Aircraft have very high-powered engines, and their performance is often a result of increased crankshaft revolutions per minute (rpm), as compared to older engine options. Consequently, aircraft often have reduction gears to limit propeller rotation speed so that there is efficient operation without the risk of damage. Whenever the speed of blade tips approaches the speed of sound, the efficiency of the propeller decreases. Therefore, reduction gearing can be used to both reduce wear and increase the efficiency of the engine. Overall, it allows the engine to operate at a higher rpm, developing more power while slowing down propeller rpm. To carry out this purpose, many types of reduction gearing are used alongside various forms of propeller shafts
Propeller Reduction Gearing
The first type of reduction gearing you may find in an aircraft engine is the spur planetary type. Combining spur gears
with a planetary alignment, this style of reduction gearing distributes the force of the rotor across several different gears. This system specifically consists of a large driving gear attached to the crankshaft, a large stationary gear, and a set of small spur planetary pinion gears mounted on a carrier ring. When the engine is operating, the sun gear rotates along with the planetary gears surrounding it. As a result, much of the rotational force is spread across many different gears, but ultimately with the planetary gears moving in the opposite direction of the central rotor. This can be very helpful for decreasing the speed of a propeller while maintaining its fluid motion.
Another common type of reduction gearing system is the bevel planetary reduction gearing system. In this device, the driving gear is designed with beveled external teeth and is attached to the crankshaft. At the end of the propeller shaft is a set of mating bevel pinion gears mounted in a connected cage. Additionally, the drive and the fixed gears are usually supported by heavy-duty ball bearings
. This type of planetary reduction assembly is more compact than the spur planetary reduction gear assembly. Therefore, it can be used where a smaller propeller gear step-down is desired.
Aside from using reduction gearing, aircraft engines may also use specially designed propeller shafts. The three major types available are tapered, splined, and flanged. As the name implies, tapered propeller shafts have a tapered end to connect to other parts in the assembly. Conversely, splined propeller shafts have slats or grooves which can help secure the assembly within a housing. Finally, flanged propeller shafts feature a flanged shaft-end with drilled holes to accept the propeller maintaining bolts. Each of these different shaft designs affect how they are fastened within an assembly. As such, they are also used in different applications. For example, flanged propeller shafts are used on most modern reciprocating and turboprop engines
due to their simple, yet secure, installation.
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