What Is Connecting Rod in an Aircraft?

For the forward movement of an engine, the reciprocating motion of a piston must be converted into the rotation of the crankshaft. To perform this particular task, you need a connecting rod. A connecting rod is an integral part of the engine, forming the connection between the piston and crankshaft.

The connecting rod is a crucial part of the engine assembly, and the smaller part of the component connects with the piston while the big end connects the crankpin. With this setup, the connecting rod transmits force between the two components of an aircraft.

The connecting rod is built using forged steel and aluminum alloy, and they consist of an I-beam cross-section. The materials have a similar weight for maintaining the engine balance. The lighter the weight of a connecting rod and piston bearing, the more power they can provide. Additionally, a lower weight assembly also leads to less vibration as the reciprocating weight is less. With the implementation of a connecting rod, the linear motion of the piston is easily transformed into a rotational motion sourcing from the crankshaft.

Types Of Connecting Rods

There are various types of connecting rods used in many engines, and these include the:

Plain rod - The plain connecting rod has a big end connected to the crankpin and is fitted with a bearing cap. This type of connecting rod is used in opposed and inline engines.
Fork and blade rod - The fork and blade connecting rod is used in V12 aircraft engines. In each pair of engine cylinders, the fork is split into two parts, and a blade rod is tapered from the opposing cylinder to fit the gap in the fork.
Master and slave rod – Master and slave connecting rods are usually employed in radial engines. One piston consists of the master rod directly attached to the crankshaft bearing, and another piston joined with their connecting rods to the edge rings of the master rod.
Billet connecting rod – Compared to others, a Billet connecting rod is lightweight and robust with a long lifespan. These connecting rods are made up of aluminum and steel.
Cast Rod –These connecting rods are preferably used and designed for handling the load of the stock engine.

Construction and Function Of Connecting Rods

A connecting rod may feature either small end or big end bearings. The big end is split into right angles at its length so that it may be assembled on a crankpin. The body of the connecting rod is provided with a cap using two bolts and nuts. Modern aircraft engines use low-carbon steel bearing shells instead of metal fused to the bore.

Though shell bearings require extra adjustments specifically for wear, they provide better control over running and side clearance, allowing the bearing cap to fit correctly. In some instances, shims, which are thin pieces of metals, are also used when spur bearings are employed. This filled thinner works as compensation for reducing the wear of the bearing while providing enough clearance between the crankshaft and connecting rod.

To reduce vibrations during operation, the weight of the connecting rod must be equal. Therefore, connecting rods and caps are matched separately and are provided with identifying numbers to avoid any mismatch if the engine is disassembled for service.

Conclusion

In a reciprocating engine, the function of the connecting rod is crucial. It is a connection that helps transmit forces between the piston and the crankshaft. This blog taught us four significant types of connecting rod assemblies: master-and-slave rods, articulated rods, plain rods, and fork and blade rods.

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