Copyright © Shindary Automotive Parts Co., Ltd. All Rights Reserved
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft.
The crankshaft connecting rod system is a mechanical system that converts the linear motion of a piston into the rotary motion of a crankshaft:
Connecting rod: Also called a "con rod", this part connects the piston to the crankshaft. It has two ends, the foot and the head, which move with the piston and crank pin, respectively. The connecting rod transmits compressive and tensile forces from the piston.
Crankshaft: A rotating element that receives circular motion from the connecting rod.
Crank: An arm attached to the crankshaft that receives or imparts circular motion.
The crankshaft connecting rod system is used in many technological applications, including: Internal combustion engines, Pumps and compressors, and Steam engines.
Connecting rods are highly stressed components in an engine, and are exposed to extreme heat, pressure, and bending. Standard connecting rods may not be suitable for tuned cars, so high-performance connecting rods made from stronger materials are used.
One of the primary reasons for connecting rod failure is improper bearing clearance. The rod bearing, located between the connecting rod and crankshaft, plays a critical role in reducing friction and supporting the load from combustion forces. It consists of an upper half attached to the big end of the rod and a lower half fastened to the rod bearing cap.
If the clearance between the bearing and crankshaft journal is too tight, it can result in insufficient lubrication and excessive friction. This can lead to the rod bearing making direct contact with the crank journal, potentially causing the bearing to spin, generating heat, and damaging the rod’s big end—both of which can have catastrophic effects on the engine.
On the other hand, excessive clearance allows the crankshaft to expel too much oil from the space between the bearing and crank journal, resulting in a drop in oil pressure. This lack of lubrication causes metal-on-metal contact, accelerating wear and increasing the risk of bearing spin, which can severely damage or destroy both the rod and crankshaft.
To prevent such issues, it's essential to use precise measuring techniques and follow manufacturer specifications during engine assembly to ensure proper bearing clearances. Regular maintenance and vigilance for warning signs like rod knock or low oil pressure indicators on the dashboard can also help prevent rod bearing failures.
Rod bolts are crucial for securing the connecting rod cap to the rod, maintaining the necessary clamping force even under extreme engine conditions. Often regarded as the most critical fasteners in the engine, rod bolts endure immense stress from the reciprocating load created by the motion of the piston and connecting rod.
Incorrect torque specifications during assembly can lead to insufficient clamping force. This can cause rod bolts to overstretch or loosen over time, eventually losing their grip on the rod and cap. As a result, the rod cap may separate from the rod, or excessive movement may occur. In the worst-case scenario, rod bolt failure could cause the connecting rod to break free, potentially puncturing the engine block, leading to a catastrophic loss of oil pressure, engine overheating, and eventual engine seizure.
Selecting bolts that aren’t suitable for the engine's power output or operating conditions also poses a significant risk. Unlike standard bolts found in everyday vehicles, high-performance engines require stronger, high-strength bolts capable of withstanding greater loads and vibrations. These specialized bolts are typically made from ultra-strong materials and are often coated to improve durability and fatigue resistance.
To avoid these issues, it’s essential to always follow the manufacturer’s specified torque settings and choose rod bolts appropriate for the engine’s horsepower, RPM, and intended use.
Engine detonation, often referred to as knock or pinging, can have severe consequences on connecting rods. Detonation occurs when the fuel-air mixture in the combustion chamber ignites prematurely, before the spark plug fires, resulting in an uncontrolled explosion. This abrupt release of energy generates a shockwave that puts extreme pressure on the connecting rods and other engine components.
Detonation can be triggered by several factors, including lean air-fuel mixtures, high compression ratios, overheating, or the use of low-octane fuel. In high-performance and racing engines, aggressive ignition timing and insufficient fuel enrichment are common culprits of detonation.
When detonation occurs, the connecting rods are subjected to intense mechanical stress due to the rapid spike in pressure. This can lead to rod bending, breaking, or even bearing and crankshaft damage. The excessive heat generated during detonation can also melt or crack piston rings, further contributing to rod failure.
To prevent detonation, measures such as retarding the ignition timing, enriching the fuel mixture, and using higher-octane fuel can be effective. Additionally, the choice of materials for the connecting rods and rod bearings plays a vital role in withstanding the shock and stress from detonation, so it’s important to use components designed to handle such extreme conditions.
Hydrolocking occurs when a cylinder fills with liquid, preventing the piston from completing its upward stroke. This can happen because of a head gasket failure, where coolant leaks into the cylinder, or a cracked or stuck open fuel injector, which causes excessive fuel to flood the cylinder. Then, when the piston tries to compress the liquid, the connecting rod may bend or fracture due to the sudden increase in resistance.
Hydrolocking places immense stress on the connecting rods and other engine components, potentially leading to severe or even permanent damage if not addressed promptly. A broken connecting rod, for instance, can detach and puncture a hole in the engine block. To help prevent hydrolocking, it's essential to regularly inspect your head gaskets and fuel injectors, monitor coolant levels, and keep an eye on engine performance.
Choosing the wrong type of connecting rod can seriously jeopardize your engine build, as engines vary in power output, RPM ranges, and the stress they place on the rods. For example, using lightweight rods designed for lower horsepower in a high-performance engine producing 2,500 horsepower and running at 50 psi of boost can quickly lead to failure. Lightweight rods may not be able to withstand the higher stress and vibrations, resulting in bending or breaking under extreme conditions.
Conversely, using overly heavy rods increases the reciprocating mass, which can negatively impact engine response and cause fatigue failure over time. When building an engine, it’s essential to match the connecting rod specifications to the engine’s power output, RPM range, and intended use. This ensures both durability and optimal performance.
Shindary Automotive Parts Co., Ltd.
