The Aircraft Reciprocating Engine: Connecting Rods

When you are operating or maintaining a reciprocating engine aircraft, one important component to be knowledgeable about is the connecting rod. The connecting rod is a component that is placed between the piston and crankshaft of the engine, allowing for forces to be transmitted between them. To operate efficiently, connecting rods need to have high strength so that they can withstand the loads that they are subjected to, as well as be light enough to mitigate inertia forces that result from the stopping of the rod and piston, the starting and ending of each stroke, and changing direction.

Types of Connecting Rods

In general, there are four types of connecting rods, those being plain, fork and blade, master and articulated, and split-type assemblies.

The plain type connecting rod is most often found in inline and opposed engines, and the end of one side of the rod is connected to a crankpin and fitted with a two-piece bearing and cap. To fasten the bearing cap to the rod, bolts and studs are used. When replacing plain connecting rods or conducting maintenance, one should always ensure that the rods are placed back within the same cylinder and at the same position as before.

Fork and blade connecting rods are typically utilized for V-type engine aircraft, and they are rarely found in modern type engines. Fork and blade connecting rods may be distinguished by a split at the crankpin, permitting for a blade rod to be placed between the prongs. To attach the crankshaft to the rod, fastening is provided through two-piece bearings.

On aircraft that feature a radial engine, a master-and-articulated rod assembly is most often used, and the crankshaft bearing utilizes a master rod in order to attach itself to one cylinder in each row of the engine. The other pistons of each row are also connected to the master rod as well, though through articulated rods. For example, if a radial engine has 18 cylinders with two rows, two master rods connect to two of the cylinders, and the other 16 cylinders are attached by 16 articulated rods. To ensure their integrity, articulated rods are composed of forged steel alloy, and they are shaped in either an I or H-shape to create a cross-sectional shaped assembly. To establish lock knuckle pin and piston pin bearings, bushings are implemented in the bores of each articulated rod.

With the master rod, the piston and crankpin can be linked together, and airulated rods are attached through flanges that are provided on the end of the crankpin. With a lock knuckle pin, the articulated rods are then attached to the master rod. If either a split-spline or split-clamp type crankshaft is used within the system, a one-piece master rod is implemented. If the crankshaft is a one-piece type, on the other hand, the larger end of the master rod and the master rod bearing are both split and the rod is installed on the crankpin. For the lock knuckle pins to operate efficiently, they are constructed solidly, albeit the oil passages that are for lubrication.

While each type of reciprocating engine type requires varying connecting rods, each assembly type provides the same benefit of power transfer from the piston to the crankshaft for operation. When it comes time to begin sourcing the connecting rod components and other aviation parts, let the experts at NSN Axis help you secure everything you need with ease. NSN Axis is a leading online distributor for aviation parts for both the civil and defense industries, and we provide customers access to over 2 billion new, used, and obsolete components. Get started on the purchasing process today and see how we can help fulfill all of your operational needs. 


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