What Is a Crankshaft in a Reciprocating Compressor? - Samco

Reciprocating compressors are positive displacement machines that compress air or gas using pistons within a cylinder. These machines are used extensively in chemical, petrochemical, and gas industrial processes. They are made of several components, each of which has considerable roles to play in the general function of the reciprocating compressor.

Some of these compressors help with the following:

  • Assist in rotation
  • Narrow gas flow
  • Manage heat
  • Enhance the durability of the machine
  • Enable other components to perform their tasks optimally and efficiently

However, this article covers the overall function of the crankshaft in a reciprocating compressor. A few other parts of reciprocating compressors will also be discussed briefly.

What is a Crankshaft?

The crankshaft. Otherwise known as the con rod, it is often referred to as the main shaft of a reciprocating compressor. The piston cannot perform any reciprocating motion within the cylinder without the rotary action of the crankshaft. 

One side of the crankshaft connects directly to the electric motor via the belt and pulley arrangement or coupling. As the motor shaft rotates, the crankshaft rotates accordingly. 

The other side of the crankshaft is connected to the connecting rod, which is connected to the piston at the other end. The rotary motion of a compressor’s crankshaft is readily converted into the reciprocating motion of the piston via the connecting rod.

Compressors with multi-cylinders do not matter. The number of connecting rods that is connected to the compressor’s crankshaft is the same as the number of cylinders.

Crankshafts in reciprocating compressors are mostly made from forged steel. They are found in large reciprocating compressors operating above 200 horsepower or 150 kW. However, a crankshaft made from ductile iron is used for a reciprocating machine that works with less than or equal to 200 horsepower.

The crankshaft in a reciprocating compressor is often subjected to enormous stresses. As a result, crankshafts for single-cylinder reciprocating compressors usually have simpler designs than those for machines with multiple cylinders.

Other Crucial Reciprocating Compressor Components That Help the Crankshaft Perform Its Duties

The crankshaft in a reciprocating compressor cannot readily function independently without the assistance of other components. Here are some of the components that assist the crankshaft in performing its duty:

  • Connecting Rod

The connecting rod – otherwise known as a con rod – is part of the piston engine connecting the piston to the crankshaft. Connecting rods are usually made from ductile iron or forged steel material, depending significantly on the reciprocating compressor’s power. 

The type of material used in constructing the connecting rod in a reciprocating compressor is also determined by weight as well as compatibility with the gas undergoing compression.

For instance, connecting rods made from forged steel are used in machines that operate above 200 horsepower (150 kW). But ductile iron connecting rods are used extensively in reciprocating compressors that function optimally at 150 kW or even less.

One side of a connecting rod is linked to the crankshaft using a connecting cap rod. The other side of the connecting rod is connected to the piston using a piston pin.

Although these connections sound straightforward, they are required in order to give the crankshaft its distinct rotary motion. The latter also helps the piston to translate within the cylinder.

In other words, the connecting rod in a compressor changes the rotary motion to reciprocating motion.

  • Piston 

The reciprocating compressor’s mode of operation relies heavily on the piston. The piston is the component that actually compresses the gas or air passing through the machine. This compressor component is also responsible for moving the energy from the crankcase to the gas or air within the cylinder with the primary goal of preventing the leakage of refrigerant through the gap.

This component is usually covered with a piston ring between the cylinder walls and the piston. The piston is usually found at the end of the piston rod. It acts as a movable barrier within the compressor cylinder.

Selecting the material the piston is made of depends significantly on its compatibility with the gas to be compressed, its strength, and its weight. However, in most cases, the piston in a reciprocating compressor is constructed with lightweight material such as cast iron, steel, or aluminum. In addition, it comes with a hollow center for considerable weight reduction.

  • Piston Rings

Piston rings are usually wound around the piston in a reciprocating compressor. As the piston moves up and down within the cylinder, the piston rings come in direct contact with the cylinder walls. 

During this reciprocating action, a considerable amount of friction is generated. This is why the piston rings require replacement from time to time, as they tend to wear off within a short period and maintain the smooth operation of the reciprocating compressor.

In many cases, a rider ring or wear band is also included as an additional piston ring. The primary function of the rider ring or wear band is to significantly minimize the probability of wear and tear occurring between the piston and cylinder.

Although the piston rings have to be somewhat softer than the wall of the cylinder, different types of rings are employed for the piston. Some piston rings are made of metal, while others are made from non-metallic materials.

  • Cylinders

Cylinders are the pressure vessels within a reciprocating compressor that contains the gas or air to be compressed. They are one of the vital components of this machine and are available in 2 major variations:

  1. Single-acting cylinders
  2. Double-acting cylinders

Single-acting cylinders generally compress gas or air only in one direction of piston travel. They can be crank head or head end. On the other hand, double-acting cylinders usually compress gas or air in both directions of piston travel. Most reciprocating compressors in active operation use double-acting cylinders.


The crankshaft is known as the main shaft of a reciprocating compressor. Without the crankshaft, the piston cannot perform any reciprocating motion within the cylinder of the compressor. This makes the crankshaft one of the most vital components of a reciprocating compressor.

Other components that contribute to the smooth operation of a reciprocating compressor include the cylinder, piston, valves, etc. All these work conjointly to ensure the compressor gives the desired output each time it is operated, provided every other factor has been measured and remains constant.

From industry standards like Ariel compressor parts to Ingersoll Rand and Worthington compressor parts, we carry compressor parts like valves, pistons, cylinders, and more that you can count on.