Title: Basic Cylinder Circuits, Double-Acting Cylinders
Course: Fluid Power Pneumatics (Sandbox) > Section: Module 5: Breaking Down a Pneumatic System / Work Side - 10 Hours

Cylinders in Circuits

In this module, we will describe how cylinders are used in different circuits. A pneumatic cylinder is an actuator that converts air power into mechanical power in a straight-line motion. Here is an example of a cylinder that we will use on our trainers.

The following video shows pneumatic cylinders in action. Notice how one pneumatic cylinder pushes the part under the press, then the pneumatic press flattens the part, and then another pneumatic cylinder pushes the part out of the press.

The Double-Acting Cylinder

A pneumatic cylinder consists of a piston/rod assembly that can move inside a barrel-shaped body. The most common type of cylinder is a double-acting cylinder. This type has two ports through which air can enter. When air flows into the cap end, the cylinder rod extends. The cylinder rod retracts when air flows into the rod end.

Look at the following diagram:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

Here is the schematic symbol for a double-acting cylinder:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

The following shows a diagram of a cylinder extending. Notice how Pascal's Law is demonstrated. As air flows in the cap end, the rod extends and air flows out of the rod end.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

The following shows a diagram of cylinder retracting; Pascal's law is demonstrated once again. As air flows in the rod end, the rod retracts and air flows out the cap end.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

At this point, you may be wondering why the air doesn't leak out of the cylinder around the rod. This is because there is a flexible seal called a rod seal that is placed around the rod, as shown in Figure 50. To hold this seal in place, a rod bushing is needed. This bushing also acts as a bearing to support the rod as it extends. In addition to the rod seal, there is also one or more piston seals. The piston seals keep air from leaking around the piston so that pressure can build up to move the load. It is very important to notice that the surfaces that the seals seal against are incredibly smooth. Any foreign debris that enters the cylinders and scratches the surfaces will cause the pneumatic components to leak. This is why we must prepare the air through filtration.

The following diagram illustrates the different seals found in a cylinder.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

In the following lab, we will hook up, extend, and retract a double-acting cylinder.

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