Pneumatic cylinders, also referred to as air cylinders, are important mechanical devices used to create force and motion in many industrial applications. Their design is pretty simple, featuring piston and piston rod, ports, barrel, and tie rod, which all work together to convert pressure energy (from compressed air) into linear motion.

Even with the simplicity of pneumatic cylinders, a lot can go wrong if engineers incorrectly specify or size air cylinders for a particular application. Oversized pneumatic cylinders cause increased air consumption and operational cost, whereas undersized cylinders cause low performance and pose significant operational safety risks.

This article presents important calculations and considerations for sizing pneumatic cylinders. But before delving into these calculations, it is important to first discuss the two common forms a pneumatic cylinder can take.

A double-acting pneumatic cylinder. Source: José-Loyer-72/Adobe Stock

Categorizing pneumatic cylinders according to function

Pneumatic cylinders can either be single-acting or double-acting.

In single-acting pneumatic cylinders, compressed air is only supplied to one side of the piston, causing the thrust (or output) force to be developed in one direction. These cylinders usually feature a single port and a mechanical spring (as shown in Figure 1) that returns the piston to its base position after the forward stroke.

Single-acting pneumatic cylinder returned by a spring. Source: Marton Kiss-Albert/CC [SA] [3.0]

Double-acting cylinders have two ports at opposite ends of the piston, allowing high-pressure compressed air to be supplied to chambers on both sides of the piston. When high-pressure air is directed to one of the chambers through the first port, it moves the piston rod forward (advance stroke). The retract stroke occurs when high-pressure air is supplied to the other chamber of the piston through the second port. This arrangement makes the double-acting cylinders ideal for pushing and pulling loads.

Operation of a double-acting cylinder. Source: Cdang/CC [SA][3.0]

Sizing pneumatic cylinders

A vital step to take when sizing cylinders is to determine the available force derived from the working pressure and the effective piston area. The total available force can be calculated using:

Where:

A = Effective piston area (m²)

P = Operating pressure (N/m²)

But due to system friction between the piston and the cylinder, the theoretically available (or actual) force is usually a fraction of the total available force. These losses are usually accounted for using an efficiency factor, n. The theoretically available force can be calculated using:

Where:

n = Efficiency factor

The effective piston area describes the surface area of the piston face where the pressure energy acts. Keep in mind that the formula for calculating this effective piston area differs according to the type of pneumatic cylinder and the type of stroke.

For the single-acting cylinder described earlier, the effective area can be calculated using:

Where:

D = cylinder diameter (m)

For the forward stroke of the double-acting cylinder, the effective piston area formula is exactly the same as with the single-acting cylinder. However, for the return stroke, engineers must realize that a portion of the piston face is covered by the piston rod, reducing the surface area that the pressure energy acts on. The effective area can then be calculated using:

Where:

D = cylinder diameter (m)

d = piston rod diameter (m)

So, consider a typical example where a double-acting pneumatic cylinder has an operating pressure of 6 bar and an efficiency factor of 0.90. If this cylinder has a diameter of 100 mm and a piston rod diameter of 20 mm, then the force on the advance stroke will be approximately 4241.7 N. In contrast, the force on the return stroke will be approximately 4072.03 N.

Cylinder air consumption

Another important parameter that product designers must determine is the cylinder’s air consumption. Knowing this value helps to estimate potential costs associated with a particular pneumatic cylinder design. Still considering the double-acting cylinder, the air consumption can be calculated using:

Where:

Q = Air consumption (l/min)

A_a = Effective piston area for advance stroke (cm²)

A_r = Effective piston area for return stroke (cm²)

s = piston stroke (cm)

n = cycles per minute

P_e = operating pressure (bar)

P_a = atmospheric pressure (bar)

Although this article presents useful information about cylinder sizing, product designers and engineers are advised to reach out to manufacturers and check several standards, including the ISO 15552 and ISO 6432 standards.