What is the Crankshaft of an Aircraft Engine?
For the reciprocating engine of an aircraft to optimally transfer power generated by the internal combustion engine to the propeller assembly, an apparatus known as the crankshaft is used. As an essential element of the power transmission system, crankshafts and their connecting rod convert the linear movement of a firing piston into rotational motion that is used to drive the propeller assembly. In this blog, we will discuss the design and functionality of the crankshaft, allowing you to better understand its use and importance in the realm of aviation.
Components of a Crankshaft
A typical crankshaft consists of numerous parts that come together to establish the assembly, and these parts include crankpin components, crank webs, balancing weights, and main journals. Generally, the connecting rod will be attached to the crankshaft’s crankpin by its large end. Upon every stroke of the piston, the center-to-center distance exhibited between the crankshaft and crankpin will be half of the piston’s displacement. As a result, two strokes of the piston are needed for one full complete revolution of the crankshaft.
As discussed before, there are a variety of parts that come together to form the crankshaft, and each holds its own unique role in standard operations. The crankpin serves as a mechanical part of the engine, implemented to firmly attach the connecting rod to the crankshaft. As the crankpin surface is cylindrical, rotational force is supplied to the connecting rod. Oftentimes, the crankpin is referred to as a connecting rod journal.
The main journals of the assembly are implemented on the engine block. Journals serve to secure the crankshaft in place while allowing for it to rotate within the engine block. Depending on the assembly, the journal bearings may also be plain bearing types, the choice differing between engine to engine based on the forces that are exhibited during standard operations. The crankweb is an essential part of the crankshaft as a whole, used to connect the crankshaft with all main bearing journals.
Importance of Counterweights
In order to ensure more stability and balance for the crankshaft during its rotation, counterweights are used to apply opposing force. Generally, the counterweights are mounted onto the crank web, making them advantageous for eliminating the reaction that results from crankshaft rotation. Beyond balancing the assembly, counterweights also have the benefit of permitting higher RPM operations for easier engine functionality.
It is important that the crankshaft is able to rotate without moving lengthwise, and thus thrust washers are placed in the assembly. Using two or more washers, such components are assembled along the machined surfaces of the crank web and crankshaft saddle. Similar to how they are used in other assemblies and applications, the thrust washers of crankshafts remove unnecessary gaps so that the crankshaft is secured in place. Oftentimes, modern assemblies will feature thrust washers that are pre-built into the main bearings, while older crankshafts will feature both as separate elements.
As lubrication is useful for the functionality and health of various crankshaft components, oil passage and oil seals are placed to permit oil movement from the main bearing journals to the big end journals. As the crankpin is facing upward and combustion forces push the connecting rod down, oil will be permitted to move between the journal and bearing components. Seals are also crucial for standard operations, ensuring that oil does not leak from the ends of the crankshaft. Generally, oil seals come in the form of front-end and rear-end types.
The final major component of standard crankshafts is the flywheel mounting flange. In many assemblies, the crankshaft is connected to the flywheel with the use of flanges. Commonly, the crankshaft wheel end will feature a larger diameter as compared to the other end, allowing for a face to be provided for the flywheel.
Conclusion
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