gearing systems, those of which include spur planetary, bevel planetary, and spur & pinion variations.

Spur planetary reduction gearing consists of a large driving gear or sun gear that has been splined or shrunk to the crankshaft, a large stationary gear called a bell gear, and a set of compact spur planetary pinion gears mounted on a carrier ring. The bell gear is bolted onto the front section of the housing. When the engine is in operation, the sun gear rotates. As the planetary gears and stationary gears are weaved in with the ring, they both rotate. At the same time, the propeller shaft also rotates in the same direction as the crankshaft, but at a reduced speed.

In some engines, the bell gear is mounted on the propeller shaft while the planetary pinion gear cage is kept stationary. The sun gear is splined to the crankshaft and serves as a driving gear. Such an arrangement produces a propeller that travels at a reduced speed in an opposition direction to the crankshaft. 

Within a bevel planetary reduction gearing system, the driving hear is machined with beveled external teeth and is affixed to the crankshaft. Meanwhile, a set of mating bevel pinion gears are mounted in a cage that is attached to the end of the propeller shaft. The pinion gears are powered by the drive gear and the stationary gear is bolted to the front section of the housing. The thrust of the bevel pinion gears is absorbed by a specially-machined thrust ball bearing. The drive and the fixed gears, on the other hand, are supported by heavy-duty ball bearings.

This type of planetary reduction assembly is far more compact than its counterparts. With this in mind, it can be utilized where a smaller propeller gear step-down is needed. In terms of gas turbine turboprop engines, multiple reduction gearing variations can be used to meet the demands of their high output speeds. Finally, many lower powered engine types can use the spur and pinion reduction gear arrangement.

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