Glossary: Connecting Rod

What is a Connecting Rod

A connecting rod is a rod which interconnects the piston and the crankshaft and transmits the gas forces from the piston to the crankshaft. The two ends of the connecting rod are called as small end and the big end. Small end is connected to the piston with the help of the piston pin and the big end is connected to the crankshaft with the help of a crank pin.

What are the Connecting Rod’s functions?

1) Fuel is combusted during the power stroke in the presence of air. The combustion of the charge(fuel and air) exerts a PRESSURE on the surface of the piston. This gas PRESSURE is transmitted to the crankshaft by means of the connecting rod.

2) It converts the reciprocating(up and down) motion of the piston into rotational motion of the crankshaft.

3) It houses the wrist pin bushes and crankshaft bearings.

What are the forces acting on the Connecting Rod?

1) forces on the piston due to gas pressure and inertia force which is transmitted to the connecting rod
2) force due to the inertia of the connecting rod or the inertia bending forces
3) force due to the friction of piston rings and of the piston
4) force due to the friction of the bearings

What are the design considerations when a Connecting Rod is designed?

1) The con rod has to be kept AS LIGHT AS POSSIBLE

This is done to reduce vibrations and inertia loading on the bearings and further to reduce frictional losses. By keeping the parts lighter in weight, the vibrations can be reduced which translates into smoother engine operation. The frictional losses are caused due to the contact of moving parts. The connecting rod has to be designed with a suitable material and has to be light to reduce the load acting on the bearings and lessen friction between the parts. The strength and rigidity of the con rod must be as high as possible and shouldn’t be compromised for reduction in weight. There should be weight consistency in the connecting rod to facilitate engine balancing.

2) Con rod must withstand mechanical and thermal loads WITHOUT MAJOR CHANGES IN DIMENSION or PROPERTIES

The con rod is under tremendous stress from the reciprocating load of the piston, expanding and contracting with every rotation, and this phenomenon increases with increase in engine speed. The material used should have high fatigue strength and rigidity. Coating of the con rod is done to increase fatigue life.

6) The material used for the con rod should not REACT with the lubricating oil at high temperatures. The lubricating oil has a tendency to break down and form carbon deposits at elevated temperatures which must be minimized as much as possible.

7) Other factors like maximum rpm, maximum torque, bore size, stroke length, type of fuel used, the expected lifespan (in hours) are considered before the designing process begins.

What are the materials used for making Con-Rods?

Almost all the con-rods found in modern engines are made up of steel. Aluminium alloy is also used in some engines. Titanium alloy is used in race car engines due to its superior properties.