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Pneumatic systems are widely used for many reasons. They’re durable, clean, affordable, and fairly easy to install and maintain. They move loads in a variety of ways: pushing, pulling, lifting, lowering, and rotating. And they can handle widely varying payloads. While not ultra-precise in terms of positioning capabilities they are, nonetheless, accurate enough for countless applications.

However, the relative simplicity of pneumatic systems can be deceptive when it comes to selecting components. There are thousands of types, sizes, and variations of cylinders and valves, from off-the-shelf versions to custom designs. The sheer number of choices can be overwhelming, especially when options such as sensors are added to the mix.

But taking the time to choose the right components for a job helps ensure good performance, lower expenses, improve cycle rates, and prolong equipment life. This article examines the parameters – load, force factor, speed, and sequencing, as well as the impact of other components – that engineers should take into account when selecting a cylinder for a pneumatic system.

Cylinder types

Although there are many types of cylinders, their construction is fairly similar from one to another. Basically, a cylinder is a sealed tube. It contains a rod, attached to a piston, that extends through an opening at one end. Compressed air enters through a port at one end of the cylinder, causing the piston rod to move. At the other end, a second port lets air escape. Understanding the basics helps to show how different applications affect the cylinder and piston rod.

Pneumatic cylinder design factors, Fig. 1

The first step in choosing a cylinder is deciding whether to use the single- or double-acting version. As the name implies, single-acting cylinders use compressed air to move the load in one direction, such as lifting an object. With single-acting cylinders, air is supplied to only one side of the piston, while the other side vents the air to the environment. A spring (or, in some cases, gravity) returns the piston to its original position once air pressure is removed.

A double-acting cylinder uses compressed air to power the rod in both directions and move a load, such as opening and closing a gate. This type of cylinder uses more energy, but it’s well suited for loads that require both pushing and pulling.

However, force calculations can get complicated. In single-acting cylinders with a spring, the spring force opposing the push or pull increases as the stroke progresses. And in double-acting cylinders, push and pull forces are not equal, as designers must account for the rod area in making force calculations. Manufacturers’ catalogs often list push and pull values for both double-acting and single-acting cylinders, with and without springs, simplifying calculations for users.

Load and speed

The load is the primary consideration when determining cylinder type and piston size. The piston area (force factor) multiplied by the air pressure in the cylinder gives the available force. A general rule is to select a force factor that will produce a force 25% greater than the load to help compensate for friction and losses. Pneumatic systems are quite forgiving in terms of oversizing, but using components that are too big adds unnecessary expenses in terms of both purchase price and energy consumption.

The bore size (force factor) determines force at a given pressure. The operating pressure, which in a plant can typically range from range from 10 to 150 psi, is the first consideration when selecting a bore size.

The next step in choosing the bore size is the amount of force that the application requires. Suppliers often provide charts to assist with calculating bore size. If the bore diameter is between sizes, fluid-power experts recommend rounding up to the next size.

It’s also important to remember the bore diameter squares the thrust delivered. For example, a two-inch diameter cylinder has four times the power of a one-inch diameter unit. Therefore, doubling the bore quadruples the thrust.

In addition to load, designers must also take into account the speed at which the load will move. When compressed air flows through a system, there are pressure losses due to friction against the tube wall, flow around bends, and restrictions in valves and fittings (to name a few issues). Higher speeds result in greater pressure loss as the air must flow faster through the valves, tubing, and ports. Attaining higher speeds also requires that the cylinder deliver more force in a shorter amount of time. A force that exceeds the load by 50% or more may be required to reliably move a load at high speeds.

For example, a typical air compressor might supply air to a system at 100 psi. In an application with a slow-moving load, the actual pressure available at the piston might be reduced to no less than 90 psi. With that same load moving at a much faster rate, the available pressure could drop as low as 70 psi.

Pressure losses can be remedied by increasing pressure, but this must be done with caution: Too much pressure creates stress on the cylinder and could possibly damage the cylinder, as well as the load. In these instances, it’s better to go with a larger cylinder. Also keep in mind that raising system pressure means the compressor must work harder, increasing energy consumption of the overall pneumatic system.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.