Connecting Rod: Parts, Function, and Types

What is Connecting Rod?

A connecting rod is the part of a piston engine that 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 connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an internal combustion engine, it allows pivoting on the piston end and rotation on the shaft end.

The predecessor to the connecting rod is a mechanical linkage used by water mills to convert the rotating motion of the water wheel into reciprocating motion. The most common usage of connecting rods is in internal combustion engines or on steam engines.

Parts of Connecting Rod

Following are the parts of connecting rod:

Parts of Connecting Rod
  • Small End: The end at which the connecting rod is attached to the face of the piston pin is known as the small end of the connecting rod.
  • Big End: The end at which the connecting rod is attached to the side of the crank pin is known as big end of the connecting rod.
  • Bush Bearing: Both ends of the connecting rod are fixed with a bush bearing. A phosphor bronze bush is fitted with the solid eye is attached to the small end of the connecting rod. The Big end is attached to the crankpin. The end is divided into two parts and is supported over the crank bearing shell.
  • Bearing Insert: In the big end of the connecting rod, there is a bearing insert that is connected to the bearing cap, it is known as a bearing insert. These are made in two parts that fit together on the crankshaft. This is the position where the connecting rod travels along the reverse direction.
  • Bolt and Nut: After the connecting rod is fitted with the crank at the bottom, both sides of the big ends are fastened by some bolts and nuts. Thus, by combining these all components the connecting rod is ready to use.
  • Shank: Furthermore, each of the bolt and nuts are employed to connect both the connecting rod and bearing cap. And a section beam is applied it is known as shank. The section of the rod may be rectangular, tubular, and a circular section.
  • Wrist Pin: The engine piston is connected to the connecting rod with the help of a hollow hardened steel tube called wrist pin. It is also known as gudgeon pin. Wrist pin goes through the short end of the connecting rod and pivots on the engaged piston.
  • Piston: The piston is connected to the crankshaft with the help of a connecting rod, which is usually shortened to the rod or Conrod. The purpose of the piston is to work as a movable plug in the cylinder, which forms the bottom of the combustion chamber.
  • Bearing Cap: Shell bearings have an adjustment for wear, but it controls the running and the side clearance allows the bearing cap to be tightened correctly.

Construction and Function of Connecting Rod

The connecting rod converts the linear up and down movement of the piston into the circular motion of the crankshaft and is therefore subject to tension, compression, bending, and buckling.

Engineering Choice | The biggest engineering Blogs

The connecting rod creates the link between the piston and crankshaft and thus transmits power. It converts the linear up and down movement of the piston into the circular motion of the crankshaft and is therefore subject to tension, compression, bending, and buckling.

The connecting rod is mounted on the crankpin of the crankshaft with a plain bearing. The connecting rod bearing cap is bolted to the big end. In most cases, the connecting rod is hollow or provided with an internal cast oil duct to supply the gudgeon pin with lubricant.

connecting rod

Materials

To obtain minimal weight and high strength, connecting rods are made of the following materials:

  • Micro alloyed steels
  • Sintered metals
  • High-grade aluminum
  • CFRP and titanium (for high-performance engines)

Mass-produced connecting rods are forged, cast or sintered. Forged connecting rods exhibit a better strength-to-weight ratio and lower costs than sintered connecting rods. Die manufacture is however comparatively expensive.

Types of Connecting Rod

Following are the types of connecting rods, used in various types of engines:

  • Plain type rod
  • Fork and blade rod
  • Master and slave rod
  • Billet conrods
  • Cast rods
  • Forged rods
  • Powered metal conrods

1. Plain Type Rods

The plain type of connecting rod is used in inline and opposed engines. The big end of the connecting rod is attached to the crankpin and fitted with a bearing cap.

The bearing cap is mounted by a bolt or stud at the end of the connecting rod. The connecting rod must be replaced in the same cylinder and in the same relative position to maintain proper fit and balance.

2. Fork and Blade Rods

These types of connecting rods are used on V-twin motorcycle engines and V12 aircraft engines. In each pair of engine cylinders, a “fork” rod is divided into two parts at the big end and a “blade” rod is tapered from the opposing cylinder to fit this gap in the fork.

This system eliminates the rocking couple that occurs when the cylinder pairs are balanced along with the crankshaft.

In the big-end bearings type of arrangement, the fork rod has a single wide-bearing sleeve that extends over the entire width of the rod, including the central gap.

The blade rod then runs directly outside this sleeve, not on the crankpin. This causes the two rods to move back and forth, this reducing the force on the bearing and the surface speed. But, the bearing speed also reciprocates instead of continuously rotating, which is a major problem for lubrication.

3. Master and Slave Rods

Radial engines typically use master-and-slave connecting rods. In this system, the one piston consists of a master rod with a direct attachment to the crankshaft. Other pistons connect their connecting rods to the rings surrounding the edge of the master rod.

The disadvantage of master-slave rods is that the stroke of the slave piston is slightly larger than that of the master piston, which increases the vibration in the V-type engine.

4. Billet rods

Billet connecting rods are designed from steel or aluminum. Compared to other types of connecting rods, they are lighter, stronger, and longer in lifespan.

It is commonly used in high-speed vehicles. It is sometimes designed to reduce stress risers and ease into the natural grain of the billet material.

5. Cast Rods

These types of connecting rods are preferred and designed by manufacturers because they can capable of handling the load of a stock engine.

Cast rods require low cost to produce and cannot be used in applications of high horsepower. The cast rods have a noticeable seam in the middle that separates them from the forged type.

6. Forged Rods

Some of the connecting rods are manufacture by forging. These types of connecting rods are made by forcing a grain of material to the shape of the end. Depending on the required properties the material may be steel alloy or aluminum.

Commonly used steel alloys are chrome and nickel alloy. The end product is not designed to be brittle. Hence, nickel or chrome alloys increase the strength of the connecting rod.

7. Powered Metal Conrods

Connecting rods are also designed from power metal as it is a suitable choice for manufacturers. It is prepared with a metal powder mixture that is pressed into the mold and heated to a high temperature. This mixture is made into a solid form.

It may require light machining but the product basically comes out of a finished product mold. Conrods of powder metal are less costly than steel and they are stronger than cast rods.

Failure of Connecting Rod

During each rotation of the crankshaft, a connecting rod is often subject to large and repetitive forces: shear forces due to the angle between the piston and the crankpin, compression forces as the piston moves downwards, and tensile forces as the piston moving upwards. These forces are proportional to the engine speed (RPM) squared.

Failure of a connecting rod often called “throwing a rod”, is one of the most common causes of catastrophic engine failure in cars, frequently driving the broken rod through the side of the crankcase and thereby rendering the engine irreparable.

Common causes of connecting rod failure are tensile failure from high engine speeds, the impact force when the piston hits a valve (due to a valvetrain problem), rod bearing failure usually due to a lubrication problem, or incorrect installation of the connecting rod.