What Is a Connecting Rod?

What Is a Connecting Rod?

A connecting rod is a vital part of an engine that transfers motion between the piston and crankshaft. It is under constant stress and subject to high forces during operation, so it requires careful design, manufacturing, operation, and monitoring for a long and incident-free service life. In order to perform effectively, it must be strong enough to transmit and convert motion efficiently but also light enough to avoid putting unnecessary stress on other engine parts. There are several types of connecting rods available, each made of different materials with unique properties to meet the requirements of specific applications. The most common material used for connecting rods is steel but aluminum and titanium are also widely utilized.

In a trunk piston engine, the connecting rod connects the piston's up-and-down motion to the crankshaft's rotational motion. In general, the connecting rod has bearings at both its big and small ends to reduce friction between it and the piston and cylinder wall during the stroke.

Typically, there is a pinhole in the connecting rod's big end so that lubricating oil can squirt out and lubricate the piston's rings, crankshaft, and bearings when the rod is in motion. If the rod is not properly lubricated, it can cause friction between the surfaces and lead to mechanical failure of the entire system.

The length of the connecting rod is determined by the length of the engine's stroke and engine power requirements. For example, a short-stroke engine requires shorter connecting rods to allow the piston to travel further down the cylinder and provide more low-end torque. Similarly, a longer-stroke engine requires longer connecting rods to allow the piston to travel farther up the cylinder.

There are several ways to manufacture connecting rods including investment casting, powder metal, and drop forging. Forged connecting rods offer the best strength-to-weight ratio and cost-effectiveness. They can be made from various metals including steel, nickel, and aluminum. Powdered metal connecting rods are less costly than forged rods but they do not have the same strength as forged rods. Investment casting is a cheap way to make connecting rods but the quality of the finished product can be compromised.

A connecting rod must be inspected, measured and reconditioned prior to assembly in order to ensure that it meets all design specifications. It must be straight, the center-to-center rod length must meet design specs, the connecting rod's big and small ends must be properly sized for crank pin and wrist pin fit, the rod must have proper oil clearance and each bore must be free of potential flaws. If a connecting rod fails, it can lead to severe damage to the engine and should be replaced as soon as possible. Symptoms of a failed connecting rod include low oil pressure and smoke from the engine. If you notice these symptoms, take your vehicle to a qualified mechanic as soon as possible.

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