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Everything You Need to Know About the Connecting Rod

If you have wondered how connecting rods, otherwise known as con rods, work, their primary functions, or even how it works in a reciprocating compressor, you’re on the right page. Here’s¬†everything you should know about the connecting rod.¬†

Everything You Need to Know About the Connecting Rod - KB Delta

A connecting rod is a rod linking the crankshaft and the piston. The rod’s big end connects to the crank pin, while the small end connects with the piston pin.

The primary purpose of a connecting rod in a reciprocating compressor is to convert the piston’s linear motion into the rotary motion of the crankshaft.

Connecting rods are generally made of forged steel and aluminum alloy and consist of an I-beam cross-section. This component is often precisely matched in sets of similar weights as it helps maintain the balance of the reciprocating compressor.

However, the lighter the piston and connecting rod, the greater or more profound the resulting power they generate, and the lesser the vibration since the reciprocating weight is also less.

Connecting rods transmit the power thrust from the piston to the crankpin. This is why they must be solid, lightweight, and rigid.


Types of Connecting Rods

Connecting rods are available in different types, depending on the types of engines that require them.

Here are the types of connecting rods you should know:


  • Plain Type Connecting Rod

This connecting rod is employed in opposed and inline engines. Its big end is usually connected to the crankpin and fitted exceptionally well with a bearing cap.

This bearing cap is mounted by a stud or bold at the end of the con rod. It must be replaced in the same relative position and cylinder for proper balance and fit.


  • Forged Connecting Rods

Forging is the process used in creating this connecting rod. It is constructed when a grain of the material is forced into the shape of the con rod’s end. These materials may be aluminum or steel alloy.

Nickel and chrome alloys are some of the well-known steel alloys that help enhance the strength of these connecting rods.


  • Powdered-Metal Connecting Rod

Powdered metal is also used in making con rods. This connecting rod is manufactured by preparing a metal powdered mixture pressed into a mold and heated to an extremely high temperature.

The finished product may require light machining, though it mostly comes out as a completed, finished product mold. Powdered metal connecting rods are less expensive than their steel counterparts but stronger than cast con rods.

Other types of connecting rods include:


  • Master and Slave Con Rods
  • Cast Connecting Rods
  • Billet Con Rods
  • Fork and Blade Connecting Rods


The Different Parts of a Connecting Rod

A connecting rod is generally composed of several parts. Here they are in no particular order:


  1. Small end
  2. Big end
  3. Piston
  4. Bearing inserts
  5. Bush Bearing
  6. Wrist pin
  7. Bearing cap
  8. Shank
  9. Bolt and nut


Here’s a closer look at each part of the connecting rod:


  • Small End

This part of the connecting rod is usually connected to the face of the piston pin.


  • Big End

This part of the connecting rod is usually connected to the side of the crank pin.


  • Piston

The piston is connected to the crankshaft via the connecting rod. The piston’s purpose is to function as a movable plug within the cylinder that forms the bottom of the combustion chamber.


  • Bearing Inserts

The bearing insert is the section connected to the bearing cap via the big end of the con rod. Bearing inserts come in 2 parts that fit seamlessly together on the crankshaft.


  • Bush Bearing

The connecting rod is connected at both ends with bush bearings. A phosphor bronze bush fitted with a solid eye is attached to the small end of the con rod.

The big end of the con rod is attached to the crankpin. The end is divided into two parts and supported over the crank-bearing shell.


  • Wrist Pin

The wrist pin, also called a gudgeon pin, is a hollow, hardened steel tube connecting the piston to the connecting rod. This component passes through the short end of the pivots and connecting rod on the engaged piston.


  • Shank

Different nuts and bolts connect the bearing cap and connecting rod. However, the applied shank can be tubular, rectangular, or circular.


  • Bolt and Nut

When a connecting rod is fitted with the crank at the bottom, both sides of the con rod’s big ends are fastened by bolts and nuts.


Faults and Common Issues: Fatigue, Pin Failure, and Hydrolock

Most connecting rods often undergo repetitive and large forces during every crankshaft rotation. These forces are directly proportional to the engine’s speed, i.e., revolutions per minute (RPM).

Since connecting rods work practically non-stop in the crankshaft, they tend to break or get severely damaged. Some faults connecting rods suffer from fatigue, pin failure, and hydrolock.


  • Fatigue

Fatigue is the most common defect connecting rods suffer from. This is because the stretch and compression of con rods occur all the time during operation. This causes the component to wear down over time until it eventually breaks.

The presence of dirt and insufficient lubricating oil can worsen this con rod problem.


  • Pin Failure

The piston pin is also prone to damage, resulting in catastrophic engine failure. This situation usually occurs when the crankshaft is bent or when the con rod moves into the engine block.

Pin failure can cause severe power loss in some engines as the latter can stop functioning suddenly when the pin breaks.


  • Hydrolock

Hydrolock can occur when water unexpectedly enters the piston chamber. This can cause the con rod to deform.

Connecting rods convert the reciprocation motion of pistons in response to the rotation of the crankshaft. They are used in several engines and are one of the most vital components in reciprocating compressors.



Connecting rods convert the reciprocation motion of pistons in response to the rotation of the crankshaft. They are used in several engines and are one of the most vital components in reciprocating compressors.

No engine can function optimally without a connecting rod. It is one of the most vital components in a reciprocating compressor. It helps minimize component wear as well as the alternating mass for safety.


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